INTRODUCTTION

Industrial control system (ICS) is a general term that encompasses several types of control

systems used in industrial production, including supervisory control and data acquisition (SCADA)

systems, distributed control systems(DCS), and other smaller control system configurations such as

skid-mounted programmable logic controllers (PLC) often found in the industrial sectors and critical

infrastructures.

ICSs are typically used in industries such as electrical, water, oil, gas and data. Based on information

received from remote stations, automated or operator-driven supervisory commands can be pushed to

remote station control devices, which are often referred to as field devices. Field devices control local

operations such as opening and closing valves and breakers, collecting data from sensor systems,

and monitoring the local environment for alarm conditions.

Industrial control system technology has evolved over the decades. DCS systems generally refer to

the particular functional distributed control system design that exist in industrial process plants (e.g., oil

and gas, refining, chemical, pharmaceutical, some food and beverage, water and wastewater, pulp

and paper, utility power, mining, metals). The DCS concept came about from a need to gather data

and control the systems on a large campus in real time on high-bandwidth, low-latency data networks.

It is common for loop controls to extend all the way to the top level controllers in a DCS, as everything

works in real time. These systems evolved from a need to extend pneumatic control systems beyond

just a small cell area of arefinery.

The PLC (programmable logic controller) evolved out of a need to replace racks of relays in ladder

form. The latter were not particularly reliable, were difficult to rewire, and were difficult to diagnose.

PLC control tends to be used in very regular, high-speed binary controls, such as controlling a high-

speed printing press. Originally, PLC equipment did not have remote I/Oracks, and many couldn't

even perform more than rudimentary analog controls.

SCADA's history is rooted in distribution applications, such as power, natural gas, and water pipelines,

where there is a need to gather remote data through potentially unreliable or intermittent low-

bandwidth/high-latency links. SCADA systems use open-loop control with sites that are widely

separated geographically. A SCADA system uses RTUs (remote terminal units, also referred to as

remote telemetry units) to send supervisory data back to a control center. Most RTU systems always

did have some limited capacity to handle local controls while the master station is not available.

However, over the years RTU systems have grown more and more capable of handling local controls.

The boundaries between these system definitions are blurring as time goes on. The technical limits

that drove the designs of these various systems are no longer as much of an issue. Many PLC

platforms can now perform quite well as a small DCS, using remote I/O and reliable that some SCADA

systems actually manage closed loop control over long distances. With the increasing speed of today's

processors, many DCS products have a full line of PLC-like subsystems that weren't offered when

they were initially developed.

This led to the concept of a PAC (programmable automation controller or process automation

controller), that is an amalgamation of these three concepts. Time and the market will determine

whether this can simplify some of the terminology and confusion that surrounds these concepts today.

DCSs

DCSs are used to control industrial processes such as electric power generation, oil and gas

refineries, water and wastewater treatment, and chemical, food, and automotive production. DCSs are

integrated as a control architecture containing a supervisory level of control, overseeing multiple

integrated sub-systems that are responsible for controlling the details of a localized process.

Product and process control are usually achieved by deploying feed back or feed forward control loops

whereby key product and/or process conditions are automatically maintained around a desired set

point. To accomplish the desired product and/or process tolerance around a specified set point, only

specific programmable controllers are used.

PLCs

PLCs provide boolean logic operations, timers, and (in some models) continuous control. The

proportional, integral, and/or differential gains of the PLC continuous control feature may be tuned to

provide the desired tolerance as well as the rate of self-correction during process upsets. DCSs are

used extensively in process-based industries. PLCs are computer-based solid-state devices that

control industrial equipment and processes. While PLCs can control system components used

throughout SCADA and DCS systems, they are often the primary components in smaller control

system configurations. They are used to provide regulatory control of discrete processes such as

automobile assembly lines and power plant soot blowercontrols and are used extensively in almost all

industrial processes.

Symbol

Toggle switch

A toggle switch is a class of electrical switches that are manually actuated by a mechanical lever,

handle, or rocking mechanism.

Toggle switches are available in many different styles and sizes, and are used in countless

applications. Many are designed to provide the simultaneous actuation of multiple sets of electrical

contacts, or the control of large amounts of electric current or mains voltages.

The word "toggle" is a reference to a kind of mechanism or joint consisting of two arms, which are

almost in line with each other, connected with an elbow-like pivot. However, the phrase "toggle switch"

is applied to a switch with a short handle and a positive snap-action, whether it actually contains a

toggle mechanism or not. Similarly, a switch where a definitive click is heard, is called a "positive on-

off switch".

Push button

The "push-button" has been utilized in calculators, push-button telephones, kitchen appliances, and

various other mechanical and electronic devices, home and commercial.

In industrial and commercial applications, push buttons can be linked together by a mechanical linkage

so that the act of pushing one button causes the other button to be released. In this way, a stop button

can "force" a start button to be released. This method of linkage is used in simple manual operations

in which the machine or process have no electrical circuits for control.

Pushbuttons are often color-coded to associate them with their function so that the operator will not

push the wrong button in error. Commonly used colors are red for stopping the machine or process

and green for starting the machine or process.

Red pushbuttons can also have large heads (called mushroom heads) for easy operation and to

facilitate the stopping of a machine. These pushbuttons are called emergency stop buttons and are

mandated by the electrical code in many jurisdictions for increased safety. This large mushroom shape

can also be found in buttons for use with operators who need to wear gloves for their work and could

not actuate a regularflush-mounted push button. As an aid for operators and users in industrial or

commercial applications, a pilot light is commonly added to draw the attention of the user and to

provide feedback if the button is pushed. Typically this light is included into the center of the

pushbutton and alens replaces the pushbutton hard center disk. The source of the energy to illuminate

the light is not directly tied to the contacts on the back of the pushbutton but to the action the

pushbutton controls. In this way a start button when pushed will cause the process or machine

operation to be started and a secondary contact designed into the operation or process will close to

turn on the pilot light and signify the action of pushing the button caused the resultant process or

action to start.

In popular culture, the phrase "the button" (sometimes capitalized) refers to a (usually fictional) button

that a military or government leader could press to launch nuclear weapons.

Dual-function operating mode selector switch PITmodeEthiopian Review | May 3rd, 2010 at 5:00 pm

Pilz has expanded its range of operator and visualisation systems with the operating mode selector switch PITmode. The PITmode provides two functions in one compact unit: selection of operating mode and authorisation control for machine access. The operating mode selector switch has been developed for use in plant and machinery with a range of control sequences and operating modes.

As an operating mode selector switch, the PITmode enables you to switch between defined operating modes.

Individual (access) authorisations can be assigned for each operator via an RFID-based key. They are assigned via identification management in the machine control system. The evaluation device safely identifies and evaluates the selected operating mode, which plays a crucial role in avoiding accidents. Mechanical wear is excluded thanks to RFID technology.

The operating mode is selected by inserting a transponder key and pressing the pushbutton defined for the relevant operating mode. Each key is individually coded, which prevents manipulations. The administrative work is reduced because the functions of several mechanical keys can be combined within one transponder key.

By combining the operating mode selector switch PITmode with Pilz control solutions such as the configurable control system PNOZmulti, the result is an economical solution for selecting the operating mode and controlling access authorisations.

SOLENOID

A solenoid valve is an electromechanical valve for use with liquid or gas. The valve is controlled by

an electric current through a solenoid: in the case of a two-port valve the flow is switched on or off; in

the case of a three-port valve, the outflow is switched between the two outlet ports. Multiple solenoid

valves can be placed together on a manifold.

Solenoid valves are the most frequently used control elements in fluidics. Their tasks are to shut off,

release, dose, distribute or mix fluids. They are found in many application areas. Solenoids offer fast

and safe switching, high reliability, long service life, good medium compatibility of the materials used,

low control power and compact design.

Besides the plunger-type actuator which is used most frequently, pivoted-armature actuators and

rocker actuators are also used.

A solenoid valve has two main parts: the solenoid and the valve. The solenoid converts electrical

energy into mechanical energy which, in turn, opens or closes the valve mechanically. Adirect

acting valve has only a small flow circuit, shown within section E of this diagram (this section is

mentioned below as a pilot valve). This diaphragm piloted valve multiplies this small flow by using it to

control the flow through a much larger orifice.

Solenoid valves may use metal seals or rubber seals, and may also have electrical interfaces to allow

for easy control. A spring may be used to hold the valve opened or closed while the valve is not

activated.

A- Input side

B- Diaphragm

C- Pressure chamber

D- Pressure relief conduit

E- Solenoid

F- Output side

The diagram to the right shows the design of a basic valve. At the top figure is the valve in its closed

state. The water under pressure enters at A. B is an elastic diaphragm and above it is a weak spring

pushing it down. The function of this spring is irrelevant for now as the valve would stay closed even

without it. The diaphragm has a pinhole through its center which allows a very small amount of water

to flow through it. This water fills the cavity C on the other side of the diaphragm so that pressure is

equal on both sides of the diaphragm, however the compressed spring supplies a net downward force.

The spring is weak and is only able to close the inlet because water pressure is equalised on both

sides of the diaphram.

In the previous configuration the small conduit D was blocked by a pin which is the armature of

the solenoid E and which is pushed down by a spring. If the solenoid is activated by drawing the pin

upwards via magnetic force from the solenoid current, the water in chamber C will flow through this

conduit D to the output side of the valve. The pressure in chamber C will drop and the incoming

pressure will lift the diaphragm thus opening the main valve. Water now flows directly from A to F.

When the solenoid is again deactivated and the conduit D is closed again, the spring needs very little

force to push the diaphragm down again and the main valve closes. In practice there is often no

separate spring, the elastomer diaphragm is moulded so that it functions as its own spring, preferring

to be in the closed shape.

From this explanation it can be seen that this type of valve relies on a differential of pressure between

input and output as the pressure at the input must always be greater than the pressure at the output

for it to work. Should the pressure at the output, for any reason, rise above that of the input then the

valve would open regardless of the state of the solenoid and pilot valve.

In some solenoid valves the solenoid acts directly on the main valve. Others use a small, complete

solenoid valve, known as a pilot, to actuate a larger valve. While the second type is actually a solenoid

valve combined with a pneumatically actuated valve, they are sold and packaged as a single unit

referred to as a solenoid valve. Piloted valves require much less power to control, but they are

noticeably slower. Piloted solenoids usually need full power at all times to open and stay open, where

a direct acting solenoid may only need full power for a short period of time to open it, and only low

power to hold it.

Types

Many variations are possible on the basic, one way, one solenoid valve described above:

one or two solenoid valves;

direct current or alternating current powered;

different number of ways and positions;

Common uses

Solenoid valves are used in fluid power pneumatic and hydraulic systems, to control cylinders, fluid

power motors or larger industrial valves. Automatic irrigation sprinkler systems also use solenoid

valves with an automatic controller. Domestic washing machines and dishwashers use solenoid valves

to control water entry to the machine. In the paintball industry, solenoid valves are usually referred to

simply as "solenoids." They are commonly used to control a larger valve used to control the propellant

(usually compressed air or CO2).Solenoid valves are used in dental chairs to control air flow. In the

industry, "solenoid" may also refer to an electromechanical solenoid commonly used to actuate a sear.

Besides controlling the flow of air and fluids solenoids are used in pharmacology experiments,

especially for patch-clamp, which can control the application of agonist or antagonist

TIMER

A timer is a specialized type of clock. A timer can be used to control the sequence of an event or process. Whereas a stopwatch counts upwards from zero for measuring elapsed time, a timer counts down from a specified time interval, like an hourglass. Timers can bemechanical, electromechanical, electronic (quartz), or even software as all modern computers include digital timers of one kind or another. When the set period expires some timers simply indicate so (e.g., by an audible signal), while others operate electrical switches, such as atime switch, which cuts electrical power.

Mechanical timers

A typical mechanical timer

Mechanical timers regulate their speed. Inaccurate, cheap mechanisms use a flat beater that spins

against air resistance. Mechanical egg-timers are sometimes of this type.

More accurate mechanisms have mechanisms similar to mechanical alarm clocks; they require no

power, and can be stored for long periods of time. The most widely-known application is to

control explosives.

Electromechanical timers

Short-period bimetallic electromechanical timers use a thermal mechanism, with a metal finger made

of strips of two metals with different rates of thermal expansion sandwiched

together; steel and bronze are common. An electric current flowing through this finger causes heating

of the metals, one side expands less than the other, and an electrical contact on the end of the finger

moves away from or towards an electrical switch contact. The most common use of this type is in the

"flasher" units that flash turn signals in automobiles, and sometimes in Christmas lights. This is a non-

electronic type of multivibrator.

An electromechanical cam timEr uses a small synchronous AC motor turning a cam against a comb

of switch contacts. The AC motor is turned at an accurate rate by the alternating current, which power

companies carefully regulate. Gears drive a shaft at the desired rate, and turn the cam. The most

common application of this timer now is in washers, driers and dishwashers. This type of timer often

has a friction clutch between the gear train and the cam, so that the cam can be turned to reset the

time.

Electromechanical timers survive in these applications because mechanical switch contacts may still

be less expensive than the semiconductor devices needed to control powerful lights, motors and

heaters.

In the past these electromechanical timers were often combined with electrical relays to create electro-

mechanical controllers. Electromechanical timers reached a high state of development in the 1950s

and 60s because of their extensive use in aerospace and weapons systems. Programmable

electromechanical timers controlled launch sequence events in early rockets and ballistic missiles. As

digital electronics has progressed and dropped in price, electronic timers have become more

advantageous.

Electronic timers

Electronic timers are essentially quartz clocks with special electronics, and can achieve higher

precision than mechanical timers. Electronic timers have digital electronics, but may have

an analog or digital display. Integrated circuits have made digital logic so inexpensive that an

electronic timer is now less expensive than many mechanical and electromechanical timers. Individual

timers are implemented as a simple single-chip computer system, similar to a watch and usually using

the same, mass-produced, technology.

Many timers are now implemented in software. Modern controllers use a programmable logic

controller rather than a box full of electromechanical parts. The logic is usually designed as if it were

relays, using a special computer language called ladder logic. In PLCs, timers are usually simulated by

the software built into the controller. Each timer is just an entry in a table maintained by the software.

Digital timers are used in safety device such as a gas timer.

Computer timers

Computer systems usually have at least one timer. These are typically digital counters that either

increment or decrement at a fixed frequency, which is often configurable, and thatinterrupt the

processor when reaching zero, or a counter with a sufficiently large word size that it will not reach its

counter limit before the end of life of the system.

More sophisticated timers may have comparison logic to compare the timer value against a specific

value, set by software, that triggers some action when the timer value matches the preset value. This

might be used, for example, to measure events or generate pulse width modulated waveforms to

control the speed of motors (using a class D digital electronic amplifier).

As the number of hardware timers in a computer system or processor is finite and limited, operating

systems and embedded systems often use a single hardware timer to implement an extensible set of

software timers. In this scenario, the hardware timer's interrupt service routine would handle house-

keeping and management of as many software timers as are required, and the hardware timer would

be set to expire when the next software timer is due to expire. At expiry, the interrupt routine would

update the hardware timer to expire when the next software timer is due, and any actions would be

triggered for the software timers that had just expired. Expired timers that are continuous would also

be reset to a new expiry time based on their timer interval, and one-shot timers would be disabled or

removed from the set of timers. While simple in concept, care must be taken with software timer

implementation if issues such as timer drift and delayed interrupts is to be minimised.

COUNTERIn digital logic and computing, a counter is a device which stores (and sometimes displays) the number of times a particular event or process has occurred, often in relationship to aclock signal.

Electronic counters

In electronics, counters can be implemented quite easily using register-type circuits such as the flip-

flop, and a wide variety of classifications exist:

Asynchronous (ripple) counter – changing state bits are used as clocks to subsequent state flip-

flops

Synchronous counter – all state bits change under control of a single clock

Decade counter – counts through ten states per stage

Up/down counter – counts both up and down, under command of a control input

Ring counter – formed by a shift register with feedback connection in a ring

Johnson counter – a twisted ring counter

Cascaded counter

Each is useful for different applications. Usually, counter circuits are digital in nature, and count

in natural binary. Many types of counter circuits are available as digital building blocks, for example a

number of chips in the 4000 series implement different counters.

Occasionally there are advantages to using a counting sequence other than the natural binary

sequence—such as the binary coded decimal counter, a linear feedback shift registercounter, or

a Gray-code counter.

Counters are useful for digital clocks and timers, and in oven timers, VCR clocks, etc.[1]

Asynchronous (ripple) counter

Asynchronous counter created from two JK flip-flops

An asynchronous (ripple) counter is a single JK-type flip-flop, with its J (data) input fed from its own

inverted output. This circuit can store one bit, and hence can count from zero to one before it

overflows (starts over from 0). This counter will increment once for every clock cycle and takes two

clock cycles to overflow, so every cycle it will alternate between a transition from 0 to 1 and a transition

from 1 to 0. Notice that this creates a new clock with a 50% duty cycle at exactly half the frequency of

the input clock. If this output is then used as the clock signal for a similarly arranged D flip-flop

(remembering to invert the output to the input), you will get another 1 bit counter that counts half as

fast. Putting them together yields a two-bit counter:

Cycl

eQ1 Q0 (Q1:Q0)dec

0 0 0 0

1 0 1 1

2 1 0 2

3 1 1 3

4 0 0 0

You can continue to add additional flip-flops, always inverting the output to its own input, and using the

output from the previous flip-flop as the clock signal. The result is called a ripple counter, which can

count to 2n − 1 where n is the number of bits (flip-flop stages) in the counter. Ripple counters suffer

from unstable outputs as the overflows "Ripple" from stage to stage, but they do find frequent

application as dividers for clock signals, where the instantaneous count is unimportant, but the

division ratio overall is (to clarify this, a 1-bit counter is exactly equivalent to a divide by two circuit; the

output frequency is exactly half that of the input when fed with a regular train of clock pulses).

The use of flip-flop outputs as clocks leads to timing skew between the count data bits, making this

ripple technique incompatible with normal synchronous circuit design styles.

Synchronous counter

A 4-bit synchronous counter using JK flip-flops

A simple way of implementing the logic for each bit of an ascending counter (which is what is depicted

in the image to the right) is for each bit to toggle when all of the less significant bits are at a logic high

state. For example, bit 1 toggles when bit 0 is logic high; bit 2 toggles when both bit 1 and bit 0 are

logic high; bit 3 toggles when bit 2, bit 1 and bit 0 are all high; and so on.

Synchronous counters can also be implemented with hardware finite state machines, which are more

complex but allow for smoother, more stable transitions.

Hardware-based counters are of this type.

Decade counter

A circuit diagram of decade counter using JK FlipFlops(74LS112D)

A decade counter is one that counts in decimal digits, rather than binary. A decade counter may have

each digit binary encoded (that is, it may count in binary-coded decimal, as the 7490 integrated circuit

did) or other binary encodings (such as the bi-quinary encoding of the 7490 integrated circuit).

Alternatively, it may have a "fully decoded" or one-hot output code in which each output goes high in

turn (the 4017 is such a circuit). The latter type of circuit finds applications in multiplexers and

demultiplexers, or wherever a scanning type of behavior is useful. Similar counters with different

numbers of outputs are also common.

The decade counter is also known as a mod-counter when it counts to ten (0, 1, 2, 3, 4, 5, 6, 7, 8, 9).

A Mod Counter that counts to 64 stops at 63 because 0 counts as a valid digit.

Up/down counter

A counter that can change state in either direction, under the control of an up/down selector input, is

known as an up/down counter. When the selector is in the up state, the counter increments its value.

When the selector is in the down state, the counter decrements the count.

Ring counter

A ring counter is a circular shift register which is initiated such that only one of its flip-flops is the state

one while others are in their zero states.

A ring counter is a Shift Register (a cascade connection of flip-flops) with the output of the last one

connected to the input of the first, that is, in a ring. Typically, a pattern consisting of a single bit is

circulated so the state repeats every n clock cycles if n flip-flops are used. It can be used as a cycle

counter of n states.

Johnson counter

A Johnson counter (or switchtail ring counter, twisted-ring counter, walking-ring counter, or Moebius

counter) is a modified ring counter, where the output from the last stage is inverted and fed back as

input to the first stage. The register cycles through a sequence of bit-patterns, whose length is equal to

twice the length of the shift register, continuing indefinitely. These counters find specialist applications,

including those similar to the decade counter, digital-to-analog conversion, etc. They can be

implemented easily using D- or JK-type flip-flops.

Computer science counters

In computability theory, a counter is considered a type of memory. A counter stores a single natural

number (initially zero) and can be arbitrarily many digits long. A counter is usually considered in

conjunction with a finite-state machine (FSM), which can perform the following operations on the

counter:

Check whether the counter is zero

Increment the counter by one.

Decrement the counter by one (if it's already zero, this leaves it unchanged).

The following machines are listed in order of power, with each one being strictly more powerful than

the one below it:

1. Deterministic or non-deterministic FSM plus two counters

2. Non-deterministic FSM plus one stack

3. Non-deterministic FSM plus one counter

4. Deterministic FSM plus one counter

5. Deterministic or non-deterministic FSM

For the first and last, it doesn't matter whether the FSM is a deterministic finite automaton or

a nondeterministic finite automaton. They have equivalent power. The first two and the last one are

levels of the Chomsky hierarchy.

The first machine, an FSM plus two counters, is equivalent in power to a Turing machine. See the

article on counter machines for a proof.

Mechanical counters

Mechanical counter wheels showing both sides. The bump on the wheel shown at the top engages the ratchet on the

wheel below every turn.

Several mechanical counters

Long before electronics became common, mechanical devices were used to count events. These are

known as tally counters. They typically consist of a series of disks mounted on an axle, with the digits

0 through 9 marked on their edge. The right most disk moves one increment with each event. Each

disk except the left-most has a protrusion that, after the completion of one revolution, moves the next

disk to the left one increment. Such counters were originally used to control manufacturing processes,

but were later used as odometers for bicycles and cars and in fuel dispensers. One of the largest

manufacturers was the Veeder-Root company, and their name was often used for this type of counter.

POTENTIOMETERA potentiometer, informally, a pot, in electronics technology is a component, a three-

terminal resistor with a sliding contact that forms an adjustable voltage divider.[1] If only two terminals

are used, one end and the wiper, it acts as a variable resistor orrheostat.

In circuit theory and measurement a potentiometer is essentially a voltage divider used for

measuring electric potential (voltage); the component is an implementation of the same principle,

hence its name.

Potentiometers are commonly used to control electrical devices such as volume controls on audio

equipment. Potentiometers operated by a mechanism can be used as position transducers, for

example, in a joystick.

Potentiometers are rarely used to directly control significant power (more than a watt), since the power

dissipated in the potentiometer would be comparable to the power in the controlled load (see infinite

switch). Instead they are used to adjust the level of analog signals (e.g. volumecontrols on audio

equipment), and as control inputs for electronic circuits. For example, a light dimmer uses a

potentiometer to control the switching of a TRIAC and so indirectly to control the brightness of lamps.

Potentiometer

A typical single-turn potentiometer

Type Passive

Electronic symbol

 (International)

 (US)

Potentiometers comprise a resistive element, a sliding contact (wiper) that moves along the element,

making good electrical contact with one part of it, electrical terminals at each end of the element, a

mechanism that moves the wiper from one end to the other, and a housing containing the element and

wiper.

The resistive element of inexpensive potentiometers is often made of graphite. Other materials used

include resistance wire, carbon particles in plastic, and a ceramic/metal mixture called cermet.

Conductive track potentiometers use conductive polymer resistor pastes that contain hard-wearing

resins and polymers, solvents, and lubricant, in addition to the carbon that provides the conductive

properties. The tracks are made by screen-printing the paste onto a paper-based phenolic substrate

and then curing it in an oven. The curing process removes all solvents and allows the conductive

polymer to polymerize and cross-link. This produces a durable track with electrical resistance which is

stable throughout its working life. Low-resistance wire-wound potentiometers may be made with

resistive wire close-wound round a former with a slider jumping from turn to turn.

Some potentiometers are designed to be operated by the user of equipment, and are fitted with a

slider or rotating shaft which extends outside the housing of the equipment using it and is fitted with a

knob; a familiar example is the volume control knob of analog audio equipment. Others are enclosed

within the equipment and are intended to be adjusted to calibrate equipment during manufacture or

repair, and not otherwise touched. They are usually physically much smaller than user-accessible

potentiometers, and may need to be operated by a screwdriver rather than having a knob. They are

usually called "preset potentiometers". Some presets are accessible by a small screwdriver poked

through a hole in the case to allow servicing without dismantling.

User-accessible rotary potentiometers can be fitted with a switch which operates usually at the anti-

clockwise extreme of rotation. Before digital electronics became the norm such a component was used

to allow radio and television receivers and other equipment to be switched on at minimum volume with

an audible click, then the volume increased, by turning a knob.

Many inexpensive potentiometers are constructed with a resistive element formed into an arc of a

circle usually a little less than a full turn, and a wiper rotating around the arc and contacting it. The

resistive element, with a terminal at each end, is flat or angled. The wiper is connected to a third

terminal, usually between the other two. On panel potentiometers, the wiper is usually the center

terminal of three. For single-turn potentiometers, this wiper typically travels just under one revolution

around the contact. The only point of ingress for contamination is the narrow space between the shaft

and the housing it rotates in.

Another type is the linear slider potentiometer, which has a wiper which slides along a linear element

instead of rotating. Contamination can potentially enter anywhere along the slot the slider moves in,

making effective sealing more difficult and compromising long-term reliability. An advantage of the

slider potentiometer is that the slider position gives a visual indication of its setting. While the setting of

a rotary potentiometer can be seen by the position of a marking on the knob, an array of sliders can

give a visual impression of, for example, the effect of a multi-channel equaliser.

Multiturn potentiometers are also operated by rotating a shaft, but by several turns rather than less

than a full turn. Some multiturn potentiometers have a linear resistive element with a slider which

moves along it moved by a worm gear; others have a helical resistive element and a wiper that turns

through 10, 20, or more complete revolutions, moving along the helix as it rotates. Multiturn

potentiometers, both user-accessible and preset, allow finer adjustments; rotation through the same

angle changes the setting by typically a tenth as much as for a simple rotary potentiometer.

A string potentiometer is a multi-turn potentiometer operated by an attached reel of wire turning

against a spring, enabling it to convert linear position to a variable resistance.

PCB mount trimmer potentiometers, or "trimpots", intended for infrequent adjustment.

Resistance–position relationship: "taper"

The relationship between slider position and resistance, known as the "taper" or "law", is controlled by

the manufacturer. In principle any relationship is possible, but for most

purposes linear or logarithmic (aka "audio taper") potentiometers are sufficient. A letter code ("A"

taper, "B" taper, etc.) may be used to identify which taper is used, but the letter code definitions are

not standardised.

Linear taper potentiometer

A linear taper potentiometer (linear describes the electrical characteristic of the device, not the

geometry of the resistive element) has a resistive element of constant cross-section, resulting in a

device where the resistance between the contact (wiper) and one end terminal isproportional to the

distance between them. Linear taper potentiometers are used when the division ratio of the

potentiometer must be proportional to the angle of shaft rotation (or slider position), for example,

controls used for adjusting the centering of an analog cathode-rayoscilloscope.

Logarithmic potentiometer

A logarithmic taper potentiometer has a resistive element that either 'tapers' in from one end to the

other, or is made from a material whose resistivity varies from one end to the other. This results in a

device where output voltage is a logarithmic function of the slider position.

Most (cheaper) "log" potentiometers are not accurately logarithmic, but use two regions of different

resistance (but constant resistivity) to approximate a logarithmic law. A logarithmic potentiometer can

also be simulated (not very accurately) with a linear one and an external resistor. True logarithmic

potentiometers are significantly more expensive.

Logarithmic taper potentiometers are often used in connection with audio amplifiers as human

perception of audio volume is logarithmic.

A high power wirewound potentiometer. Any potentiometer may be connected as a rheostat.

Rheostat

The most common way to vary the resistance in a circuit is to use a rheostat,[2] a two-terminal variable

resistor. For low-power applications (less than about 1 watt) a three-terminal potentiometer is often

used, with one terminal unconnected or connected to the wiper.

Where the rheostat must be rated for higher power (more than about 1 watt), they may be built with a

resistance wire wound around a semicircular insulator, with the wiper sliding from one turn of the wire

to the next. Sometimes a rheostat is made from resistance wire wound on a heat-resisting cylinder,

with the slider made from a number of metal fingers that grip lightly onto a small portion of the turns of

resistance wire. The "fingers" can be moved along the coil of resistance wire by a sliding knob thus

changing the "tapping" point. Wire-wound rheostats made with ratings up to several thousand watts

are used in applications such as DC motor drives, electric welding controls, or in the controls for

generators.

Digital potentiometer

A digital potentiometer is an electronic component that mimics the functions of analog potentiometers.

Through digital input signals, the resistance between two terminals can be adjusted, just as in an

analog potentiometer.

Membrane potentiometer

A membrane potentiometer uses a conductive membrane that is deformed by a sliding element to

contact a resistor voltage divider. Linearity can range from 0.5% to 5% depending on the material,

design and manufacturing process. The repeat accuracy is typically between 0.1mm and 1.0mm with a

theoretically infinite resolution. The service life of these types of potentiometers is typically 1 million to

20 million cycles depending on the materials used during manufacturing and the actuation method;

contact and contactless (magnetic) methods are available. Many different material variations are

available such as PET(foil), FR4, and Kapton. Membrane potentiometer manufacturers offer linear,

rotary, and application-specific variations. The linear versions can range from 9mm to 1000mm in

length and the rotary versions range from 0° to multiple full turns, with each having a height of 0.5mm.

Membrane potentiometers can be used for position sensing.

Potentiometer applications

Preset potentiometers are widely used throughout electronics wherever adjustments must be made

during manufacturing or servicing.

User-actuated potentiometers are widely used as user controls, and may control a very wide variety of

equipment functions. The widespread use of potentiometers in consumer electronics declined in the

1990s, with digital controls now more common. However they remain in many applications, such as

volume controls and as position sensors.

Audio control

Linear potentiometers ("faders")

Low-power potentiometers, both linear and rotary, are used to control audio equipment, changing

loudness, frequency attenuation and other characteristics of audio signals.

The 'log pot' is used as the volume control in audio amplifiers, where it is also called an "audio taper

pot", because the amplitude response of the human ear is approximately logarithmic. It ensures that

on a volume control marked 0 to 10, for example, a setting of 5 sounds subjectively half as loud as a

setting of 10. There is also an anti-log pot or reverse audio taper which is simply the reverse of a

logarithmic potentiometer. It is almost always used in a ganged configuration with a logarithmic

potentiometer, for instance, in an audio balance control.

Potentiometers used in combination with filter networks act as tone controls or equalizers.

Television

Potentiometers were formerly used to control picture brightness, contrast, and color response. A

potentiometer was often used to adjust "vertical hold", which affected the synchronization between the

receiver's internal sweep circuit (sometimes a multivibrator) and the received picture signal, along with

other things such as audio-video carrier offset, tuning frequency (for push-button sets) and so on.

Transducers

Potentiometers are also very widely used as a part of displacement transducers because of the

simplicity of construction and because they can give a large output signal.

Computation

In analog computers, high precision potentiometers are used to scale intermediate results by desired

constant factors, or to set initial conditions for a calculation. A motor-driven potentiometer may be used

as a function generator, using a non-linear resistance card to supply approximations to trigonometric

functions. For example, the shaft rotation might represent an angle, and the voltage division ratio can

be made proportional to the cosine of the angle.

TRANSFORMERA transformer is a device that transfers electrical energy from one circuit to another

through inductively coupled conductors—the transformer's coils. A varying current in the first

or primary winding creates a varying magnetic flux in the transformer's core and thus a

varying magnetic fieldthrough the secondary winding. This varying magnetic field induces a

varying electromotive force (EMF), or "voltage", in the secondary winding. This effect is

called inductive coupling.

If a load is connected to the secondary, current will flow in the secondary winding, and electrical

energy will be transferred from the primary circuit through the transformer to the load. In an ideal

transformer, the induced voltage in the secondary winding (Vs) is in proportion to the primary voltage

(Vp) and is given by the ratio of the number of turns in the secondary (Ns) to the number of turns in the

primary (Np) as follows:

By appropriate selection of the ratio of turns, a transformer thus enables an alternating current

(AC) voltage to be "stepped up" by making Nsgreater than Np, or "stepped down" by

making Ns less than Np.

In the vast majority of transformers, the windings are coils wound around a ferromagnetic

core, air-core transformers being a notable exception.

Transformers range in size from a thumbnail-sized coupling transformer hidden inside a

stage microphone to huge units weighing hundreds of tons used to interconnect portions of power

grids. All operate on the same basic principles, although the range of designs is wide. While new

technologies have eliminated the need for transformers in some electronic circuits, transformers

are still found in nearly all electronic devices designed for household ("mains") voltage.

Transformers are essential for high-voltage electric power transmission, which makes long-

distance transmission economically practical.

Discovery

Faraday's experiment with induction between coils of wire 

The phenomenon of electromagnetic induction was discovered independently by Michael

Faraday and Joseph Henry in 1831. However, Faraday was the first to publish the results of his

experiments and thus receive credit for the discovery. The relationship betweenelectromotive

force (EMF) or "voltage" and magnetic flux was formalized in an equation now referred to as

"Faraday's law of induction":

.

where   is the magnitude of the EMF in volts and ΦB is the magnetic flux through the circuit

in webers.

Faraday performed the first experiments on induction between coils of wire, including winding a

pair of coils around an iron ring, thus creating the first toroidal closed-core transformer.

Induction coils

The first type of transformer to see wide use was the induction coil, invented by Rev. Nicholas

Callan of Maynooth College, Ireland in 1836. He was one of the first researchers to realize that

the more turns the secondary winding has in relation to the primary winding, the larger is the

increase in EMF. Induction coils evolved from scientists' and inventors' efforts to get higher

voltages from batteries. Since batteries produce direct current (DC) rather thanalternating

current (AC), induction coils relied upon vibrating electrical contacts that regularly interrupted the

current in the primary to create the flux changes necessary for induction. Between the 1830s and

the 1870s, efforts to build better induction coils, mostly by trial and error, slowly revealed the basic

principles of transformers.

Faraday's ring transformer

By the 1870s, efficient generators that produced alternating current (alternators) were available,

and it was found that alternating current could power an induction coil directly, without

an interrupter. In 1876, Russian engineer Pavel Yablochkov invented a lighting system based on a

set of induction coils where the primary windings were connected to a source of alternating

current and the secondary windings could be connected to several "electric candles" (arc lamps)

of his own design. The coils Yablochkov employed functioned essentially as transformers.

In 1878, the Ganz Company in Hungary began manufacturing equipment for electric lighting and,

by 1883, had installed over fifty systems inAustria-Hungary. Their systems used alternating

current exclusively and included those comprising both arc and incandescent lamps, along

with generators and other equipment.

Lucien Gaulard and John Dixon Gibbs first exhibited a device with an open iron core called a

"secondary generator" in London in 1882, then sold the idea to the Westinghouse company in

the United States. They also exhibited the invention in Turin, Italy in 1884, where it was adopted

for an electric lighting system. However, the efficiency of their open-core bipolar apparatus

remained very low.

Induction coils with open magnetic circuits are inefficient for transfer of power to loads. Until about

1880, the paradigm for AC power transmission from a high voltage supply to a low

voltage load was a series circuit. Open-core transformers with a ratio near 1:1 were connected

with their primaries in series to allow use of a high voltage for transmission while presenting a low

voltage to the lamps. The inherent flaw in this method was that turning off a single lamp affected

the voltage supplied to all others on the same circuit. Many adjustable transformer designs were

introduced to compensate for this problematic characteristic of the series circuit, including those

employing methods of adjusting the core or bypassing the magnetic flux around part of a

coil. Efficient, practical transformer designs did not appear until the 1880s, but within a decade,

the transformer would be instrumental in the "War of Currents", and in seeing AC distribution

systems triumph over their DC counterparts, a position in which they have remained dominant

ever since.

Closed-core transformers and parallel power distribution

Shell-form transformer. Sketch used by Uppenborn to describe Z.B.D. engineers' 1885 patents and earliest articles.

Core-form, front; shell-form, back. Earliest specimens of Z.B.D.-designed high-efficiency constant-potential transformers

manufactured at the Ganz factory in 1885.

Stanley's 1886 design for adjustable gap open-core induction coils[11]

In the autumn of 1884, Károly Zipernowsky, Ottó Bláthy and Miksa Déri (Z.B.D.), three engineers

associated with the Ganz factory, had determined that open-core devices were impracticable, as they

were incapable of reliably regulating voltage. In their joint 1885 patent applications for novel

transformers (later called Z.B.D. transformers), they described two designs with closed magnetic

circuits where copper windings were either a) wound around iron wire ring core or b) surrounded by

iron wire core. The two designs were the first application of the two basic transformer construction

types in common use to this day which can as a class all be termed as either core-form or shell-form

(or alternatively, core-type or shell-type), as in a) or b), respectively (see images). The Ganz

factory had also in the autumn of 1884 made delivery of the world's first five high-efficiency AC

transformers, the first of these units having been shipped on September 16, 1884.This first unit had

been manufactured to the following specifications: 1,400 W, 40 Hz, 120:72 V, 11.6:19.4 A, ratio 1.67:1,

one-phase, shell-form. In both designs, the magnetic flux linking the primary and secondary windings

traveled almost entirely within the confines of the iron core, with no intentional path through air (see

'Toroidal cores' below). The new transformers were 3.4 times more efficient than the open core bipolar

devices of Gaulard and Gibbs. Their patents included two other major interrelated innovations: one

concerning the use of parallel connected, instead of series connected, utilization loads, the other

concerning the ability to have high turns ratio transformers such that the supply network voltage could

be much higher (initially 1,400 to 2,000 V) than the voltage of utilization loads (100 V initially

preferred).  When they employed them in parallel connected electric distribution systems, closed-core

transformers finally made it technically and economically feasible to provide electric power for lighting

in homes, businesses and public spaces. Bláthy had suggested the use of closed-cores, Zipernowsky

the use of parallel shunt connections, and Déri had performed the experiments;. The vast majority of

transformers in use today are based on principles discovered by the three engineers. They also

popularized the word "transformer" to describe a device for altering the EMF of an electric

current, although the term had already been in use by 1882. In 1886, the Z.B.D. engineers designed,

and the Ganz factory supplied electrical equipment for, the world's first power station that used

AC generators to power a parallel-connected common electrical network, the steam-powered Rome-

Cerchi power plant.

Although George Westinghouse had bought Gaulard and Gibbs' patents in 1885, the Edison Electric

Light Company held an option on the U.S. rights for the Z.B.D. transformers, requiring Westinghouse

to pursue alternative designs on the same principles. He assigned to William Stanley the task of

developing a device for commercial use in United States. Stanley's first patented design was for

induction coils with single cores of soft iron and adjustable gaps to regulate the EMF present in the

secondary winding (see image). This design  was first used commercially in the U.S. in 1886 but

Westinghouse was intent on improving the Stanley design to make it (unlike the Z.B.D. type) easy and

cheap to produce. Westinghouse, Stanley and a few other associates had soon developed a core

consisting of a stack of thin "E-shaped" iron plates, separated individually or in pairs by thin sheets of

paper or other insulating material. Prewound copper coils could then be slid into place, and straight

iron plates laid in to create a closed magnetic circuit. Westinghouse applied for a patent for the new

design in December 1886; it was granted in July 1887.

Other early transformers

In 1889, Russian-born engineer Mikhail Dolivo-Dobrovolsky developed the first three-

phase transformer at the Allgemeine Elektricitäts-Gesellschaft ("General Electricity Company") in

Germany.

In 1891, Nikola Tesla invented the Tesla coil, an air-cored, dual-tuned resonant transformer for

generating very high voltages at high frequency.

Audio frequency transformers ("repeating coils") were used by early experimenters in the development of the telephone.

Basic principles

An ideal transformer. The secondary current arises from the action of the secondary EMF on the (not shown) load

impedance.

The transformer is based on two principles: first, that an electric current can produce a magnetic

field (electromagnetism) and second that a changing magnetic field within a coil of wire induces a

voltage across the ends of the coil (electromagnetic induction). Changing the current in the primary

coil changes the magnetic flux that is developed. The changing magnetic flux induces a voltage in the

secondary coil.

An ideal transformer is shown in the adjacent figure. Current passing through the primary coil creates

a magnetic field. The primary and secondary coils are wrapped around a core of very high magnetic

permeability, such as iron, so that most of the magnetic flux passes through both the primary and

secondary coils. If a load is connected to the secondary winding, the load current and voltage will be in

the directions indicated, given the primary current and voltage in the directions indicated (each will

be alternating current in practice).

Induction law

The voltage induced across the secondary coil may be calculated from Faraday's law of induction,

which states that:

where Vs is the instantaneous voltage, Ns is the number of turns in the secondary coil and Φ is

the magnetic flux through one turn of the coil. If the turns of the coil are oriented perpendicularly to the

magnetic field lines, the flux is the product of the magnetic flux density B and the area A through which

it cuts. The area is constant, being equal to the cross-sectional area of the transformer core, whereas

the magnetic field varies with time according to the excitation of the primary. Since the same magnetic

flux passes through both the primary and secondary coils in an ideal transformer, the instantaneous

voltage across the primary winding equals

Taking the ratio of the two equations for Vs and Vp gives the basic equation for stepping up or stepping

down the voltage

Np/Ns is known as the turns ratio, and is the primary functional characteristic of any transformer. In the

case of step-up transformers, this may sometimes be stated as the reciprocal,Ns/Np. Turns ratio is

commonly expressed as an irreducible fraction or ratio: for example, a transformer with primary and

secondary windings of, respectively, 100 and 150 turns is said to have a turns ratio of 2:3 rather than

0.667 or 100:150.

Ideal power equation

The ideal transformer as a circuit element

If the secondary coil is attached to a load that allows current to flow, electrical power is transmitted

from the primary circuit to the secondary circuit. Ideally, the transformer is perfectly efficient. All the

incoming energy is transformed from the primary circuit to themagnetic field and into the secondary

circuit. If this condition is met, the input electric power must equal the output power:

giving the ideal transformer equation

This formula is a reasonable approximation for most commercial built transformers today.

If the voltage is increased, then the current is decreased by the same factor. The impedance in one

circuit is transformed by thesquare of the turns ratio. For example, if an impedance Zs is attached

across the terminals of the secondary coil, it appears to the primary circuit to have an impedance of

(Np/Ns)2Zs. This relationship is reciprocal, so that the impedance Zp of the primary circuit appears to the

secondary to be (Ns/Np)2Zp.

Detailed operation

The simplified description above neglects several practical factors, in particular, the primary current

required to establish a magnetic field in the core, and the contribution to the field due to current in the

secondary circuit.

Models of an ideal transformer typically assume a core of negligible reluctance with two windings of

zero resistance. When a voltage is applied to the primary winding, a small current flows,

driving flux around the magnetic circuit of the core.: The current required to create the flux is termed

the magnetizing current. Since the ideal core has been assumed to have near-zero reluctance, the

magnetizing current is negligible, although still required, to create the magnetic field.

The changing magnetic field induces an electromotive force (EMF) across each winding. Since the

ideal windings have no impedance, they have no associated voltage drop, and so the voltages VP and

VS measured at the terminals of the transformer, are equal to the corresponding EMFs. The primary

EMF, acting as it does in opposition to the primary voltage, is sometimes termed the "back EMF". This

is in accordance with Lenz's law, which states that induction of EMF always opposes development of

any such change in magnetic field.

Practical considerations

Leakage flux

Leakage flux of a transformer

The ideal transformer model assumes that all flux generated by the primary winding links all the turns

of every winding, including itself. In practice, some flux traverses paths that take it outside the

windings. Such flux is termed leakage flux, and results inleakage inductance in series with the

mutually coupled transformer windings. Leakage results in energy being alternately stored in and

discharged from the magnetic fields with each cycle of the power supply. It is not directly a power loss

(see "Stray losses" below), but results in inferior voltage regulation, causing the secondary voltage to

not be directly proportional to the primary voltage, particularly under heavy load. Transformers are

therefore normally designed to have very low leakage inductance. Nevertheless, it is impossible to

eliminate all leakage flux because it plays an essential part in the operation of the transformer. The

combined effect of the leakage flux and the electric field around the windings is what transfers energy

from the primary to the secondary.

In some applications increased leakage is desired, and long magnetic paths, air gaps, or magnetic

bypass shunts may deliberately be introduced in a transformer design to limit the short-circuit current it

will supply. Leaky transformers may be used to supply loads that exhibit negative resistance, such

as electric arcs, mercury vapor lamps, and neon signs or for safely handling loads that become periodically short-circuited such as electric arc welders.

Air gaps are also used to keep a transformer from saturating, especially audio-frequency transformers

in circuits that have a direct current component flowing through the windings

Leakage inductance is also helpful when transformers are operated in parallel. It can be shown that if

the "per-unit" inductance of two transformers is the same (a typical value is 5%), they will automatically

split power "correctly" (e.g. 500 kVA unit in parallel with 1,000 kVA unit, the larger one will carry twice

the current)

Effect of frequency

Transformer universal EMF equation

If the flux in the core is purely sinusoidal, the relationship for either winding between

its rmsvoltage Erms of the winding, and the supply frequency f, number of turns N, core cross-sectional

area a and peak magnetic flux density B is given by the universal EMF equation:

If the flux does not contain even harmonics the following equation can be used for half-cycle average

voltage Eavg of any waveshape:

The time-derivative term in Faraday's Law shows that the flux in the core is the integral with respect to

time of the applied voltage. Hypothetically an ideal transformer would work with direct-current

excitation, with the core flux increasing linearly with time. In practice, the flux rises to the point

where magnetic saturation of the core occurs, causing a large increase in the magnetizing current and

overheating the transformer. All practical transformers must therefore operate with alternating (or

pulsed direct) current.

The EMF of a transformer at a given flux density increases with frequency. By operating at higher

frequencies, transformers can be physically more compact because a given core is able to transfer

more power without reaching saturation and fewer turns are needed to achieve the same impedance.

However, properties such as core loss and conductor skin effect also increase with frequency. Aircraft

and military equipment employ 400 Hz power supplies which reduce core and winding

weight. Conversely, frequencies used for some railway electrification systems were much lower (e.g.

16.7 Hz and 25 Hz) than normal utility frequencies (50 – 60 Hz) for historical reasons concerned

mainly with the limitations of early electric traction motors. As such, the transformers used to step

down the high over-head line voltages (e.g. 15 kV) were much heavier for the same power rating than

those designed only for the higher frequencies.

Operation of a transformer at its designed voltage but at a higher frequency than intended will lead to

reduced magnetizing current. At a lower frequency, the magnetizing current will increase. Operation of

a transformer at other than its design frequency may require assessment of voltages, losses, and

cooling to establish if safe operation is practical. For example, transformers may need to be equipped

with "volts per hertz" over-excitation relays to protect the transformer from overvoltage at higher than

rated frequency.

One example of state-of-the-art design is transformers used for electric multiple unit high speed trains,

particularly those required to operate across the borders of countries using different electrical

standards. The position of such transformers is restricted to being hung below the passenger

compartment. They have to function at different frequencies (down to 16.7 Hz) and voltages (up to 25

kV) whilst handling the enhanced power requirements needed for operating the trains at high speed.

Knowledge of natural frequencies of transformer windings is necessary for the determination of

winding transient response and switching surge voltages.

Energy losses

An ideal transformer would have no energy losses, and would be 100% efficient. In practical

transformers, energy is dissipated in the windings, core, and surrounding structures. Larger

transformers are generally more efficient, and those rated for electricity distribution usually perform

better than 98%.

Experimental transformers using superconducting windings achieve efficiencies of 99.85%. The

increase in efficiency can save considerable energy, and hence money, in a large heavily loaded

transformer; the trade-off is in the additional initial and running cost of the superconducting design.

Losses in transformers (excluding associated circuitry) vary with load current, and may be expressed

as "no-load" or "full-load" loss. Winding resistance dominates load losses,

whereashysteresis and eddy currents losses contribute to over 99% of the no-load loss. The no-load

loss can be significant, so that even an idle transformer constitutes a drain on the electrical supply and

a running cost. Designing transformers for lower loss requires a larger core, good-quality silicon steel,

or even amorphous steel for the core and thicker wire, increasing initial cost so that there is a trade-

off between initial cost and running cost (also see energy efficient transformer).

Transformer losses are divided into losses in the windings, termed copper loss, and those in the

magnetic circuit, termed iron loss. Losses in the transformer arise from:

Winding resistance

Current flowing through the windings causes resistive heating of the conductors. At higher

frequencies, skin effect and proximity effect create additional winding resistance and losses.

Hysteresis losses

Each time the magnetic field is reversed, a small amount of energy is lost due to hysteresis within the

core. For a given core material, the loss is proportional to the frequency, and is a function of the peak

flux density to which it is subjected.

Eddy currents

Ferromagnetic materials are also good conductors and a core made from such a material also

constitutes a single short-circuited turn throughout its entire length. Eddy currentstherefore circulate

within the core in a plane normal to the flux, and are responsible for resistive heating of the core

material. The eddy current loss is a complex function of the square of supply frequency and inverse

square of the material thickness. Eddy current losses can be reduced by making the core of a stack of

plates electrically insulated from each other, rather than a solid block; all transformers operating at low

frequencies use laminated or similar cores.

Magnetostriction

Magnetic flux in a ferromagnetic material, such as the core, causes it to physically expand and

contract slightly with each cycle of the magnetic field, an effect known asmagnetostriction. This

produces the buzzing sound commonly associated with transformers[35] that can cause losses due to

frictional heating. This buzzing is particularly familiar from low-frequency (50 Hz or 60 Hz) mains hum,

and high-frequency (15,734 Hz (NTSC) or 15,625 Hz (PAL)) CRT noise.

Mechanical losses

In addition to magnetostriction, the alternating magnetic field causes fluctuating forces between the

primary and secondary windings. These incite vibrations within nearby metalwork, adding to

the buzzing noise and consuming a small amount of power.

Stray losses

Leakage inductance is by itself largely lossless, since energy supplied to its magnetic fields is returned

to the supply with the next half-cycle. However, any leakage flux that intercepts nearby conductive

materials such as the transformer's support structure will give rise to eddy currents and be converted

to heat. There are also radiative losses due to the oscillating magnetic field but these are usually

small.

Dot convention

It is common in transformer schematic symbols for there to be a dot at the end of each coil within a

transformer, particularly for transformers with multiple primary and secondary windings. The dots

indicate the direction of each winding relative to the others. Voltages at the dot end of each winding

are in phase; current flowing into the dot end of a primary coil will result in current flowing out of the

dot end of a secondary coil.

[edit]Core form and shell form transformers

Core form = core type; shell form = shell type

As first mentioned in regard to earliest ZBD closed-core transformers, transformers are generally

considered to be either core form or shell form in design depending on the type of magnetic circuit

used in winding construction (see image). That is, when winding coils are wound around the core,

transformers are termed as being of core form design; when winding coils are surrounded by the core,

transformers are termed as being of shell form design. Shell form design may be more prevalent than

core form design for distribution transformer applications due to the relative ease in stacking the core

around winding coils. Core form design tends to, as a general rule, be more economical, and therefore

more prevalent, than shell form design for high voltage power transformer applications at the lower

end of their voltage and power rating ranges (less than or equal to, nominally, 230 kV or 75 MVA). At

higher voltage and power ratings, shell form transformers tend to be more prevalent. Shell form design

tends to be preferred for extra high voltage and higher MVA applications because, though more labor

intensive to manufacture, shell form transformers are characterized as having inherently better kVA-to-

weight ratio, better short-circuit strength characteristics and higher immunity to transit damage.

Equivalent circuit

Refer to the diagram below

The physical limitations of the practical transformer may be brought together as an equivalent circuit

model (shown below) built around an ideal lossless transformer. Power loss in the windings is current-

dependent and is represented as in-series resistances Rp and Rs. Flux leakage results in a fraction of

the applied voltage dropped without contributing to the mutual coupling, and thus can be modeled as

reactances of each leakage inductance Xp and Xs in series with the perfectly coupled region.

Iron losses are caused mostly by hysteresis and eddy current effects in the core, and are proportional

to the square of the core flux for operation at a given frequency. Since the core flux is proportional to

the applied voltage, the iron loss can be represented by a resistance RC in parallel with the ideal

transformer.

A core with finite permeability requires a magnetizing current Im to maintain the mutual flux in the core.

The magnetizing current is in phase with the flux. Saturation effects cause the relationship between

the two to be non-linear, but for simplicity this effect tends to be ignored in most circuit

equivalents. With a sinusoidal supply, the core flux lags the induced EMF by 90° and this effect can be

modeled as a magnetizing reactance (reactance of an effective inductance) Xm in parallel with the core

loss component, Rc. Rc and Xm are sometimes together termed the magnetizing branch of the model. If

the secondary winding is made open-circuit, the current I0 taken by the magnetizing branch represents

the transformer's no-load current.

The secondary impedance Rs and Xs is frequently moved (or "referred") to the primary side after

multiplying the components by the impedance scaling factor (Np/Ns)2.

Transformer equivalent circuit, with secondary impedances referred to the primary side

The resulting model is sometimes termed the "exact equivalent circuit", though it retains a number of

approximations, such as an assumption of linearity.[55] Analysis may be simplified by moving the

magnetizing branch to the left of the primary impedance, an implicit assumption that the magnetizing

current is low, and then summing primary and referred secondary impedances, resulting in so-called

equivalent impedance.

The parameters of equivalent circuit of a transformer can be calculated from the results of two

transformer tests: open-circuit test and short-circuit test.

Types

For more details on this topic, see Transformer types.

A wide variety of transformer designs are used for different applications, though they share several

common features. Important common transformer types are described below.

Autotransformer

A variable autotransformer

In an autotransformer portions of the same winding act as both the primary and secondary. The

winding has at least three taps where electrical connections are made. An autotransformer can be

smaller, lighter and cheaper than a standard dual-winding transformer however the autotransformer

does not provide electrical isolation.

As an example of the material saving an autotransformer can provide, consider a double wound 2 kVA

transformer designed to convert 240 volts to 120 volts. Such a transformer would require 8 amp wire

for the 240 volt primary and 16 amp wire for the secondary. If constructed as an autotransformer, the

output is a simple tap at the centre of the 240 volt winding. Even though the whole winding can be

wound with 8 amp wire, 16 amps can nevertheless be drawn from the 120 volt tap. This comes about

because the 8 amp 'primary' current is of opposite phase to the 16 amp 'secondary' current and thus it

is the difference current that flows in the common part of the winding (8 amps). There is also

considerable potential for savings on the core material as the apertures required to hold the windings

are smaller. The advantage is at its greatest with a 2:1 ratio transformer and becomes smaller as the

ratio is greater or smaller.

Autotransformers are often used to step up or down between voltages in the             110-117-

120       volt range and voltages in the 220-230-240 volt range, e.g., to output either 110 or 120V (with

taps) from 230V input, allowing equipment from a 100 or 120V region to be used in a 230V region.

A variable autotransformer is made by exposing part of the winding coils and making the secondary

connection through a sliding brush, giving a variable turns ratio.[58] Such a device is often referred to by

the trademark name Variac.

Three-phase step-down transformer mounted between two utility poles

Screenshot of a FEM simulation of the magnetic flux inside a three-phase power transformer (full animation)

Polyphase transformers

For three-phase supplies, a bank of three individual single-phase transformers can be used, or all

three phases can be incorporated as a single three-phase transformer. In this case, the magnetic

circuits are connected together, the core thus containing a three-phase flow of flux. A number of

winding configurations are possible, giving rise to different attributes and phase shifts. One particular

polyphase configuration is thezigzag transformer, used for grounding and in the suppression

of harmonic currents.

Leakage transformers

Leakage transformer

A leakage transformer, also called a stray-field transformer, has a significantly higher leakage

inductance than other transformers, sometimes increased by a magnetic bypass or shunt in its core

between primary and secondary, which is sometimes adjustable with a set screw. This provides a

transformer with an inherent current limitation due to the loose coupling between its primary and the

secondary windings. The output and input currents are low enough to prevent thermal overload under

all load conditions—even if the secondary is shorted.

Uses

Leakage transformers are used for arc welding and high voltage discharge lamps (neon lights and cold

cathode fluorescent lamps, which are series-connected up to 7.5 kV AC). It acts then both as a voltage

transformer and as a magnetic ballast.

Other applications are short-circuit-proof extra-low voltage transformers for toys

or doorbell installations.

Resonant transformers

A resonant transformer is a kind of leakage transformer. It uses the leakage inductance of its

secondary windings in combination with external capacitors, to create one or more resonant circuits.

Resonant transformers such as the Tesla coil can generate very high voltages, and are able to provide

much higher current than electrostatic high-voltage generation machines such as the Van de Graaff

generator. One of the applications of the resonant transformer is for the CCFL inverter. Another

application of the resonant transformer is to couple between stages of a superheterodyne receiver,

where the selectivity of the receiver is provided by tuned transformers in the intermediate-frequency

amplifiers.[63]

Audio transformers

Audio transformers are those specifically designed for use in audio circuits. They can be used to block

radio frequency interference or the DC component of an audio signal, to split or combine audio

signals, or to provide impedance matching between high and low impedance circuits, such as between

a high impedance tube (valve) amplifier output and a low impedanceloudspeaker, or between a high

impedance instrument output and the low impedance input of a mixing console.

Such transformers were originally designed to connect different telephone systems to one another

while keeping their respective power supplies isolated, and are still commonly used to

interconnect professional audio systems or system components.

Being magnetic devices, audio transformers are susceptible to external magnetic fields such as those

generated by AC current-carrying conductors. "Hum" is a term commonly used to describe unwanted

signals originating from the "mains" power supply (typically 50 or 60 Hz). Audio transformers used for

low-level signals, such as those from microphones, often include shielding to protect against

extraneous magnetically coupled signals.

Instrument transformers

Instrument transformers are used for measuring voltage and current in electrical power systems, and

for power system protection and control. Where a voltage or current is too large to be conveniently

used by an instrument, it can be scaled down to a standardized low value. Instrument transformers

isolate measurement, protection and control circuitry from the high currents or voltages present on the

circuits being measured or controlled.

Current transformers, designed for placing around conductors

A current transformer is a transformer designed to provide a current in its secondary coil proportional

to the current flowing in its primary coil.

Voltage transformers (VTs), also referred to as "potential transformers" (PTs), are designed to have an

accurately known transformation ratio in both magnitude and phase, over a range of measuring circuit

impedances. A voltage transformer is intended to present a negligible load to the supply being

measured. The low secondary voltage allows protective relay equipment and measuring instruments

to be operated at a lower voltages.

Both current and voltage instrument transformers are designed to have predictable characteristics on

overloads. Proper operation of over-currentprotective relays requires that current transformers provide

a predictable transformation ratio even during a short-circuit.

Classification

Electrical machines are generally understand to include not only rotating and linear electro-mechanical

machines but transformers as well. Transformers can be further classified according to such key

parameters as follow:

Power capacity: from a fraction of a volt-ampere (VA) to over a thousand MVA;

Duty of a transformer: continuous, short-time, intermittent, periodic, varying;

Frequency range: power-, audio-, or radio frequency;

Voltage class: from a few volts to hundreds of kilovolts;

Cooling type: (dry and liquid-immersed) self-cooled, forced air-cooled; (liquid-immersed) forced oil-

cooled, water-cooled;

Application: such as power supply, impedance matching, output voltage and current stabilizer or circuit

isolation;

Purpose: distribution, rectifier, arc furnace, amplifier output, etc.;

Basic magnetic form: core form, shell form;

Constant-potential transformer descriptor: power, step-up, step-down, isolation, high-voltage, low

voltage;

Three phase winding configuration: autotransformer, delta, wye, zigzag;[66]

Rectifier input phase-shift configuration: (n-winding -> p-pulse) 2-wdg -> 6-p, 3-wdg -> 12-p, . . . n-wdg

-> [n-1]*6-p; polygon; etc.)

System characteristics: ungrounded, solidly grounded, high or low resistance grounded, reactance

grounded;

Efficiency, losses and regulation: excitation, impedance & total losses, resistance, reactance &

impedance drop, regulation.

Construction

Cores

Laminated steel cores

Laminated core transformer showing edge of laminations at top of photo

Transformers for use at power or audio frequencies typically have cores made of

high permeability silicon steel. The steel has a permeability many times that of free space and the core

thus serves to greatly reduce the magnetizing current and confine the flux to a path which closely

couples the windings. Early transformer developers soon realized that cores constructed from solid

iron resulted in prohibitive eddy-current losses, and their designs mitigated this effect with cores

consisting of bundles of insulated iron wires. Later designs constructed the core by stacking layers of

thin steel laminations, a principle that has remained in use. Each lamination is insulated from its

neighbors by a thin non-conducting layer of insulation. The universal transformer equation indicates a

minimum cross-sectional area for the core to avoid saturation.

The effect of laminations is to confine eddy currents to highly elliptical paths that enclose little flux, and

so reduce their magnitude. Thinner laminations reduce losses, but are more laborious and expensive

to construct. Thin laminations are generally used on high frequency transformers, with some types of

very thin steel laminations able to operate up to 10 kHz.

Laminating the core greatly reduces eddy-current losses

One common design of laminated core is made from interleaved stacks of E-shaped steel sheets

capped with I-shaped pieces, leading to its name of "E-I transformer". Such a design tends to exhibit

more losses, but is very economical to manufacture. The cut-core or C-core type is made by winding a

steel strip around a rectangular form and then bonding the layers together. It is then cut in two, forming

two C shapes, and the core assembled by binding the two C halves together with a steel strap. They

have the advantage that the flux is always oriented parallel to the metal grains, reducing reluctance.

A steel core's remanence means that it retains a static magnetic field when power is removed. When

power is then reapplied, the residual field will cause a high inrush current until the effect of the

remaining magnetism is reduced, usually after a few cycles of the applied alternating

current. Overcurrent protection devices such as fuses must be selected to allow this harmless inrush

to pass. On transformers connected to long, overhead power transmission lines, induced currents due

to geomagnetic disturbances during solar storms can cause saturation of the core and operation of

transformer protection devices.

Distribution transformers can achieve low no-load losses by using cores made with low-loss high-

permeability silicon steel or amorphous (non-crystalline) metal alloy. The higher initial cost of the core

material is offset over the life of the transformer by its lower losses at light load.

Solid cores

Powdered iron cores are used in circuits such as switch-mode power supplies that operate above

mains frequencies and up to a few tens of kilohertz. These materials combine high

magnetic permeability with high bulk electrical resistivity. For frequencies extending beyond the VHF

band, cores made from non-conductive magnetic ceramic materials called ferritesare common. Some

radio-frequency transformers also have movable cores (sometimes called 'slugs') which allow

adjustment of the coupling coefficient (and bandwidth) of tuned radio-frequency circuits.

Toroidal cores

Small toroidal core transformer

Toroidal transformers are built around a ring-shaped core, which, depending on operating frequency,

is made from a long strip of silicon steel orpermalloy wound into a coil, powdered iron, or ferrite. A strip

construction ensures that the grain boundaries are optimally aligned, improving the transformer's

efficiency by reducing the core's reluctance. The closed ring shape eliminates air gaps inherent in the

construction of an E-I core. The cross-section of the ring is usually square or rectangular, but more

expensive cores with circular cross-sections are also available. The primary and secondary coils are

often wound concentrically to cover the entire surface of the core. This minimizes the length of wire

needed, and also provides screening to minimize the core's magnetic field from

generating electromagnetic interference.

Toroidal transformers are more efficient than the cheaper laminated E-I types for a similar power level.

Other advantages compared to E-I types, include smaller size (about half), lower weight (about half),

less mechanical hum (making them superior in audio amplifiers), lower exterior magnetic field (about

one tenth), low off-load losses (making them more efficient in standby circuits), single-bolt mounting,

and greater choice of shapes. The main disadvantages are higher cost and limited power capacity

(see "Classification" above). Because of the lack of a residual gap in the magnetic path, toroidal

transformers also tend to exhibit higher inrush current, compared to laminated E-I types.

Ferrite toroidal cores are used at higher frequencies, typically between a few tens of kilohertz to

hundreds of megahertz, to reduce losses, physical size, and weight of a switched-mode power supply.

A drawback of toroidal transformer construction is the higher labor cost of winding. This is because it is

necessary to pass the entire length of a coil winding through the core aperture each time a single turn

is added to the coil. As a consequence, toroidal transformers are uncommon above ratings of a few

kVA. Small distribution transformers may achieve some of the benefits of a toroidal core by splitting it

and forcing it open, then inserting a bobbin containing primary and secondary windings.

Air cores

A physical core is not an absolute requisite and a functioning transformer can be produced simply by

placing the windings near each other, an arrangement termed an "air-core" transformer. The air which

comprises the magnetic circuit is essentially lossless, and so an air-core transformer eliminates loss

due to hysteresis in the core material. The leakage inductance is inevitably high, resulting in very poor

regulation, and so such designs are unsuitable for use in power distribution. They have however very

high bandwidth, and are frequently employed in radio-frequency applications, for which a satisfactory

coupling coefficient is maintained by carefully overlapping the primary and secondary windings.

They're also used for resonant transformers such as Tesla coils where they can achieve reasonably

low loss in spite of the high leakage inductance.

Windings

Windings are usually arranged concentrically to minimize flux leakage.

The conducting material used for the windings depends upon the application, but in all cases the

individual turns must be electrically insulated from each other to ensure that the current travels

throughout every turn. For small power and signal transformers, in which currents are low and the

potential difference between adjacent turns is small, the coils are often wound from enamelled magnet

wire, such as Formvar wire. Larger power transformers operating at high voltages may be wound with

copper rectangular strip conductors insulated by oil-impregnated paper and blocks of pressboard.

Cut view through transformer windings. White: insulator. Green spiral: Grain oriented silicon steel. Black: Primary

winding made of oxygen-free copper. Red: Secondary winding. Top left: Toroidal transformer. Right: C-core, but E-core

would be similar. The black windings are made of film. Top: Equally low capacitance between all ends of both windings.

Since most cores are at least moderately conductive they also need insulation. Bottom: Lowest capacitance for one end

of the secondary winding needed for low-power high-voltage transformers. Bottom left: Reduction ofleakage

inductance would lead to increase of capacitance.

High-frequency transformers operating in the tens to hundreds of kilohertz often have windings made

of braided Litz wire to minimize the skin-effect and proximity effectlosses. Large power transformers

use multiple-stranded conductors as well, since even at low power frequencies non-uniform

distribution of current would otherwise exist in high-current windings. Each strand is individually

insulated, and the strands are arranged so that at certain points in the winding, or throughout the

whole winding, each portion occupies different relative positions in the complete conductor. The

transposition equalizes the current flowing in each strand of the conductor, and reduces eddy current

losses in the winding itself. The stranded conductor is also more flexible than a solid conductor of

similar size, aiding manufacture.

For signal transformers, the windings may be arranged in a way to minimize leakage inductance and

stray capacitance to improve high-frequency response. This can be done by splitting up each coil into

sections, and those sections placed in layers between the sections of the other winding. This is known

as a stacked type or interleaved winding.

Power transformers often have internal connections or taps at intermediate points on the winding,

usually on the higher voltage winding side, for voltage regulation control purposes. Such taps are

normally manually operated, automatic on-load tap changers being reserved, for cost and reliability

considerations, to higher power rated or specialized transformers supplying transmission or

distribution circuits or certain utilization loads such as furnace transformers. Audio-frequency

transformers, used for the distribution of audio to public address loudspeakers, have taps to allow

adjustment of impedance to each speaker. A center-tapped transformer is often used in the output

stage of an audio power amplifier in a push-pull circuit. Modulation transformers in AM transmitters are

very similar.

Certain transformers have the windings protected by epoxy resin. By impregnating the transformer

with epoxy under a vacuum, one can replace air spaces within the windings with epoxy, thus sealing

the windings and helping to prevent the possible formation of corona and absorption of dirt or water.

This produces transformers more suited to damp or dirty environments, but at increased

manufacturing cost.

Cooling

Cutaway view of oil-filled power transformer. The conservator (reservoir) at top provides oil-to-atmosphere isolation.

Tank walls' cooling fins provide required heat dissipation balance.

Though it is not uncommon for oil-filled transformers to have today been in operation for over fifty

years high temperature damages winding insulation, the accepted rule of thumb being that transformer

life expectancy is halved for every 8 degree C increase in operating temperature. At the lower end of

the power rating range, dry and liquid-immersed transformers are often self-cooled by natural

convection and radiation heat dissipation. As power ratings increase, transformers are often cooled by

such other means as forced-air cooling, force-oil cooling, water-cooling, or a combinations of these.

The dielectic coolant used in many outdoor utility and industrial service transformers is transformer

oil that both cools and insulates the windings. Transformer oil is a highly refined mineral oil that

inherently helps thermally stabilize winding conductor insulation, typically paper, within acceptable

insulation temperature rating limitations. However, the heat removal problem is central to all electrical

apparatus such that in the case of high value transfomer assets, this often translates in a need to

monitor, model, forecast and manage oil and winding conductor insulation temperature conditions

under varying, possibly difficult, power loading conditions. Indoor liquid-filled transformers are required

by building regulations in many jurisdictions to either use a non-flammable liquid or to be located in

fire-resistant rooms. Air-cooled dry transformers are preferred for indoor applications even at capacity

ratings where oil-cooled construction would be more economical, because their cost is offset by the

reduced building construction cost.

The oil-filled tank often has radiators through which the oil circulates by natural convection. Some

large transformers employ electric-operated fans or pumps for forced-air or forced-oil cooling or heat

exchanger-based water-cooling. Oil-filled transformers undergo prolonged drying processes to ensure

that the transformer is completely free of water vapor before the cooling oil is introduced. This helps

prevent electrical breakdown under load. Oil-filled transformers may be equipped with Buchholz

relays, which detect gas evolved during internal arcing and rapidly de-energize the transformer to

avert catastrophic failure. Oil-filled transformers may fail, rupture, and burn, causing power outages

and losses. Installations of oil-filled transformers usually includes fire protection measures such as

walls, oil containment, and fire-suppression sprinkler systems.

Polychlorinated biphenyls have properties that once favored their use as a dielectic coolant, though

concerns over their environmental persistence led to a widespread ban on their use. Today, non-toxic,

stable silicone-based oils, or fluorinated hydrocarbons may be used where the expense of a fire-

resistant liquid offsets additional building cost for a transformer vault. Before 1977, even transformers

that were nominally filled only with mineral oils may also have been contaminated with polychlorinated

biphenyls at 10-20 ppm. Since mineral oil and PCB fluid mix, maintenance equipment used for both

PCB and oil-filled transformers could carry over small amounts of PCB, contaminating oil-filled

transformers.

Some "dry" transformers (containing no liquid) are enclosed in sealed, pressurized tanks and cooled

by nitrogen or sulfur hexafluoride gas.

Experimental power transformers in the 2 MVA range have been built with superconducting windings

which eliminates the copper losses, but not the core steel loss. These are cooled byliquid

nitrogen or helium.

Insulation drying

Construction of oil-filled transformers requires that the insulation covering the windings be thoroughly

dried before the oil is introduced. There are several different methods of drying. Common for all is that

they are carried out in vacuum environment. The vacuum makes it difficult to transfer energy (heat) to

the insulation. For this there are several different methods. The traditional drying is done by circulating

hot air over the active part and cycle this with periods of hot-air vacuum (HAV) drying. More common

for larger transformers is to use evaporated solvent which condenses on the colder active part. The

benefit is that the entire process can be carried out at lower pressure and without influence of added

oxygen. This process is commonly called vapour-phase drying (VPD).

For distribution transformers, which are smaller and have a smaller insulation weight, resistance

heating can be used. This is a method where current is injected in the windings to heat the insulation.

The benefit is that the heating can be controlled very well and it is energy efficient. The method is

called low-frequency heating (LFH) since the current is injected at a much lower frequency than the

nominal of the grid, which is normally 50 or 60 Hz. A lower frequency reduces the effect of the

inductance in the transformer, so the voltage needed to induce the current can be reduced. The LFH

drying method is also used for service of older transformers.

Terminals

Very small transformers will have wire leads connected directly to the ends of the coils, and brought

out to the base of the unit for circuit connections. Larger transformers may have heavy bolted

terminals, bus bars or high-voltage insulated bushings made of polymers or porcelain. A large bushing

can be a complex structure since it must provide careful control of theelectric field gradient without

letting the transformer leak oil.

Applications

Image of an electrical substation in Melbourne, Australiashowing 3 of 5 220kV/66kV transformers, each with a capacity

of 185MVA

A major application of transformers is to increase voltage before transmitting electrical energy over

long distances through wires. Wires have resistance and so dissipate electrical energy at a rate

proportional to the square of the current through the wire. By transforming electrical power to a high-

voltage (and therefore low-current) form for transmission and back again afterward, transformers

enable economical transmission of power over long distances. Consequently, transformers have

shaped theelectricity supply industry, permitting generation to be located remotely from points

of demand. All but a tiny fraction of the world's electrical power has passed through a series of

transformers by the time it reaches the consumer.

Image of transformer at the Limestone Generating Station in Manitoba, Canada

Transformers are also used extensively in electronic products to step down the supply voltage to a

level suitable for the low voltage circuits they contain. The transformer also electrically isolates the end

user from contact with the supply voltage.

Signal and audio transformers are used to couple stages of amplifiers and to match devices such

as microphones and record players to the input of amplifiers. Audio transformers

allowed telephone circuits to carry on a two-way conversation over a single pair of wires.

A balun transformer converts a signal that is referenced to ground to a signal that has balanced

voltages to ground, such as between external cables and internal circuits.

The principle of open-circuit (unloaded) transformer is widely used for characterisation of soft magnetic

materials, for example in the internationally standardised Epstein frame method.

RELAY

A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching

mechanism mechanically, but other operating principles are also used. Relays are used where it is

necessary to control a circuit by a low-power signal (with complete electrical isolation between control

and controlled circuits), or where several circuits must be controlled by one signal. The first relays

were used in long distance telegraph circuits, repeating the signal coming in from one circuit and re-

transmitting it to another. Relays were used extensively in telephone exchanges and early computers

to perform logical operations.

A type of relay that can handle the high power required to directly control an electric motor or other

loads is called a contactor. Solid-state relayscontrol power circuits with no moving parts, instead using

a semiconductor device to perform switching. Relays with calibrated operating characteristics and

sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in

modern electric power systems these functions are performed by digital instruments still called

"protective relays".

Basic design and operation

Simple electromechanical relay.

Small "cradle" relay often used in electronics. The "cradle" term refers to the shape of the relay's armature.

A simple electromagnetic relay consists of a coil of wire wrapped around a soft iron core, an iron yoke

which provides a low reluctance path for magnetic flux, a movable iron armature, and one or more sets

of contacts (there are two in the relay pictured). The armature is hinged to the yoke and mechanically

linked to one or more sets of moving contacts. It is held in place by a spring so that when the relay is

de-energized there is an air gap in the magnetic circuit. In this condition, one of the two sets of

contacts in the relay pictured is closed, and the other set is open. Other relays may have more or

fewer sets of contacts depending on their function. The relay in the picture also has a wire connecting

the armature to the yoke. This ensures continuity of the circuit between the moving contacts on the

armature, and the circuit track on the printed circuit board (PCB) via the yoke, which is soldered to the

PCB.

When an electric current is passed through the coil it generates a magnetic field that activates the

armature, and the consequent movement of the movable contact(s) either makes or breaks

(depending upon construction) a connection with a fixed contact. If the set of contacts was closed

when the relay was de-energized, then the movement opens the contacts and breaks the connection,

and vice versa if the contacts were open. When the current to the coil is switched off, the armature is

returned by a force, approximately half as strong as the magnetic force, to its relaxed position. Usually

this force is provided by a spring, but gravity is also used commonly in industrial motor starters. Most

relays are manufactured to operate quickly. In a low-voltage application this reduces noise; in a high

voltage or current application it reduces arcing.

When the coil is energized with direct current, a diode is often placed across the coil to dissipate the

energy from the collapsing magnetic field at deactivation, which would otherwise generate a voltage

spike dangerous to semiconductor circuit components. Some automotive relays include a diode inside

the relay case. Alternatively, a contact protection network consisting of a capacitor and resistor in

series (snubber circuit) may absorb the surge. If the coil is designed to be energized with alternating

current (AC), a small copper "shading ring" can be crimped to the end of the solenoid, creating a small

out-of-phase current which increases the minimum pull on the armature during the AC cycle.

A solid-state relay uses a thyristor or other solid-state switching device, activated by the control signal,

to switch the controlled load, instead of a solenoid. An optocoupler (a light-emitting diode (LED)

coupled with a photo transistor) can be used to isolate control and controlled circuits.

Types

Latching relay

Latching relay with permanent magnet

A latching relay has two relaxed states (bistable). These are also called "impulse", "keep", or "stay"

relays. When the current is switched off, the relay remains in its last state. This is achieved with

a solenoid operating a ratchet and cam mechanism, or by having two opposing coils with an over-

center spring or permanent magnet to hold the armature and contacts in position while the coil is

relaxed, or with a remanent core. In the ratchet and cam example, the first pulse to the coil turns the

relay on and the second pulse turns it off. In the two coil example, a pulse to one coil turns the relay on

and a pulse to the opposite coil turns the relay off. This type of relay has the advantage that one coil

consumes power only for an instant, while it is being switched, and the relay contacts retain this

setting across a power outage. A remanent core latching relay requires a current pulse of opposite

polarity to make it change state.

Reed relay

A reed relay is a reed switch enclosed in a solenoid. The switch has a set of contacts inside

an evacuated or inert gas-filled glass tube which protects the contacts against atmospheric corrosion;

the contacts are made of magnetic material that makes them move under the influence of the field of

the enclosing solenoid. Reed relays can switch faster than larger relays, require only little power from

the control circuit, but have low switching current and voltage ratings. In addition, the reeds can

become magnetized over time, which makes them stick 'on' even when no current is present;

changing the orientation of the reeds with respect to the solenoid's magnetic field will fix the problem.

Top, middle: reed switches, bottom: reed relay

Mercury-wetted relay

A mercury-wetted reed relay is a form of reed relay in which the contacts are wetted with mercury.

Such relays are used to switch low-voltage signals (one volt or less) where the mercury reduces the

contact resistance and associated voltage drop, for low-current signals where surface contamination

may make for a poor contact, or for high-speed applications where the mercury eliminates contact

bounce. Mercury wetted relays are position-sensitive and must be mounted vertically to work properly.

Because of the toxicity and expense of liquid mercury, these relays are now rarely used. See

also mercury switch.

Polarized relay

A polarized relay placed the armature between the poles of a permanent magnet to increase

sensitivity. Polarized relays were used in middle 20th Century telephone exchanges to detect faint

pulses and correct telegraphic distortion. The poles were on screws, so a technician could first adjust

them for maximum sensitivity and then apply a bias spring to set the critical current that would operate

the relay.

External links

Schematic diagram of a polarized relay used in a teletype machine.

Machine tool relay

A machine tool relay is a type standardized for industrial control of machine tools, transfer machines,

and other sequential control. They are characterized by a large number of contacts (sometimes

extendable in the field) which are easily converted from normally-open to normally-closed status,

easily replaceable coils, and a form factor that allows compactly installing many relays in a control

panel. Although such relays once were the backbone of automation in such industries as automobile

assembly, the programmable logic controller(PLC) mostly displaced the machine tool relay from

sequential control applications.

A relay allows circuits to be switched by electrical equipment: for example, a timer circuit with a relay

could switch power at a preset time. For many years relays were the standard method of controlling

industrial electronic systems. A number of relays could be used together to carry out complex

functions (relay logic). The principle of relay logic is based on relays which energize and de-energize

associated contacts. Relay logic is the predecessor of ladder logic, which is commonly used

in Programmable logic controllers.

Ratchet relay

This is again a clapper type relay which does not need continuous current through its coil to retain its

operation.

Contactor relay

A contactor is a very heavy-duty relay used for switching electric motors and lighting loads, although

contactors are not generally called relays. Continuous current ratings for common contactors range

from 10 amps to several hundred amps. High-current contacts are made with alloys containing silver.

The unavoidable arcing causes the contacts to oxidize; however,silver oxide is still a good

conductor. Such devices are often used for motor starters. A motor starter is a contactor with overload

protection devices attached. The overload sensing devices are a form of heat operated relay where a

coil heats a bi-metal strip, or where a solder pot melts, releasing a spring to operate auxiliary contacts.

These auxiliary contacts are in series with the coil. If the overload senses excess current in the load,

the coil is de-energized. Contactor relays can be extremely loud to operate, making them unfit for use

where noise is a chief concern.

Solid-state relay

Solid state relay with no moving parts

25 A or 40 A solid state contactors

A solid state relay (SSR) is a solid state electronic component that provides a similar function to

an electromechanical relay but does not have any moving components, increasing long-term reliability.

With early SSR's, the tradeoff came from the fact that every transistor has a small voltage drop across

it. This voltage drop limited the amount of current a given SSR could handle. The minimum voltage

drop for such a relay is equal to the voltage drop across one transistor (~0.6-2.0 volts), and is a

function of the material used to make the transistor (typically silicon). As transistors improved, higher

current SSR's, able to handle 100 to 1,200 Amperes, have become commercially available. Compared

to electromagnetic relays, they may be falsely triggered by transients.

Solid state contactor relay

A solid state contactor is a heavy-duty solid state relay, including the necessary heat sink, used for

switching electric heaters, small electric motors and lighting loads; where frequent on/off cycles are

required. There are no moving parts to wear out and there is no contact bounce due to vibration. They

are activated by AC control signals or DC control signals from Programmable logic controller (PLCs),

PCs, Transistor-transistor logic (TTL) sources, or other microprocessor and microcontroller controls.

Buchholz relay

A Buchholz relay is a safety device sensing the accumulation of gas in large oil-filled transformers,

which will alarm on slow accumulation of gas or shut down the transformer if gas is produced rapidly in

the transformer oil.

Forced-guided contacts relay

A forced-guided contacts relay has relay contacts that are mechanically linked together, so that

when the relay coil is energized or de-energized, all of the linked contacts move together. If one set of

contacts in the relay becomes immobilized, no other contact of the same relay will be able to move.

The function of forced-guided contacts is to enable the safety circuit to check the status of the relay.

Forced-guided contacts are also known as "positive-guided contacts", "captive contacts", "locked

contacts", or "safety relays".

Overload protection relay

Electric motors need overcurrent protection to prevent damage from over-loading the motor, or to

protect against short circuits in connecting cables or internal faults in the motor windings.[3] One type

of electric motor overload protection relay is operated by a heating element in series with the electric

motor. The heat generated by the motor current heats a bimetallic strip or melts solder, releasing a

spring to operate contacts. Where the overload relay is exposed to the same environment as the

motor, a useful though crude compensation for motor ambient temperature is provided.

Pole and throw

Circuit symbols of relays. (C denotes the common terminal in SPDT and DPDT types.)

Since relays are switches, the terminology applied to switches is also applied to relays. A relay will

switch one or more poles, each of whose contacts can be thrown by energizing the coil in one of three

ways:

Normally-open (NO) contacts connect the circuit when the relay is activated; the circuit is

disconnected when the relay is inactive. It is also called a Form A contact or "make"

contact. NO contacts can also be distinguished as "early-make" or NOEM, which means that the

contacts will close before the button or switch is fully engaged.

Normally-closed (NC) contacts disconnect the circuit when the relay is activated; the circuit is

connected when the relay is inactive. It is also called a Form B contact or "break"

contact. NC contacts can also be distinguished as "late-break" or NCLB, which means that the

contacts will stay closed until the button or switch is fully disengaged.

Change-over (CO), or double-throw (DT), contacts control two circuits: one normally-open contact

and one normally-closed contact with a common terminal. It is also called a Form C contact or

"transfer" contact ("break before make"). If this type of contact utilizes a "make before break"

functionality, then it is called a Form D contact.

The following designations are commonly encountered:

SPST – Single Pole Single Throw. These have two terminals which can be connected or

disconnected. Including two for the coil, such a relay has four terminals in total. It is ambiguous

whether the pole is normally open or normally closed. The terminology "SPNO" and "SPNC" is

sometimes used to resolve the ambiguity.

SPDT – Single Pole Double Throw. A common terminal connects to either of two others. Including

two for the coil, such a relay has five terminals in total.

DPST – Double Pole Single Throw. These have two pairs of terminals. Equivalent to two SPST

switches or relays actuated by a single coil. Including two for the coil, such a relay has six

terminals in total. The poles may be Form A or Form B (or one of each).

DPDT – Double Pole Double Throw. These have two rows of change-over terminals. Equivalent to

two SPDT switches or relays actuated by a single coil. Such a relay has eight terminals, including

the coil.

The "S" or "D" may be replaced with a number, indicating multiple switches connected to a

single actuator. For example 4PDT indicates a four pole double throw relay (with 14 terminals).

EN 50005 are among applicable standards for relay terminal numbering; a typical EN 50005-compliant

SPDT relay's terminals would be numbered 11, 12, 14, A1 and A2 for the C, NC, NO, and coil

connections, respectively.

[edit]Applications

Relays are used to and for:

Amplify a digital signal, switching a large amount of power with a small operating power. Some

special cases are:

A telegraph relay, repeating a weak signal received at the end of a long wire

Controlling a high-voltage circuit with a low-voltage signal, as in some types of modems or

audio amplifiers,

Controlling a high-current circuit with a low-current signal, as in the starter solenoid of

an automobile,

Detect and isolate faults on transmission and distribution lines by opening and closing circuit

breakers (protection relays),

A DPDT AC coil relay with "ice cube" packaging

Isolate the controlling circuit from the controlled circuit when the two are at different potentials, for

example when controlling a mains-powered device from a low-voltage switch. The latter is often

applied to control office lighting as the low voltage wires are easily installed in partitions, which

may be often moved as needs change. They may also be controlled by room occupancy detectors

to conserve energy,

Logic functions. For example, the boolean AND function is realised by connecting normally open

relay contacts in series, the OR function by connecting normally open contacts in parallel. The

change-over or Form C contacts perform the XOR (exclusive or) function. Similar functions for

NAND and NOR are accomplished using normally closed contacts. The Ladder programming

language is often used for designing relay logic networks.

The application of Boolean Algebra to relay circuit design was formalized by Claude

Shannon in A Symbolic Analysis of Relay and Switching Circuits

Early computing. Before vacuum tubes and transistors, relays were used as logical elements

in digital computers. See electro-mechanical computers such as ARRA (computer), Harvard

Mark II, Zuse Z2, and Zuse Z3.

Safety-critical logic. Because relays are much more resistant than semiconductors to nuclear

radiation, they are widely used in safety-critical logic, such as the control panels of radioactive

waste-handling machinery.

Time delay functions. Relays can be modified to delay opening or delay closing a set of contacts.

A very short (a fraction of a second) delay would use a copper disk between the armature and

moving blade assembly. Current flowing in the disk maintains magnetic field for a short time,

lengthening release time. For a slightly longer (up to a minute) delay, a dashpot is used. A

dashpot is a piston filled with fluid that is allowed to escape slowly. The time period can be varied

by increasing or decreasing the flow rate. For longer time periods, a mechanical clockwork timer is

installed.

Vehicle battery isolation. A 12v relay is often used to isolate any second battery in cars, 4WDs,

RVs and boats.

Switching to a standby power supply.

Relay application considerations

A large relay with two coils and many sets of contacts, used in an old telephone switching system.

Several 30-contact relays in "Connector" circuits in mid 20th century 1XB switch and5XB switch telephone exchanges;

cover removed on one

Selection of an appropriate relay for a particular application requires evaluation of many different

factors:

Number and type of contacts – normally open, normally closed, (double-throw)

Contact sequence – "Make before Break" or "Break before Make". For example, the old style

telephone exchanges required Make-before-break so that the connection didn't get dropped while

dialing the number.

Rating of contacts – small relays switch a few amperes, large contactors are rated for up to 3000

amperes, alternating or direct current

Voltage rating of contacts – typical control relays rated 300 VAC or 600 VAC, automotive types to

50 VDC, special high-voltage relays to about 15 000 V

Operating lifetime, useful life - the number of times the relay can be expected to operate reliably.

There is both a mechanical life and a contact life; the contact life is naturally affected by the kind

of load being switched.

Coil voltage – machine-tool relays usually 24 VAC, 120 or 250 VAC, relays for switchgear may

have 125 V or 250 VDC coils, "sensitive" relays operate on a few milliamperes

Coil current - including minimum current required to operate reliably and minimum current to hold.

Also effects of power dissipation on coil temperature at various duty cycles.

Package/enclosure – open, touch-safe, double-voltage for isolation between circuits, explosion

proof, outdoor, oil and splash resistant, washable for printed circuit board assembly

Operating environment - minimum and maximum operating temperatures and other environmental

considerations such as effects of humidity and salt

Assembly – Some relays feature a sticker that keeps the enclosure sealed to allow PCB post

soldering cleaning, which is removed once assembly is complete.

Mounting – sockets, plug board, rail mount, panel mount, through-panel mount, enclosure for

mounting on walls or equipment

Switching time – where high speed is required

"Dry" contacts – when switching very low level signals, special contact materials may be needed

such as gold-plated contacts

Contact protection – suppress arcing in very inductive circuits

Coil protection – suppress the surge voltage produced when switching the coil current

Isolation between coil contacts

Aerospace or radiation-resistant testing, special quality assurance

Expected mechanical loads due to acceleration – some relays used in aerospace applications are

designed to function in shock loads of 50 g or more

Accessories such as timers, auxiliary contacts, pilot lamps, test buttons

Regulatory approvals

Stray magnetic linkage between coils of adjacent relays on a printed circuit board.

There are many considerations involved in the correct selection of a control relay for a particular

application. These considerations include factors such as speed of operation, sensitivity,

and hysteresis. Although typical control relays operate in the 5 ms to 20 ms range, relays with

switching speeds as fast as 100 us are available. Reed relays which are actuated by low currents and

switch fast are suitable for controlling small currents.

As for any switch, the current through the relay contacts (unrelated to the current through the coil)

must not exceed a certain value to avoid damage. In the particular case of high-inductance circuits

such as motors other issues must be addressed. When a power source is connected to an inductance,

an input surge current which may be several times larger than the steady current exists. When the

circuit is broken, the current cannot change instantaneously, which creates a potentially damaging

spark across the separating contacts.

Consequently for relays which may be used to control inductive loads we must specify the maximum

current that may flow through the relay contacts when it actuates, the make rating; the continuous

rating; and the break rating. The make rating may be several

times larger than the continuous rating, which is itself larger

than the break rating.

Derating factors

Control relays should not be operated above rated temperature

because of resulting increased degradation and fatigue.

Common practice is to derate 20 degrees Celsius from the

maximum rated temperature limit. Relays operating at rated

load are also affected by their environment. Oil vapors may

greatly decrease the contact tip life, and dust or dirt may cause

the tips to burn before their normal life expectancy. Control relay

life cycle varies from 50,000 to over one million cycles

depending on the electrical loads of the contacts, duty cycle,

application, and the extent to which the relay is derated. When a

control relay is operating at its derated value, it is controlling a

lower value of current than its maximum make and break ratings. This is often done to extend the

operating life of the control relay. The table lists the relay derating factors for typical industrial control

applications.

Undesired arcing

Without adequate contact protection, the occurrence of electric current arcing causes significant

degradation of the contacts in relays, which suffer significant and visible damage. Every time a relay

transitions either from a closed to an open state (break arc) or from an open to a closed state (make

arc & bounce arc), under load, an electrical arc can occur between the two contact points (electrodes)

of the relay. The break arc is typically more energetic and thus more destructive.

The heat energy contained in the resulting electrical arc is very high (tens of thousands of degrees

Fahrenheit), causing the metal on the contact surfaces to melt, pool and migrate with the current. The

extremely high temperature of the arc cracks the surrounding gas molecules creating ozone, carbon

monoxide, and other compounds. The arc energy slowly destroys the contact metal, causing some

material to escape into the air as fine particulate matter. This very activity causes the material in the

contacts to degrade quickly, resulting in device failure. This contact degradation drastically limits the

overall life of a relay to a range of about 10,000 to 100,000 operations, a level far below the

mechanical life of the same device, which can be in excess of 20 million operations.

Type of

load% of rated value

Resistive 75

Inductive 35

Motor 20

Filament 10

Capacitive 75

Protective relays

For protection of electrical apparatus and transmission lines, electromechanical relays with accurate

operating characteristics were used to detect overload, short-circuits, and other faults. While many

such relays remain in use, digital devices now provide equivalent protective functions.

Railway signalling

Part of a relay interlocking using UK Q-style miniature plug-in relays.

UK Q-style signalling relay and base.

Railway signalling relays are very big and cumbersome compared to the mostly small voltages (less

than 120 V) and currents (perhaps 100 mA) that they switch. Contacts are widely spaced to prevent

dangerous flashovers and short circuits over a lifetime that may exceed fifty years. BR930 series plug-

in relays are widely used on railways following British practice. These are 120 mm high, 180 mm deep

and 56 mm wide and weigh about 1400 g, and can have up to 16 separate contacts, say 12 make and

4 break contacts.

Since rail signal circuits must be highly reliable, special techniques are used to detect and prevent

failures in the relay system. To protect against false feeds, double switching relay contacts are often

used on both the positive and negative side of a circuit, so that two false feeds are needed to cause a

false signal. Not all relay circuits can be proved so there is reliance on construction features such as

carbon to silver contacts to resist lightning induced contact welding and to provide AC immunity.

Opto-isolators are also used in some instances with railway signalling, especially where only a single

contact is to be switched.

Signalling relays and their circuits come in a number of schools, including:

 British

 American

 German

 France

American signaling relays are the origin of the 19 inch rack.

History

A simple device, which we now call a relay, was included in the original

1840 telegraph patent of Samuel Morse. The mechanism described acted as a digital amplifier,

repeating the telegraph signal, and thus allowing signals to be propagated as far as desired. This

overcame the problem of limited range of earlier telegraphy schemes.

The earlier ‘relay’ or ‘repeater’ of Edward Davy of 1837/1838 was used in his electric telegraph.

A schematic diagram represents the elements of a system using abstract, graphic symbols rather than realistic pictures. A schematic usually omits all details that are not relevant to the information the schematic is intended to convey, and may add unrealistic elements that aid comprehension. For example, a subway map intended for riders may represent a subway station with a dot; the dot doesn't resemble the actual station at all but gives the viewer information without unnecessary visual clutter. A schematic diagram of a chemical process uses symbols to represent the vessels, piping, valves, pumps, and other equipment of the system, emphasizing their interconnection paths and suppressing physical details. In an electroniccircuit diagram, the layout of the symbols may not resemble the layout in the physical circuit. In the schematic diagram, the symbolic elements are arranged to be more easily interpreted by the viewer.

Electrical and electronic industry

In the electrical industry, a schematic diagram is often used to describe the design of equipment.

Schematic diagrams are often used for the maintenance and repair of electronic and

electromechanical systems. Original schematics were done by hand, using standardized templates or

pre-printed adhesive symbols, but today Electrical CAD software is often used.

The circuit diagram for a 4 bit TTL counter, a type of state machine

In electronic design automation, until the 1980s schematics were virtually the only formal

representation for circuits. More recently, with the progress of computer technology, other

representations were introduced and specialized computer languages were developed, since with the

explosive growth of the complexity of electronic circuits, traditional schematics are becoming less

practical. For example,hardware description languages are indispensable for modern digital

circuit design.

Schematics for electronic circuits are prepared by designers using EDA (Electronic Design

Automation) tools called schematic capturetools or schematic entry tools. These tools go beyond

simple drawing of devices and connections. Usually they are integrated into the whole IC design flow

and linked to other EDA tools for verification and simulation of the circuit under design.

In electric power systems design, a schematic drawing called a one-line diagram is frequently used to

represent substations, distribution systems or even whole electrical power grids. These diagrams

simplify and compress the details that would be repeated on each phase of a three-phase system,

showing only one element instead of three. Electrical diagrams for switchgear often have common

device functions designate by standard function numbers.

Schematics in repair manuals

Schematic diagrams are used extensively in repair manuals to help users understand the

interconnections of parts, and to provide graphical instruction to assist in taking apart and rebuilding

mechanical assemblies. Many automotive and motorcycle repair manuals devote a significant number

of pages to schematic diagrams.

LADDER LOGICLadder logic is a programming language that represents a program by a graphical diagram based on the circuit diagrams of relay logichardware. It is primarily used to develop software for programmable logic controllers (PLCs) used in industrial control applications. The name is based on the observation that programs in this language resemble ladders, with two vertical rails and a series of horizontal rungs between them.

Overview

An argument that aided the initial adoption of ladder logic was that a wide variety of engineers and

technicians would be able to understand and use it without much additional training, because of the

resemblance to familiar hardware systems. This argument has become less relevant given that most

ladder logic programmers have a software background in more conventional programming languages,

and in practice implementations of ladder logic have characteristics, such as sequential execution and

support for control flow features, that make the analogy to hardware somewhat inaccurate.

Ladder logic is widely used to program PLCs, where sequential control of a process or manufacturing

operation is required. Ladder logic is useful for simple but critical control systems or for reworking

old hardwired relay circuits. As programmable logic controllers became more sophisticated it has also

been used in very complex automation systems. Often the ladder logic program is used in conjunction

with an HMI program operating on a computer workstation.

Manufacturers of programmable logic controllers generally also provide associated ladder logic

programming systems. Typically the ladder logic languages from two manufacturers will not be

completely compatible; ladder logic is better thought of as a set of closely related programming

languages rather than one language. (The IEC 61131-3 standard has helped to reduce unnecessary

differences, but translating programs between systems still requires significant work.) Even different

models of programmable controllers within the same family may have different ladder notation such

that programs cannot be seamlessly interchanged between models.

Ladder logic can be thought of as a rule-based language rather than a procedural language. A "rung"

in the ladder represents a rule. When implemented with relays and other electromechanical devices,

the various rules "execute" simultaneously and immediately. When implemented in a programmable

logic controller, the rules are typically executed sequentially by software, in a continuous loop (scan).

By executing the loop fast enough, typically many times per second, the effect of simultaneous and

immediate execution is relatively achieved to within the tolerance of the time required to execute every

rung in the "loop" (the "scan time"). It is somewhat similar to other rule-based languages,

likespreadsheets or SQL. However, proper use of programmable controllers requires understanding

the limitations of the execution order of rungs.

Example of a simple ladder logic program

The language itself can be seen as a set of connections between logical checkers (contacts) and

actuators (coils). If a path can be traced between the left side of the rung and the output, through

asserted (true or "closed") contacts, the rung is true and the output coil storage bit is asserted (1) or

true. If no path can be traced, then the output is false (0) and the "coil" by analogy to electro-

mechanical relays is considered "de-energized". The analogy between logical propositions and relay

contact status is due to Claude Shannon.

Ladder logic has contacts that make or break circuits to control coils. Each coil or contact corresponds

to the status of a single bit in the programmable controller's memory. Unlike electromechanical relays,

a ladder program can refer any number of times to the status of a single bit, equivalent to a relay with

an indefinitely large number of contacts.

So-called "contacts" may refer to physical ("hard") inputs to the programmable controller from physical

devices such as pushbuttons and limit switches via an integrated or external input module, or may

represent the status of internal storage bits which may be generated elsewhere in the program.

Each rung of ladder language typically has one coil at the far right. Some manufacturers may allow

more than one output coil on a rung.

—( )— A regular coil, energized whenever its rung is closed.

—(\)— A "not" coil, energized whenever its rung is open.

—[ ]— A regular contact, closed whenever its corresponding coil or an input which controls it is

energized.

—[\]— A "not" contact, open whenever its corresponding coil or an input which controls it is

energized.

The "coil" (output of a rung) may represent a physical output which operates some device connected

to the programmable controller, or may represent an internal storage bit for use elsewhere in the

program.

Examples

Here is an example of what one rung in a ladder logic program might look like. In real world

applications, there may be hundreds or thousands of rungs.

For example:

1. ----[ ]---------|--[ ]--|------( )

X | Y | S

| |

|--[ ]--|

Z

The above realizes the function: S = X AND ( Y OR Z )

Typically, complex ladder logic is 'read' left to right and top to bottom. As each of the lines (or rungs)

are evaluated the output coil of a rung may feed into the next stage of the ladder as an input. In a

complex system there will be many "rungs" on a ladder, which are numbered in order of evaluation.

1. ----[ ]-----------|---[ ]---|----( )

X | Y | S

| |

|---[ ]---|

Z

2. ----[ ]----[ ]-------------------( )

S X T

2. T = S AND X

This represents a slightly more complex system for rung 2. After the first line has been evaluated, the

output coil (S) is fed into rung 2, which is then evaluated and the output coil T could be fed into an

output device (buzzer, light etc..) or into rung 3 on the ladder. (Note that the contact X on the second

rung serves no useful purpose, as X is already defined in the 'AND' function of S from the 1st rung.)

This system allows very complex logic designs to be broken down and evaluated.

For more practical examples see below:

------[ ]--------------[ ]----------------( )

Key Switch 1 Key Switch 2 Door Motor

This circuit shows two key switches that security guards might use to activate an electric motor on a

bank vault door. When the normally open contacts of both switches close, electricity is able to flow to

the motor which opens the door. This is a logical AND.

+-------+

----------------------------+ +----

+-------+

Remote Receiver

--|-------[ ]-------+-----------------( )

| |

|-------[ ]-------|

Interior Unlock

This circuit shows the two things that can trigger a car's power door locks. The remote receiver is

always powered. The lock solenoid gets power when either set of contacts is closed. This is a logical

OR.

Often we have a little green "start" button to turn on a motor, and we want to turn it off with a big red

"stop" button. The stop button itself is wired as a normally closed switch. This means that when the

stop button is in its normal state (not pushed), the PLC input will be true. When the stop button is

pushed, the input will go false. This will make the rung false and stop the "run" output. A normally-

open contact must be used in the ladder diagram, since this input is normally turned on through the

normally closed pushbutton contact, and turns off when the button is pressed.

--+----[ ]--+----[ ]----( )

| start | stop run

| |

+----[ ]--+

run

-------[ ]--------------( )

run motor

This latch configuration is a common idiom in ladder logic. In ladder logic it is referred to as seal-in

logic. The key to understanding the latch is in recognizing that "start" switch is a momentary switch

(once the user releases the button, the switch is open again). As soon as the "run" solenoid engages,

it closes the "run" switch, which latches the solenoid on. The "start" switch opening up then has no

effect.

Additional functionality

Additional functionality can be added to a ladder logic implementation by the PLC manufacturer as a

special block. When the special block is powered, it executes code on predetermined arguments.

These arguments may be displayed within the special block.

+-------+

-----[ ]--------------------+ A +----

Remote Unlock +-------+

Remote Counter

+-------+

-----[ ]--------------------+ B +----

Interior Unlock +-------+

Interior Counter

+--------+

--------------------+ A + B +-----------

+ into C +

+--------+

Adder

In this example, the system will count the number of times that the interior and remote unlock buttons

are pressed. This information will be stored in memory locations A and B. Memory location C will hold

the total number of times that the door has been unlocked electronically.

PLCs have many types of special blocks. They include timers, arithmetic operators and comparisons,

table lookups, text processing, PID control, and filtering functions. More powerful PLCs can operate on

a group of internal memory locations and execute an operation on a range of addresses, for

example,to simulate a physical sequential drum controller or a finite state machine. In some cases,

users can define their own special blocks, which effectively are subroutines or macros. The large

library of special blocks along with high speed execution has allowed use of PLCs to implement very

complex automation systems.

Limitations and successor languages

Ladder notation is best suited to control problems where only binary variables are required and where

interlocking and sequencing of binary is the primary control problem. Since execution of rungs is

sequential within a program and may be undefined or obscure within a rung, some logic race

conditions are possible which may produce unexpected results; complex rungs are best broken into

several simpler steps to avoid this problem. Some manufacturers, e.g. Omron, avoid this problem by

explicitly and completely defining the execution order of a rung, however programmers may still have

problems fully grasping the resulting complex semantics.

Analog quantities and arithmetical operations are clumsy to express in ladder logic and each

manufacturer has different ways of extending the notation for these problems. There is usually limited

support for arrays and loops, often resulting in duplication of code to express cases which in other

languages would call for use of indexed variables.

As microprocessors have become more powerful, notations such as sequential function

charts and function block diagrams can replace ladder logic for some limited applications. Very large

programmable controllers may have all or part of the programming carried out in a dialect that

resembles BASIC or C or other programming language with bindings appropriate for a real-time

application environment.

WIRING DIAGRAM

A wiring diagram is a simplified conventional pictorial representation of an electrical circuit. It shows

the components of the circuit as simplified shapes, and the power and signal connections between the

devices. A wiring diagram usually gives more information about the relative position and arrangement

of devices and terminals on the devices, to help in building the device. This is unlike a schematic

diagram where the arrangement of the components interconnections on the diagram does not

correspond to their physical locations in the finished device. A pictorial diagram would show more

detail of the physical appearance, whereas a wiring diagram uses a more symbolic notation to

emphasize interconnections over physical appearance.

A wiring diagram for parts of an electric guitar, showing semi-pictorial representation of devices arranged in roughly the

same locations they would have in the guitar.

A wiring diagram is used to troubleshoot problems and to make sure that all the connections have

been made and that everything is present.

An automotive wiring diagram, showing useful information such as crimp connection locations and wire colors. These

details may not be so easily found on a more schematic drawing.

Architectural wiring diagrams

Architectural wiring diagrams show the approximate locations and interconnections of receptacles,

lighting, and permanent electrical services in a building. Interconnecting wire routes may be shown

approximately, where particular receptacles or fixtures must be on a common circuit.

Architectural style wiring diagram, with lamps and switches shown symbolically in their physical locations on the plan

view of the building.

Wiring diagrams use standard symbols for wiring devices, usually different from those used

on schematic diagrams. The electrical symbols not only show where something is to be installed, but

also what type of device is being installed. For example, a surface ceiling light is shown by one

symbol, a recessed ceiling light has a different symbol, and a surface fluorescent light has another

symbol. Each type of switch has a different symbol and so do the various outlets. There are symbols

that show the location of smoke detectors, the doorbell chime, and thermostat. On large projects

symbols may be numbered to show, for example, the panel board and circuit to which the device

connects, and also to identify which of several types of fixture are to be installed at that location.

A set of wiring diagrams may be required by the electrical inspection authority to approve connection

of the residence to the public electrical supply system.

Wiring diagrams will also include panel schedules for circuit breaker panelboards, and riser diagrams

for special services such as fire alarm orclosed circuit television or other special services.

PRINTED CIRCUT BOARD

A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic

components using conductivepathways, tracks or signal

traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to

as printed wiring board (PWB) or etched wiring board.

A PCB populated with electronic components is a printed circuit assembly (PCA), also known as

a printed circuit board assembly orPCB Assembly (PCBA). Printed circuit boards are used in

virtually all but the simplest commercially produced electronic devices.

Alternatives to PCBs include wire wrap and point-to-point construction. PCBs are often less expensive

and more reliable than these alternatives, though they require more layout effort and higher initial cost.

PCBs are much cheaper and faster for high-volume production since production and soldering of

PCBs can be done by automated equipment. Much of the electronics industry's PCB design,

assembly, and quality control needs are set by standards that are published by the IPC organization.

Development of the methods used in modern printed circuit boards started early in the 20th century. In

1903, a German inventor, Albert Hanson, described flat foil conductors laminated to an insulating

board, in multiple layers. Thomas Edison experimented with chemical methods of plating conductors

onto linen paper in 1904. Arthur Berry in 1913 patented a print-and-etch method in Britain, and in the

United States Max Schoop obtained a patent[1] to flame-spray metal onto a board through a patterned

mask. Charles Durcase in 1927 patented a method of electroplating circuit patterns. [2]

The Austrian Jewish engineer Paul Eisler invented the printed circuit while working in England around

1936 as part of a radio set. Around 1943 the USA began to use the technology on a large scale to

make proximity fuses for use in World War II [2]. After the war, in 1948, the USA released the invention

for commercial use. Printed circuits did not become commonplace in consumer electronics until the

mid-1950s, after the Auto-Sembly process was developed by the United States Army.

Before printed circuits (and for a while after their invention), point-to-point construction was used. For

prototypes, or small production runs, wire wrap or turret board can be more efficient. Predating the

printed circuit invention, and similar in spirit, was John Sargrove's 1936–1947 Electronic Circuit

Making Equipment (ECME) which sprayed metal onto a Bakelite plastic board. The ECME could

produce 3 radios per minute.

During World War II, the development of the anti-aircraft proximity fuse required an electronic circuit

that could withstand being fired from a gun, and could be produced in quantity. The Centralab Division

of Globe Union submitted a proposal which met the requirements: a ceramic plate would

be screenprinted with metallic paint for conductors and carbon material forresistors, with ceramic disc

capacitors and subminiature vacuum tubes soldered in place.

Originally, every electronic component had wire leads, and the PCB had holes drilled for each wire of

each component. The components' leads were then passed through the holes andsoldered to the PCB

trace. This method of assembly is called through-hole construction. In 1949, Moe Abramson and

Stanislaus F. Danko of the United States Army Signal Corpsdeveloped the Auto-Sembly process in

which component leads were inserted into a copper foil interconnection pattern and dip soldered. The

patent they obtained in 1956 was assigned to the U.S. Army.  With the development of board

lamination and etching techniques, this concept evolved into the standard printed circuit board

fabrication process in use today. Soldering could be done automatically by passing the board over a

ripple, or wave, of molten solder in a wave-soldering machine. However, the wires and holes are

wasteful since drilling holes is expensive and the protruding wires are merely cut off.

In recent years, the use of surface mount parts has gained popularity as the demand for smaller

electronics packaging and greater functionality has grown.

Manufacturing

Materials

A PCB as a design on a computer (left) and realized as a board assembly populated with components (right). The board

is double sided, with through-hole plating, green solder resist, and white silkscreen printing. Both surface mount and

through-hole components have been used.

A PCB in a computer mouse. The Component Side (left) and the printed side (right).

The Component Side of a PCB in a computer mouse; some examples for common components and their reference

designations on the silk screen.

Component and solderside

Conducting layers are typically made of thin copper foil. Insulating layers dielectric are typically

laminated together with epoxy resin prepreg . The board is typically coated with a solder mask that is

green in color. Other colors that are normally available are blue, black, white and red. There are quite

a few different dielectrics that can be chosen to provide different insulating values depending on the

requirements of the circuit. Some of these dielectrics are polytetrafluoroethylene (Teflon), FR-4, FR-1,

CEM-1 or CEM-3. Well known prepreg materials used in the PCB industry are FR-2 (Phenolic cotton

paper), FR-3 (Cotton paper and epoxy), FR-4 (Woven glass and epoxy), FR-5 (Woven glass and

epoxy), FR-6 (Matte glass and polyester), G-10 (Woven glass and epoxy), CEM-1 (Cotton paper and

epoxy), CEM-2 (Cotton paper and epoxy), CEM-3 (Non-woven glass and epoxy), CEM-4 (Woven

glass and epoxy), CEM-5 (Woven glass and polyester). Thermal expansion is an important

consideration especially with ball grid array (BGA) and naked die technologies, and glass fiber offers

the best dimensional stability.

FR-4 is by far the most common material used today. The board with copper on it is called "copper-

clad laminate".

Copper foil thickness can be specified in ounces per square foot or micrometres. One ounce per

square foot is 1.344 mils or 34 micrometres.

Patterning (etching)

The vast majority of printed circuit boards are made by bonding a layer of copper over the entire

substrate, sometimes on both sides, (creating a "blank PCB") then removing unwanted copper after

applying a temporary mask (e.g., by etching), leaving only the desired copper traces. A few PCBs are

made by adding traces to the bare substrate (or a substrate with a very thin layer of copper) usually by

a complex process of multiple electroplating steps. The PCB manufacturing method primarily depends

on whether it is for production volume or sample/prototype quantities. Double-sided boards or multi-

layer boards use plated-through holes, called vias, to connect traces on either side of the substrate.

Large volume

Silk screen printing–the main commercial method.

Photographic methods–used when fine linewidths are required.

Small volume

Print onto transparent film and use as photomask along with photo-sensitized boards. (i.e., pre-

sensitized boards), then etch. (Alternatively, use a film photoplotter).

Laser resist ablation: Spray black paint onto copper clad laminate, place into CNC laser plotter.

The laser raster-scans the PCB and ablates (vaporizes) the paint where no resist is wanted. Etch.

(Note: laser copper ablation is rarely used and is considered experimental.)

Use a CNC-mill with a spade-shaped (i.e., a flat-ended cone) cutter or miniature end-mill

to rout away the undesired copper, leaving only the traces.

Hobbyist

Laser-printed resist: Laser-print onto transparency film, heat-transfer with an iron or modified

laminator onto bare laminate, touch up with a marker, then etch.

Other labor-intensive techniques exist, only suitable for one-off boards (vinyl film and resist, non-

washable marker, and others).

There are three common "subtractive" methods (methods that remove copper) used for the production

of printed circuit boards:

1. Silk screen printing uses etch-resistant inks to protect the copper foil. Subsequent etching

removes the unwanted copper. Alternatively, the ink may be conductive, printed on a blank

(non-conductive) board. The latter technique is also used in the manufacture of hybrid

circuits.

2. Photoengraving uses a photomask and developer to selectively remove

a photoresist coating. The remaining photoresist protects the copper foil. Subsequent etching

removes the unwanted copper. The photomask is usually prepared with a photoplotter from

data produced by a technician using CAM, or computer-aided manufacturing software. Laser-

printed transparencies are typically employed for phototools; however, direct laser imaging

techniques are being employed to replace phototools for high-resolution requirements.

3. PCB milling uses a two or three-axis mechanical milling system to mill away the copper foil

from the substrate. A PCB milling machine (referred to as a 'PCB Prototyper') operates in a

similar way to a plotter, receiving commands from the host software that control the position

of the milling head in the x, y, and (if relevant) z axis. Data to drive the Prototyper is extracted

from files generated in PCB design software and stored in HPGL or Gerber file format.

"Additive" processes also exist. The most common is the "semi-additive" process. In this version, the

unpatterned board has a thin layer of copper already on it. A reverse mask is then applied. (Unlike a

subtractive process mask, this mask exposes those parts of the substrate that will eventually become

the traces.) Additional copper is then plated onto the board in the unmasked areas; copper may be

plated to any desired weight. Tin-lead or other surface platings are then applied. The mask is stripped

away and a brief etching step removes the now-exposed original copper laminate from the board,

isolating the individual traces. Some boards with plated through holes but still single sided were made

with a process like this. General Electric made consumer radio sets in the late 1960s using boards like

these.

The additive process is commonly used for multi-layer boards as it facilitates the plating-through of the

holes (to produce conductive vias) in the circuit board.

PCB copper electroplating machine for adding copper to the in-process PCB

 

PCBs in process of adding copper via electroplating

The dimensions of the copper conductors of the printed circuit board is related to the amount of

current the conductor must carry. Each trace consists of a flat, narrow part of the copperfoil that

remains after etching. Signal traces are usually narrower than power or ground traces because their

current carrying requirements are usually much less. In a multi-layer board one entire layer may be

mostly solid copper to act as a ground plane for shielding and power return. For printed circuit boards

that contain microwave circuits, transmission lines can be laid out in the form

of stripline and microstrip with carefully controlled dimensions to assure a consistent impedance. In

radio-frequency circuits the inductance and capacitance of the printed circuit board conductors can be

used as a deliberate part of the circuit design, obviating the need for additional discrete components.

Chemical etching

Chemical etching is done with ferric chloride, ammonium persulfate, or sometimes hydrochloric acid.

For PTH (plated-through holes), additional steps of electroless deposition are done after the holes are

drilled, then copper is electroplated to build up the thickness, the boards are screened, and plated with

tin/lead. The tin/lead becomes the resist leaving the bare copper to be etched away.

The simplest method, used for small-scale production and often by hobbyists, is immersion etching, in

which the board is submerged in etching solution such as ferric chloride. Compared with methods

used for mass production, the etching time is long. Heat and agitation can be applied to the bath to

speed the etching rate. In bubble etching, air is passed through the etchant bath to agitate the solution

and speed up etching. Splash etching uses a motor-driven paddle to splash boards with etchant; the

process has become commercially obsolete since it is not as fast as spray etching. In spray etching,

the etchant solution is distributed over the boards by nozzles, and recirculated by pumps. Adjustment

of the nozzle pattern, flow rate, temperature, and etchant composition gives predictable control of

etching rates and high production rates. 

As more copper is consumed from the boards, the etchant becomes saturated and less effective;

different etchants have different capacities for copper, with some as high as 150 grams of copper per

litre of solution. In commercial use, etchants can be regenerated to restore their activity, and the

dissolved copper recovered and sold. Small-scale etching requires attention to disposal of used

etchant, which is corrosive and toxic due to its metal content.

The etchant removes copper on all surfaces exposed by the resist. "Undercut" occurs when etchant

attacks the thin edge of copper under the resist; this can reduce conductor widths and cause open-

circuits. Careful control of etch time is required to prevent undercut. Where metallic plating is used as

a resist, it can "overhang" which can cause short-circuits between adjacent traces when closely

spaced. Overhang can be removed by wire-brushing the board after etching. 

Lamination

Some PCBs have trace layers inside the PCB and are called multi-layer PCBs. These are formed by

bonding together separately etched thin boards.

Drilling

Holes through a PCB are typically drilled with small-diameter drill bits made of solid coated tungsten

carbide. Coated tungsten carbide is recommended since many board materials are very abrasive and

drilling must be high RPM and high feed to be cost effective. Drill bits must also remain sharp so as

not to mar or tear the traces. Drilling with high-speed-steel is simply not feasible since the drill bits will

dull quickly and thus tear the copper and ruin the boards. The drilling is performed

by automated drilling machines with placement controlled by a drill tape or drill file. These computer-

generated files are also called numerically controlled drill (NCD) files or "Excellon files". The drill file

describes the location and size of each drilled hole. These holes are often filled with annular rings

(hollow rivets) to create vias. Vias allow the electrical and thermal connection of conductors on

opposite sides of the PCB.

When very small vias are required, drilling with mechanical bits is costly because of high rates of wear

and breakage. In this case, the vias may be evaporated by lasers. Laser-drilled vias typically have an

inferior surface finish inside the hole. These holes are called micro vias.

It is also possible with controlled-depth drilling, laser drilling, or by pre-drilling the individual sheets of

the PCB before lamination, to produce holes that connect only some of the copper layers, rather than

passing through the entire board. These holes are called blind vias when they connect an internal

copper layer to an outer layer, or buried vias when they connect two or more internal copper layers

and no outer layers.

The walls of the holes, for boards with 2 or more layers, are made conductive then plated with copper

to form plated-through holes that electrically connect the conducting layers of the PCB. For multilayer

boards, those with 4 layers or more, drilling typically produces a smear of the high temperature

decomposition products of bonding agent in the laminate system. Before the holes can be plated

through, this smear must be removed by a chemical de-smear process, or by plasma-etch. Removing

(etching back) the smear also reveals the interior conductors as well.

Exposed conductor plating and coating

PCBs are plated with solder, tin, or gold over nickel as a resist for etching away the unneeded

underlying copper.

After PCBs are etched and then rinsed with water, the soldermask is applied, and then any exposed

copper is coated with solder, nickel/gold, or some other anti-corrosion coating.

Matte solder is usually fused to provide a better bonding surface or stripped to bare copper.

Treatments, such as benzimidazolethiol, prevent surface oxidation of bare copper. The places to

which components will be mounted are typically plated, because untreated bare copper oxidizes

quickly, and therefore is not readily solderable. Traditionally, any exposed copper was coated with

solder by hot air solder levelling (HASL). The HASL finish prevents oxidation from the underlying

copper, thereby guaranteeing a solderable surface. This solder was atin-lead alloy, however new

solder compounds are now used to achieve compliance with the RoHS directive in the EU and US,

which restricts the use of lead. One of these lead-free compounds is SN100CL, made up of 99.3% tin,

0.7% copper, 0.05% nickel, and a nominal of 60ppm germanium.

It is important to use solder compatible with both the PCB and the parts used. An example is Ball Grid

Array (BGA) using tin-lead solder balls for connections losing their balls on bare copper traces or using

lead-free solder paste.

Other platings used are OSP (organic surface protectant), immersion silver (IAg), immersion tin,

electroless nickel with immersion gold coating (ENIG), and direct gold plating (over nickel). Edge

connectors, placed along one edge of some boards, are often nickel plated then gold plated. Another

coating consideration is rapid diffusion of coating metal into Tin solder. Tin forms intermetallics such

as Cu5Sn6 and Ag3Cu that dissolve into the Tin liquidus or solidus(@50C), stripping surface coating or

leaving voids.

Electrochemical migration (ECM) is the growth of conductive metal filaments on or in a printed circuit

board (PCB) under the influence of a DC voltage bias. Silver, zinc, and aluminum are known to grow

whiskers under the influence of an electric field. Silver also grows conducting surface paths in the

presence of halide and other ions, making it a poor choice for electronics use. Tin will grow "whiskers"

due to tension in the plated surface. Tin-Lead or Solder plating also grows whiskers, only reduced by

the percentage Tin replaced. Reflow to melt solder or tin plate to relieve surface stress lowers whisker

incidence. Another coating issue is tin pest, the transformation of tin to a powdery allotrope at low

temperature.

Solder resist

Areas that should not be soldered may be covered with a polymer solder resist (solder mask) coating.

The solder resist prevents solder from bridging between conductors and creating short circuits. Solder

resist also provides some protection from the environment. Solder resist is typically 20–30

micrometres thick.

Screen printing

Line art and text may be printed onto the outer surfaces of a PCB by screen printing. When space

permits, the screen print text can indicate component designators, switch setting requirements, test

points, and other features helpful in assembling, testing, and servicing the circuit board.

Screen print is also known as the silk screen, or, in one sided PCBs, the red print.

Lately some digital printing solutions have been developed to substitute the traditional screen printing

process. This technology allows printing variable data onto the PCB, including serialization and

barcode information for traceability purposes.

Test

Unpopulated boards may be subjected to a bare-board test where each circuit connection (as defined

in a netlist) is verified as correct on the finished board. For high-volume production, a bed of nails

tester, a fixture or a rigid needle adapter is used to make contact with copper lands or holes on one or

both sides of the board to facilitate testing. A computer will instructthe electrical test unit to apply a

small voltage to each contact point on the bed-of-nails as required, and verify that such voltage

appears at other appropriate contact points. A "short" on a board would be a connection where there

should not be one; an "open" is between two points that should be connected but are not. For small- or

medium-volume boards, flying probeand flying-grid testers use moving test heads to make contact

with the copper/silver/gold/solder lands or holes to verify the electrical connectivity of the board under

test. Another method for testing is industrial CT scanning, which can generate a 3D rendering of the

board along with 2D image slices and can show details such a soldered paths and connections.

Printed circuit assembly

PCB with test connection pads

After the printed circuit board (PCB) is completed, electronic components must be attached to form a

functional printed circuit assembly, or PCA (sometimes called a "printed circuit board assembly"

PCBA). In through-hole construction, component leads are inserted in holes. Insurface-

mount construction, the components are placed on pads or lands on the outer surfaces of the PCB. In

both kinds of construction, component leads are electrically and mechanically fixed to the board with a

molten metal solder.

There are a variety of soldering techniques used to attach components to a PCB. High volume

production is usually done with SMT placement machine and bulk wave soldering or reflow ovens, but

skilled technicians are able to solder very tiny parts (for instance 0201 packages which are 0.02 in. by

0.01 in.) by hand under a microscope, using tweezers and a fine tip soldering iron for small volume

prototypes. Some parts may be extremely difficult to solder by hand, such as BGA packages.

Often, through-hole and surface-mount construction must be combined in a single assembly because

some required components are available only in surface-mount packages, while others are available

only in through-hole packages. Another reason to use both methods is that through-hole mounting can

provide needed strength for components likely to endure physical stress, while components that are

expected to go untouched will take up less space using surface-mount techniques.

After the board has been populated it may be tested in a variety of ways:

While the power is off, visual inspection, automated optical inspection. JEDEC guidelines for PCB

component placement, soldering, and inspection are commonly used to maintainquality control in

this stage of PCB manufacturing.

While the power is off, analog signature analysis, power-off testing.

While the power is on, in-circuit test, where physical measurements (i.e. voltage, frequency) can

be done.

While the power is on, functional test, just checking if the PCB does what it had been designed to

do.

To facilitate these tests, PCBs may be designed with extra pads to make temporary connections.

Sometimes these pads must be isolated with resistors. The in-circuit test may also exercise boundary

scan test features of some components. In-circuit test systems may also be used to program

nonvolatile memory components on the board.

In boundary scan testing, test circuits integrated into various ICs on the board form temporary

connections between the PCB traces to test that the ICs are mounted correctly. Boundary scan testing

requires that all the ICs to be tested use a standard test configuration procedure, the most common

one being the Joint Test Action Group (JTAG) standard. The JTAG test architecture provides a means

to test interconnects between integrated circuits on a board without using physical test

probes. JTAG tool vendors provide various types of stimulus and sophisticated algorithms, not only to

detect the failing nets, but also to isolate the faults to specific nets, devices, and pins.[17]

When boards fail the test, technicians may desolder and replace failed components, a task known

as rework.

Protection and packaging

PCBs intended for extreme environments often have a conformal coating, which is applied by dipping

or spraying after the components have been soldered. The coat prevents corrosion and leakage

currents or shorting due to condensation. The earliest conformal coats were wax; modern conformal

coats are usually dips of dilute solutions of silicone rubber, polyurethane, acrylic, or epoxy. Another

technique for applying a conformal coating is for plastic to be sputtered onto the PCB in a vacuum

chamber. The chief disadvantage of conformal coatings is that servicing of the board is rendered

extremely difficult.

Many assembled PCBs are static sensitive, and therefore must be placed in antistatic bags during

transport. When handling these boards, the user must be grounded (earthed). Improper handling

techniques might transmit an accumulated static charge through the board, damaging or destroying

components. Even bare boards are sometimes static sensitive. Traces have become so fine that it's

quite possible to blow an etch off the board (or change its characteristics) with a static charge. This is

especially true on non-traditional PCBs such as MCMs and microwave PCBs.

Design

Printed circuit board design was initially a fully manual process, where an initial schematic diagram

was converted into a layout of parts, then traces were routed between package terminals to provide

the required interconnections. Pre-printed non-reproducing mylar grids assisted in layout, and rub-on

dry transfers of common arrangements of circuit elements (pads, contact fingers, integrated circuit

profiles, and so on) helped standardize the layout. The finished layout "artwork" wasn then

photographically reproduced on the resist layers of the blank coated copper-clad boards.

Modern practice is less labor intensive since computers can inexpensively and accurately perform

many of the layout steps. The general progression for a commercial printed circuit board design would

include:

1. Schematic capture through an Electronic design automation tool.

2. Card dimensions and template are decided based on required circuitry and case of the PCB.

Determine the fixed components and heat sinks if required.

3. Deciding stack layers of the PCB. 4 to 12 layers or more depending on design

complexity. Ground plane and power plane are decided. Signal planes where signals are

routed are in top layer as well as internal layers.[19]

4. Line impedance determination using dielectric layer thickness, routing copper thickness and

trace-width. Trace separation also taken into account in case of differential

signals.Microstrip, stripline or dual stripline can be used to route signals.

5. Placement of the components. Thermal considerations and geometry are taken into

account. Vias and lands are marked.

6. Routing the signal trace. For optimal EMI performance high frequency signals are routed in

internal layers between power or ground planes as power plane behaves as ground for AC.

7. Gerber file generation for manufacturing.

Heavy copper

The printed circuit board industry defines heavy copper as layers exceeding 3 ounces of copper, or

approximately 0.0042 inches (4.2 mils) of copper thickness. PCB designers and fabricators often use

heavy copper when design and manufacturing circuit boards in order to increase current-carrying

capacity as well as resistance to thermal strains. Heavy copper plated vias transfer heat to external

heat sinks. The IPC 2152 is a standard for determining current-carrying capacity in printed circuit

board design.

An ounce of copper thickness on a PCB is defined as the thickness of one ounce (weight) of copper

rolled out to an area of one square foot. An ounce of copper is approximately 1.4 mils (0.0014 inch) or

35 um of thickness.

Safety certification (US)

Safety Standard UL 796 covers component safety requirements for printed wiring boards for use as

components in devices or appliances. Testing analyzes characteristics such as flammability,

maximum operating temperature, electrical tracking, heat deflection, and direct support of live

electrical parts.

"Cordwood" construction

A cordwood module

Cordwood construction can save significant space and was often used with wire-ended components in

applications where space was at a premium (such as missile guidance and telemetry systems) and in

high-speed computers, where short traces were important. In "cordwood" construction, axial-leaded

components were mounted between two parallel planes. The components were either soldered

together with jumper wire, or they were connected to other components by thin nickel ribbon welded at

right angles onto the component leads. To avoid shorting together different interconnection layers, thin

insulating cards were placed between them. Perforations or holes in the cards allowed component

leads to project through to the next interconnection layer. One disadvantage of this system was that

special nickelleaded components had to be used to allow the interconnecting welds to be made. Some

versions of cordwood construction used single sided PCBs as the interconnection method (as

pictured). This meant that normal leaded components could be used. Another disadvantage of this

system is that components located in the interior are difficult to replace.

Before the advent of integrated circuits, this method allowed the highest possible component packing

density; because of this, it was used by a number of computer vendors includingControl Data

Corporation. The cordwood method of construction now appears to have fallen into disuse, probably

because higher packing densities are possible using integrated circuits, and as older units failed they

were replaced rather than repaired at great expense.

Multiwire boards

Multiwire is a patented technique of interconnection which uses machine-routed insulated wires

embedded in a non-conducting matrix (often plastic resin). It was used during the 1980s and 1990s.

(Kollmorgen Technologies Corp, U.S. Patent 4,175,816 filed 1978) Multiwire is still available in 2010

through Hitachi. There are other competitive discrete wiring technologies that have been developed

(Jumatech , layered sheets).

Since it was quite easy to stack interconnections (wires) inside the embedding matrix, the approach

allowed designers to forget completely about the routing of wires (usually a time-consuming operation

of PCB design): Anywhere the designer needs a connection, the machine will draw a wire in straight

line from one location/pin to another. This led to very short design times (no complex algorithms to use

even for high density designs) as well as reduced crosstalk (which is worse when wires run parallel to

each other—which almost never happens in Multiwire), though the cost is too high to compete with

cheaper PCB technologies when large quantities are needed.

Surface-mount technology

Surface mount components, including resistors, transistors and an integrated circuit

Surface-mount technology emerged in the 1960s, gained momentum in the early 1980s and became

widely used by the mid 1990s. Components were mechanically redesigned to have small metal tabs or

end caps that could be soldered directly on to the PCB surface. Components became much smaller

and component placement on both sides of the board became more common than with through-hole

mounting, allowing much higher circuit densities. Surface mounting lends itself well to a high degree of

automation, reducing labour costs and greatly increasing production and quality rates. Carrier Tapes

provide a stable and protective environment for Surface mount devices (SMDs) which can be one-

quarter to one-tenth of the size and weight, and passive components can be one-half to one-quarter of

the cost of corresponding through-hole parts. However, integrated circuits are often priced the same

regardless of the package type, because the chip itself is the most expensive part. As of 2006, some

wire-ended components, such as small-signal switch diodes, e.g., 1N4148, are actually significantly

cheaper than corresponding SMD versions.

Through-hole technology

Through-hole (leaded) resistors

Through-hole technology refers to mounting electronic components by leads inserted into holes.

Horizontal installation of axial-leaded through-hole parts (e.g., resistor, capacitors, and diodes) is done

by bending the leads 90 degrees in the same direction, inserting part in the board, bending leads

located on the back of the board in opposing directions to improve the part's mechanical strength;

finally, soldering the leads such that the solder seeps through to both sides of the board. Later the

assembler cuts off the excess leads. Leads are soldered to pads on the opposite side either manually

or by a wave soldering machine.

Through-hole technology almost completely replaced earlier electronics assembly techniques such

as point-to-point construction. From thesecond generation of computers in the 1950s until surface-

mount technology became popular in the late 1980s, every component on a typical PCB was a

through-hole component.

While through-hole mounting provides strong mechanical bonds when compared to surface-mount

technology techniques, the additional drilling required makes the boards more expensive to produce.

They also limit the available routing area for signal traces on layers immediately below the top layer on

multilayer boards since the holes must pass through all layers to the opposite side. To that end,

through-hole mounting is now usually reserved for bulky components such as electrolytic capacitors or

large semiconductors packages such as the TO220 that require the additional mounting strength, or

for components such as plug connectors or electromechanical relays that require great strength in

support.

Through-hole devices mounted on the circuit board of a mid-1980's home computer

 

A box of drill bits used for making holes in printed circuit boards. While tungsten-carbide bits are very hard, they

eventually wear out or break. Making holes is a considerable part of the cost of a through-hole printed circuit

board.

BLOCK

Block diagram is a diagram of a system, in which the principal parts or functions are represented by

blocks connected by lines, that show the relationships of the blocks. They are heavily used in the

engineering world in hardware design, electronic design, software design, andprocess flow diagrams.

The block diagram is typically used for a higher level, less detailed description aimed more at

understanding the overall concepts and less at understanding the details of implementation. Contrast

this with the schematic diagram and layout diagram used in the electrical engineering world, where the

schematic diagram shows the details of each electrical component and the layout diagram shows the

details of physical construction. Because block diagrams are a visual language for describing actions

in a complex system, it is possible to formalize them into a specialized programmable logic controller

(PLC) programming language. A Function block diagram is one of five programming languages

defined in part 3 of the IEC 61131 (see IEC 61131-3) standard. Since this is a real, bona fide computer

programming language, it is highly formalized (see formal system) with strict rules for how diagrams

are to be built. Directed lines are used to connect input variables to function inputs, function outputs to

output variables, and function outputs to inputs of other functions. These blocks portray mathematical

or logical operations that occur in time sequence. They do not represent the physical entities, such as

processors or relays, that perform those operations. Each block is therefore a black box. The rules

require the logical sequence to go from left to right and top to bottom.

Usage examples

As an example, a block diagram of a radio is not expected to show each and every wire and dial and

switch, but the schematic diagram is. The schematic diagram of a radio does not show the width of

each wire in the printed circuit board, but the layout diagram does.

To make an analogy to the map making world, a block diagram is similar to a highway map of an

entire nation. The major cities (functions) are listed but the minor county roads and city streets are not.

When troubleshooting, this high level map is useful in narrowing down and isolating where a problem

or fault is.

Block diagrams rely on the principle of the black box where the contents are hidden from view either to

avoid being distracted by the details or because the details are not known. We know what goes in, we

know what goes out, but we can't see how the box does its work.

In electrical engineering, a design will often begin as a very high level block diagram, becoming more

and more detailed block diagrams as the design progresses, finally ending in block diagrams detailed

enough that each individual block can be easily implemented (at which point the block diagram is also

a schematic diagram). This is known as top down design. Geometric shapes are often used in the

diagram to aid interpretation and clarify meaning of the process or model. The geometric shapes are

connected by lines to indicate association and direction/order of traversal. Each engineering discipline

has their own meaning for each shape.

In Biology there is an increasing use of engineering principles,techniques of analysis and methods of

diagramming. There is some similarity between the block diagram and what is calledSystems Biology

Graphical Notation. As it is there is use made in systems biology of the block diagram technique

harnessed by control engineering where the latter itself is a an application of control theory.this

diagram is describes total information about processing.