Variable Resistor

8/3/2019 Variable Resistor

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Variable Resistor

Adjustable resistors

A resistor may have one or more fixed tapping points so that the resistance can be changed by

moving the connecting wires to different terminals. Some wirewound power resistors have a

tapping point that can slide along the resistance element, allowing a larger or smaller part of the

resistance to be used.

Where continuous adjustment of the resistance value during operation of equipment is required,

the sliding resistance tap can be connected to a knob accessible to an operator. Such a device is

called arheostatand has two terminals.

Potentiometers

A common element in electronic devices is a three-terminal resistor with a continuously

adjustable tapping point controlled by rotation of a shaft or knob. These variable resistors are

known aspotentiometerswhen all three terminals are present, since they act as a continuously

adjustablevoltage divider. A common example is a volume control for a radio receiver.[9]

Accurate, high-resolution panel-mounted potentiometers (or "pots") have resistance elements

typically wire wound on a helical mandrel, although some include a conductive-plastic resistance

coating over the wire to improve resolution. These typically offer ten turns of their shafts tocover their full range. They are usually set with dials that include a simple turns counter and a

graduated dial. Electronic analog computers used them in quantity for setting coefficients, and

delayed-sweep oscilloscopes of recent decades included one on their panels.

Resistance decade boxes

A resistance decade box or resistor substitution box is a unit containing resistors of many values,

with one or more mechanical switches which allow any one of various discrete resistances

offered by the box to be dialed in. Usually the resistance is accurate to high precision, ranging

from laboratory/calibration grade accuracy of 20 parts per million, to field grade at 1%.

Inexpensive boxes with lesser accuracy are also available. All types offer a convenient way of

selecting and quickly changing a resistance in laboratory, experimental and development work

without needing to attach resistors one by one, or even stock each value. The range of resistance
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provided, the maximum resolution, and the accuracy characterize the box. For example, one box

offers resistances from 0 to 24 megohms, maximum resolution 0.1 ohm, accuracy 0.1%.

Special devices

There are various devices whose resistance changes with various quantities. The resistance

ofthermistorsexhibit a strong negative temperature coefficient, making them useful for

measuring temperatures. Since their resistance can be large until they are allowed to heat up due

to the passage of current, they are also commonly used to prevent excessivecurrent surgeswhen

equipment is powered on.Metal oxide varistorsdrop to a very low resistance when a high

voltage is applied, making them useful for protecting electronic equipment by absorbing

dangerousvoltage surges. One sort of photo detector, thephoto resistor, has a resistance which

varies with illumination.

Thestrain gauge, invented byEdward E. SimmonsandArthur C. Rugein 1938, is a type of

resistor that changes value with applied strain. A single resistor may be used, or a pair (half

bridge), or four resistors connected in aWheatstone bridgeconfiguration. The strain resistor is

bonded with adhesive to an object that will be subjected tomechanical strain. With the strain

gauge and a filter, amplifier, and analog/digital converter, the strain on an object can be

measured.

A related but more recent invention uses aQuantum Tunnelling Compositeto sense mechanicalstress. It passes a current whose magnitude can vary by a factor of 1012 in response to changes in

applied pressure.

CAPACITOR
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Capacitor, device for storing an electrical charge, sometimes called a condenser. In its simplest form a

capacitor consists of two metal plates separated by a non-conducting layer called the dielectric. The

dielectric may be air, plastic, waxed paper, or another substance such as the mineral mica. When one

plate of a capacitor is charged using a battery or other source of direct current, the other plate becomes

charged with the opposite sign; that is, positive if the original charge is negative, and negative if the

original charge is positive.

The electrical size of a capacitor is its capacitance, that is the amount of electric charge it can hold per

unit potential difference across its platesC = Q/V. The SI unit of capacitance is the farad (F). Because

this is such a large unit, capacitors commonly have their size expressed in F (1 microfarad = 10 -6 F) or pF

(1 picofarad = 10-9

F). The capacitance of a parallel plate capacitor can be calculated from the

relationship:

where A is the area of the plates, d is the distance between them, 0 is the permittivity of free space,

and r is the relative permittivity of the dielectric between the two plates.

Capacitors can hold a limited amount of electric charge. As more and more charge is added to the plates

of a capacitor, the potential difference between the plates increases. Eventually this potential difference

becomes so great that the atomic structure of the dielectric breaks down, and charge leaks through it.

Capacitors can conduct direct current for only an instant but are able to act as conductors in alternating-

current circuits, as they constantly charge and discharge as the direction of the current constantly

changes. This property makes them useful when direct current must be prevented from entering some

part of an electric circuit. Fixed-capacity and variable-capacity capacitors are used with coils in resonant

circuits in radios and other electronic equipment.

Because the dielectric of a capacitor may break down, there is a limit to the potential difference that

may be applied across a capacitor. Capacitors are therefore labelled not only with their capacitance but

also with their working potential difference in order to prevent breakdown of the dielectric in use.


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BUZZER

A buzzer or beeper is anaudiosignaling device, which may bemechanical,electromechanical,

orelectronic. Typical uses of buzzers and beepers includealarms,timersand confirmation of

user input such as a mouse click or keystroke.

Piezoelectric disk beepe

Mechanical:

Ajoy buzzeris an example of a purely mechanical buzzer.

Electromechanical:

Early devices were based on an electromechanical system identical to anelectric bellwithout the

metal gong. Similarly, arelaymay be connected to interrupt its own actuatingcurrent, causing

thecontactsto buzz. Often these units were anchored to a wall or ceiling to use it as a sounding board.

The word "buzzer" comes from the rasping noise that electromechanical buzzers made.

Electronic

Apiezoelectricelement may be driven by anoscillatingelectronic circuit or other audio signal source.

Sounds commonly used to indicate that a button has been pressed are a click, a ring or a beep.

Electronic buzzers find many applications in modern days.
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RESISTOR

A resistor is a two-terminalelectronic componentthat produces avoltageacross its terminals

that isproportionalto theelectric currentthrough it in accordance withOhm's law:

V= IR

A typical axial-lead resistor

Resistors are elements ofelectrical networksand electronic circuits and are ubiquitous in most

electronic equipment. Practical resistors can be made of various compounds and films, as well

asresistance wire(wire made of a high-resistivity alloy, such as nickel-chrome).

The primary characteristics of a resistor are theresistance, thetolerance, the maximum working

voltage and thepowerrating. Other characteristics includetemperature coefficient,noise,

andinductance. Less well-known iscritical resistance, the value below which power dissipation

limits the maximum permitted current, and above which the limit is applied voltage. Critical

resistance is determined by the design, materials and dimensions of the resistor.

Resistors can be integrated intohybridandprinted circuits, as well asintegrated circuits. Size,

and position of leads (or terminals), are relevant to equipment designers; resistors must bephysically large enough not to overheat when dissipating their power.
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Partially exposed Tesla TR-212 1 k carbon film resistor

UNITS

Theohm(symbol:) is theSIunit ofelectrical resistance, named afterGeorg Simon Ohm.

Commonly used multiples and submultiples in electrical and electronic usage are the milliohm(1x10

3), kilohm (1x103), and megohm (1x106).

Theory of operation

Ohm's law

The behavior of an ideal resistor is dictated by the relationship specified in Ohm's law:

Ohm's law states that the voltage (V) across a resistor is proportional to the current (I) through it

where the constant of proportionality is the resistance (R).

Equivalently, Ohm's law can be stated:

This formulation of Ohm's law states that, when a voltage (V) is maintained across a resistance

(R), a current (I) will flow through the resistance.

This formulation is often used in practice. For example, if V is 12voltsand R is 400ohms, a

current of 12 / 400 = 0.03ampereswill flow through the resistance R.

Series and parallel resistors

Resistors in aparallelconfiguration each have the same potential difference (voltage). To find

their total equivalent resistance (Req):
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The parallel property can be represented in equations by two vertical lines "||" (as in geometry) to

simplify equations. For two resistors,

The current through resistors inseriesstays the same, but the voltage across each resistor can be

different. The sum of the potential differences (voltage) is equal to the total voltage. To find their

total resistance:

A resistor network that is a combination of parallel and series can be broken up into smaller parts

that are either one or the other. For instance,
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However, many resistor networks cannot be split up in this way. Consider acube, each edge of

which has been replaced by a resistor. For example, determining the resistance between two

opposite vertices requires additional transforms, such as theY- transform, or elsematrix

methodsmust be used for the general case. However, if all twelve resistors are equal, the corner-

to-corner resistance is 56 of any one of them.

The practical application to resistors is that a resistance of any non-standard value can be

obtained by connecting standard values in series or in parallel .

Power dissipation

The power dissipated by a resistor (or the equivalent resistance of a resistor network) is

calculated using the following:

All three equations are equivalent. The first is derived fromJoule's first law. Ohms Law derives

the other two from that.
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The total amount of heat energy released is the integral of the power over time:

If the average power dissipated is more than the resistor can safely dissipate, the resistor may

depart from its nominal resistance and may become damaged by overheating. Excessive power

dissipation may raise the temperature of the resistor to a point where it burns out, which could

cause a fire in adjacent components and materials. There are flameproof resistors that fail (open

circuit) before they overheat dangerously.

Note that the nominal power rating of a resistor is not the same as the power that it can safely

dissipate in practical use. Air circulation and proximity to a circuit board, ambient temperature,

and other factors can reduce acceptable dissipation significantly. Rated power dissipation may begiven for an ambient temperature of 25 C in free air. Inside an equipment case at 60 C, rated

dissipation will be significantly less; a resistor dissipating a bit less than the maximum figure

given by the manufacturer may still be outside thesafe operating areaand may prematurely fail.

CONSTRUCTION

Lead arrangements

Through-holecomponents typically have leads leaving the body axially. Others have leads

coming off their body radially instead of parallel to the resistor axis. Other components may

beSMT(surface mount technology) while high power resistors may have one of their leads

designed into theheat sink.

Carbon composition

Carbon composition resistors consist of a solid cylindrical resistive element with embedded wire

leads or metal end caps to which the lead wires are attached. The body of the resistor is protected

with paint or plastic. Early 20th-century carbon composition resistors had uninsulated bodies; the

lead wires were wrapped around the ends of the resistance element rod and soldered. The

completed resistor was painted for color coding of its value.
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A single in line (SIL) resistor package with 8 individual, 47 ohm resistors. One end of each resistor is connected to a separate pin and the

other ends are all connected together to the remaining (common) pin - pin 1, at the end identified by the white dot.

The resistive element is made from a mixture of finely ground (powdered) carbon and an

insulating material (usually ceramic). A resin holds the mixture together. The resistance is

determined by the ratio of the fill material (the powdered ceramic) to the carbon. Higher

concentrations of carbon, a weak conductor, result in lower resistance. Carbon composition

resistors were commonly used in the 1960s and earlier, but are not so popular for general usenow as other types have better specifications, such as tolerance, voltage dependence, and stress

(carbon composition resistors will change value when stressed with over-voltages). Moreover, if

internal moisture content (from exposure for some length of time to a humid environment) is

significant, soldering heat will create a non-reversible change in resistance value. These resistors,

however, if never subjected toovervoltagenor overheating were remarkably reliable considering

the component's size[1]

They are still available, but comparatively quite costly. Values ranged from fractions of an ohm

to 22 megohms. Because of the high price, these resistors are no longer used in most

applications. However, carbon resistors are used in power supplies and welding controls[1].

Resistors with wire leads for through-hole mounting
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Carbon film

A carbon film is deposited on an insulating substrate, and a helix cut in it to create a long, narrow

resistive path. Varying shapes, coupled with theresistivityof carbon, (ranging from 90 to

400 nm) can provide a variety of resistances.

[2]

Carbon film resistors feature a power ratingrange of 0.125 W to 5 W at 70 C. Resistances available range from 1 ohm to 10 megohm. The

carbon film resistor has anoperating temperaturerange of -55 C to 155 C. It has 200 to 600

volts maximum working voltage range. Special carbon film resistors are used in applications

requiring high pulse stability.

Thick and thin film

Thick film resistors became popular during the 1970s, and mostSMD(surface mount device)

resistors today are of this type. The principal difference between thin film and thick film resistors

is not the actual thickness of the film, but rather how the film is applied to the cylinder (axial

resistors) or the surface (SMD resistors).

Thin film resistors are made bysputtering(a method ofvacuum deposition) the resistive material

onto an insulating substrate. The film is then etched in a similar manner to the old (subtractive)

process for making printed circuit boards; that is, the surface is coated with aphoto-sensitive

material, then covered by a pattern film, irradiated withultravioletlight, and then the exposed

photo-sensitive coating is developed, and underlying thin film is etched away.

Thick film resistors are manufactured using screen and stencil printing processes[1].

Because the time during which the sputtering is performed can be controlled, the thickness of the

thin film can be accurately controlled. The type of material is also usually different consisting of

one or more ceramic (cermet) conductors such astantalum nitride(TaN),ruthenium

dioxide(RuO2),lead oxide(PbO),bismuth ruthenate(Bi2Ru2O7),nickel chromium(NiCr),

and/orbismuth iridate(Bi2Ir2O7).

The resistance of both thin and thick film resistors after manufacture is not highly accurate; they

are usually trimmed to an accurate value by abrasive orlaser trimming. Thin film resistors are

usually specified with tolerances of 0.1, 0.2, 0.5, or 1%, and with temperature coefficients of 5 to

25 ppm/K.
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Thick film resistors may use the same conductive ceramics, but they are mixed

withsintered(powdered) glass and some kind of liquid so that the composite can bescreen-

printed. This composite of glass and conductive ceramic (cermet) material is then fused (baked)

in an oven at about 850 C.

Thick film resistors, when first manufactured, had tolerances of 5%, but standard tolerances have

improved to 2% or 1% in the last few decades. Temperature coefficients of thick film resistors

are high, typically 200 or 250 ppm/K; a 40kelvin(70 F) temperature change can change the

resistance by 1%.

Thin film resistors are usually far more expensive than thick film resistors. For example, SMD

thin film resistors, with 0.5% tolerances, and with 25 ppm/K temperature coefficients, when

bought in full size reel quantities, are about twice the cost of 1%, 250 ppm/K thick film resistors.

Metal film

A common type of axial resistor today is referred to as a metal-film resistor. Metal electrode

leadless face (MELF) resistors often use the same technology, but are a cylindrically shaped

resistor designed for surface mounting. Note that other types of resistors (e.g., carbon

composition) are also available in MELF packages.

Metal film resistors are usually coated with nickel chromium (NiCr), but might be coated with

any of the cermet materials listed above for thin film resistors. Unlike thin film resistors, the

material may be applied using different techniques than sputtering (though that is one such

technique). Also, unlike thin-film resistors, the resistance value is determined by cutting a helix

through the coating rather than by etching. (This is similar to the way carbon resistors are made.)

The result is a reasonable tolerance (0.5, 1, or 2%) and a temperature coefficient that is generally

between 50 and 100 ppm/K.[4]

. Metal film resistors possess good noise characteristics and low

non-linearity due to a low voltage coefficient. Also beneficial are the components efficient

tolerance, temperature coefficient and stability.

Metal Oxide film

Metal-Oxide film resistors resemble Metal film types, but are made of metal oxides such as tin

oxide. This results in a higher operating temperature and greater stability/reliability than Metal

film. They are used in applications with high endurance demands.
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Wire wound

Wire wound resistors are commonly made by winding a metal wire, usuallynichrome, around a

ceramic, plastic, or fiberglass core. The ends of the wire are soldered or welded to two caps orrings, attached to the ends of the core. The assembly is protected with a layer of paint, molded

plastic, or anenamelcoating baked at high temperature. Because of the very high surface

temperature these resistors can withstand temperatures of up to +450 C[1]. Wire leads in low

power wirewound resistors are usually between 0.6 and 0.8 mm in diameter and tinned for ease

of soldering. For higher power wirewound resistors, either a ceramic outer case or an aluminum

outer case on top of an insulating layer is used. The aluminum-cased types are designed to be

attached to a heat sink to dissipate the heat; the rated power is dependent on being used with a

suitable heat sink, e.g., a 50 W power rated resistor will overheat at a fraction of the power

dissipation if not used with a heat sink. Large wirewound resistors may be rated for 1,000 watts

or more.

Types of windings in wire resistors:

1 - Common

2 -Bifilar

3 - Common on a thin former

4 -Ayrton-Perry
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Because wirewound resistors arecoilsthey have more undesirableinductancethan other types of

resistor, although winding the wire in sections with alternately reversed direction can minimize

inductance. Other techniques employbifilar winding, or a flat thin former (to reduce cross-

section area of the coil). For most demanding circuits resistors withAyrton-Perry windingare

used.

Applications of wirewound resistors are similar to those of composition resistors with the

exception of the high frequency. The high frequency of wirewound resistors is substantially

worse than that of a composition resistor.

Foil resistor

The primary resistance element of a foil resistor is a special alloy foil several micrometresthick.

Since their introduction in the 1960s, foil resistors have had the best precision and stability of

any resistor available. One of the important parameters influencing stability is the temperature

coefficient of resistance (TCR). The TCR of foil resistors is extremely low, and has been further

improved over the years. One range of ultra-precision foil resistors offers a TCR of 0.14 ppm/C,

tolerance 0.005%, long-term stability (1 year) 25 ppm, (3 year) 50 ppm (further improved 5-

fold by hermetic sealing), stability under load (2000 hours) 0.03%, thermal EMF 0.1 V/C,

noise -42 dB, voltage coefficient 0.1 ppm/V, inductance 0.08 H, capacitance 0.5 pF.[5]

Ammeter shunts

Anammeter shuntis a special type of current-sensing resistor, having four terminals and a value

in milliohms or even micro-ohms. Current-measuring instruments, by themselves, can usually

accept only limited currents. To measure high currents, the current passes through the shunt,

where the voltage drop is measured and interpreted as current. A typical shunt consists of two

solid metal blocks, sometimes brass, mounted on to an insulating base. Between the blocks, and

soldered or brazed to them, are one or more strips of lowtemperature coefficient of

resistance(TCR)manganinalloy. Large bolts threaded into the blocks make the currentconnections, while much-smaller screws provide voltage connections. Shunts are rated by full-

scale current, and often have a voltage drop of 50 mV at rated current. Such meters are adapted

to the shunt full current rating by using an appropriately marked dial face; no change need be

made to the other parts of the meter.
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Grid resistor

In heavy-duty industrial high-current applications, a grid resistor is a large convection-cooled

lattice of stamped metal alloy strips connected in rows between two electrodes. Such industrial

grade resistors can be as large as a refrigerator; some designs can handle over 500 amperes ofcurrent, with a range of resistances extending lower than 0.04 ohms. They are used in

applications such asdynamic brakingandload bankingforlocomotivesand trams, neutral

grounding for industrial AC distribution, control loads for cranes and heavy equipment, load

testing of generators and harmonic filtering for electric substations.

The term grid resistoris sometimes used to describe a resistor of any type connected to

thecontrol gridof avacuum tube. This is not a resistor technology; it is an electronic circuit

topology.

Measurement

The value of a resistor can be measured with anohmmeter, which may be one function of

amultimeter. Usually, probes on the ends of test leads connect to the resistor. A

simpleohmmetermay apply a voltage from a battery across the unknown resistor (with an

internal resistor of a known value in series) producing a current which drives a meter movement.

The current flow, in accordance withOhm's Law, is inversely proportional to the sum of the

internal resistance and the resistor being tested, resulting in an analog meter scale which is very

non-linear, calibrated from infinity to 0 ohms. A digital multimeter, using active electronics, may

instead pass a specified current through the test resistance. The voltage generated across the test

resistance in that case is linearly proportional to its resistance, which is measured and displayed.

In either case the low-resistance ranges of the meter pass much more current through the test

leads than do high-resistance ranges, in order for the voltages present to be at reasonable levels

(generally below 10 volts) but still measurable.

Measuring low-value resistors, such as fractional-ohm resistors, with acceptable accuracy

requiresfour-terminal connections. One pair of terminals applies a known, calibrated current to

the resistor, while the other pair senses the voltage drop across the resistor. Some laboratory

quality ohmmeters, especially milliohmmeters, and even some of the better digital multimeters

sense using four input terminals for this purpose, which may be used with special test leads. Each
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of the two so-calledKelvin clipshas a pair of jaws insulated from each other. One side of each

clip applies the measuring current, while the other connections are only to sense the voltage

drop. The resistance is again calculated usingOhm's Lawas the measured voltage divided by the

applied current.

Resistor making

Most axial resistors use a pattern of colored stripes to indicate resistance.Surface-mountresistors

are marked numerically, if they are big enough to permit marking; more-recent small sizes are

impractical to mark. Cases are usually tan, brown, blue, or green, though other colors are

occasionally found such as dark red or dark gray.

Early 20th century resistors, essentially uninsulated, were dipped in paint to cover their entire

body for color coding. A second color of paint was applied to one end of the element, and a color

dot (or band) in the middle provided the third digit. The rule was "body, tip, dot", providing two

significant digits for value and the decimal multiplier, in that sequence. Default tolerance was

20%. Closer-tolerance resistors had silver (10%) or gold-colored (5%) paint on the other end.

Four-band resistors

Four-band identification is the most commonly used color-coding scheme on resistors. It consists

of four colored bands that are painted around the body of the resistor. The first two bands encode

the first two significant digits of the resistance value, the third is a power-of-ten multiplier or

number-of-zeroes, and the fourth is thetolerance accuracy, or acceptable error, of the value. The

first three bands are equally spaced along the resistor; the spacing to the fourth band is wider.

Sometimes a fifth band identifies the thermal coefficient, but this must be distinguished from the

true 5-color system, with 3 significant digits.

For example, green-blue-yellow-red is 56104 = 560k 2%. An easier description can be as

followed: the first band, green, has a value of 5 and the second band, blue, has a value of 6, and

is counted as 56. The third band, yellow, has a value of 104, which adds four 0's to the end,

creating 560,000 at 2% tolerance accuracy. 560,000 changes to 560k 2% (as a kilo- is

103).
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Each color corresponds to a certain digit, progressing from darker to lighter colors, as shown in

the chart below.

TRANSISTOR

A transistor is asemiconductordeviceused toamplifyand switchelectronicsignals. It is made

of a solid piece ofsemiconductormaterial, with at least three terminals for connection to an

external circuit. A voltage or current applied to one pair of the transistor's terminals changes the

current flowing through another pair of terminals. Because the controlled (output)powercan be

much more than the controlling (input) power, the transistor providesamplificationof a signal.

Today, some transistors are packaged individually, but many more are found embedded

inintegrated circuits.
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Assorted discrete transistors. Packages in order from top to bottom: TO-3, TO-126, TO-92, SOT-23

The transistor is the fundamental building block of modernelectronic devices, and is ubiquitous

in modern electronic systems. Following its release in the early 1950s the transistor

revolutionised the field of electronics, and paved the way for smaller and

cheaperradios,calculators, andcomputers, amongst other things.

History

PhysicistJulius Edgar Lilienfeldfiled the first patent for a transistor inCanadain 1925,describing a device similar to aField Effect Transistoror "FET".

[1]However, Lilienfeld did not

publish any research articles about his devices, nor did his patent cite any examples of devices

actually constructed. In 1934, German inventorOskar Heilpatented a similar device.

From 1942Herbert Matarexperimented with so-calledDuodiodes while working on a detector

for a DopplerRADARsystem. The duodiodes built by him had two separate but very close

metal contacts on the semiconductor substrate. He discovered effects that could not be explained

by two independently operating diodes and thus formed the basic idea for the later point contacttransistor.

In 1947,John BardeenandWalter BrattainatAT&T'sBell Labsin theUnited Statesobserved

that when electrical contacts were applied to a crystal ofgermanium, the output power was larger

than the input. Solid State Physics Group leaderWilliam Shockleysaw the potential in this, and
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over the next few months worked to greatly expand the knowledge of semiconductors. The

term transistorwas coined byJohn R. Pierce.[3]

According to physicist/historianRobert Arns,

legal papers from the Bell Labs patent show that William Shockley and Gerald Pearson had built

operational versions from Lilienfeld's patents, yet they never referenced this work in any of their

later research papers or historical articles.

A replica of the first working transistor.

The name transistoris aportmanteauof the term "transfer resistor".

The first silicon transistor was produced byTexas Instrumentsin 1954.[6]

This was the work

ofGordon Teal, an expert in growing crystals of high purity, who had previously worked at Bell

Labs.[7]The firstMOStransistor actually built was by Kahng and Atalla at Bell Labs in 1960.

Importance

The transistor is the key active component in practically all modernelectronics, and is

considered by many to be one of the greatest inventions of the twentieth century.[9]

Its

importance in today's society rests on its ability to bemass producedusing a highly automated

process (semiconductor device fabrication) that achieves astonishingly low per-transistor costs.

Although several companies each produce over a billion individually packaged (known

asdiscrete) transistors every year,[10]the vast majority of transistors now produced are

inintegrated circuits(often shortened toIC, microchips or simply chips), along

withdiodes,resistors,capacitorsand otherelectronic components, to produce complete

electronic circuits. Alogic gateconsists of up to about twenty transistors whereas an advanced
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microprocessor, as of 2009, can use as many as 2.3 billion transistors (MOSFETs).[11]

"About 60

million transistors were built this year [2002] ... for [each] man, woman, and child on Earth."[12]

The transistor's low cost, flexibility, and reliability have made it a ubiquitous device.

Transistorizedmechatroniccircuits have replacedelectromechanical devicesin controllingappliances and machinery. It is often easier and cheaper to use a standardmicrocontrollerand

write acomputer programto carry out a control function than to design an equivalent mechanical

control function.

Uses

Thebipolar junction transistor, or BJT, was the most commonly used transistor in the 1960s and

70s. Even after MOSFETs became widely available, the BJT remained the transistor of choice

for many analog circuits such as simple amplifiers because of their greater linearity and ease of

manufacture. Desirable properties of MOSFETs, such as their utility in low-power devices,

usually in theCMOSconfiguration, allowed them to capture nearly all market share for digital

circuits; more recently MOSFETs have captured most analog and power applications as well,

including modern clocked analog circuits, voltage regulators, amplifiers, power transmitters,

motor drivers, etc.

BC 548 transistor

The BC548 is a general purpose silicon, NPN, bipolar junctiontransistorfound commonly in

European electronic equipment. It is electrically similar to the North American2N3904and

Japanese 2SC1815 but has different lead assignments.

If theTO-92package is held in front of one's face with the flat side facing toward you and the

leads downward, (see picture) the order of the leads, from left to right is collector, base, emitter.

Note that the pin assignment of the complementary PNP device BC558 is exactly the same.
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BC548 transistor

Historical roots

Before the silicon "BC" devices were developed, the oldergermaniumtechnology "OC" series

devices were used. These generally date between about 1958-1970. The most commonly

encountered are the glass encapsulated OC44, OC45,OC71and OC75. These were very low

power devices with a Vcbo generally in the 12-16 volt range and Ic values of less than 50mA.

The OC44 and OC45 were the first common British/Europeanradio frequencydevices with an ft

around 1 MHz. The OC71 and OC75 were audio devices with an ft of about 150 kHz. A "power"

version, the OC25, with an Ic of 3 Amperes was sometimes seen in aTO-3package. All of these

earlier germanium devices were generally PNP, although NPN versions were made. These older

germanium devices containedindium, a metal with a very low melting point which limited the

power dissipation of the devices to a very low level and rendered them unreliable in harshenvironments, such as use in aircraft where wide temperature variations are encountered. The

silicon technology based "BC" devices appeared and superseded the older germanium based

devices. The doped silicon from which the newer devices were fabricated could withstand much

greater temperature variations and allowed much greater power dissipations. The main limiting

factor of the newer siliconBJTswasthermal runaway, a condition where the current gain

("beta") of a BJT increases as it gets hotter. This increases the collector current (Ic) despite the

base current being constant. An increase in Ic makes the chip "die" hotter, increasing the "beta"

and thus Ic, and so on until the transistor is cooled externally or it burns out. Thisthermal

runawaycan be overcome by using an emitter resistor in combination with a voltage divider

providing the base bias current, or by using a resistor between the collector and the base (sliding

bias), we call all these measures againstthermal runawayBipolar transistor biasing.

Specification
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The exact specs of a given device depend on the manufacturer. It is important to check the

datasheet for the exact device and brand you are dealing with. Philips and Telefunken are two

manufacturers of the BC548.

Vcbo = 30 Volts, Ic = 100mA, Ptotal = 50 mW and ft = 300 MHz

Relationship to the family of BC devices

The BC548 is a member of a larger group of similarly numbered transistors. Its complement is

the BC558, which is similar to the North American2N3906and the Japanese 2SA1015. The

BC548 is flanked by two similar transistors, the BC547 and the BC549. These are similar to the

BC548 but the 547 has a greater Vcbo of 50 volts and the 549 has the same Vcbo of 30 volts but

a lower noise figure. The 547 and 549 have complementary PNP versions numbered 557 and

559. A 556 device also exists with a Vcbo of 80 volts, which device finds extensive use in

thecurrent mirrorinput stages of medium quality audio amplifiers with relatively high rail

voltages. A family of older "BC" transistors predates the TO-92 BC54x series, the BC107, 108

and 109, (with complements BC177, 178 and 179). These are generally housed in the TO-18

metal package, the same as what the North American2N2222uses. These older transistors have

similar characteristics as the TO-92 BC5xx devices and are generally interchangeable. For

example, a damaged BC178 could be replaced with a BC558, taking the usual precautions to

ensure that the three leads are correctly oriented.

The BC337, 338 and 339 are a range of higher current, slower devices with complementary PNP

versions BC327, 328 and 329. These are similar to the North American 2N2222 and 2N2907 in

Ic and ft values and have the same Vcbo ratings as the BC547, 548 and 549. The BC635, 637

and 639 possess an Ic value of 1A, a Vcbo of between 45 and 80 volts and an ft of 50 to

130 MHz. These devices have a different lead configuration, with the collector lead in the

middle. The complementary PNP versions are BC636, BC638 and BC640. There are many other

devices based on the BC54x family, such as the surface-mount versions of the BC547, 548 and

549, the BC847, 848 and 849.

Advantages

The key advantages that have allowed transistors to replace their vacuum tube predecessors in

most applications are
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Small size and minimal weight, allowing the development of miniaturized electronic devices. Highly automated manufacturing processes, resulting in low per-unit cost. Lower possible operating voltages, making transistors suitable for small, battery-powered

applications.

No warm-up period for cathode heaters required after power application. Lower power dissipation and generally greater energy efficiency. Higher reliability and greater physical ruggedness. Extremely long life. Some transistorized devices have been in service for more than 50 years. Complementary devices available, facilitating the design ofcomplementary-symmetrycircuits,

something not possible with vacuum tubes.

Insensitivity to mechanical shock and vibration, thus avoiding the problem ofmicrophonicsin audioapplications.

Limitations

Silicon transistors do not operate at voltages higher than about 1,000volts(SiCdevices can beoperated as high as 3,000 volts). In contrast, electron tubes have been developed that can be

operated at tens of thousands of volts.

High power, high frequency operation, such as that used in over-the-airtelevision broadcasting, isbetter achieved in electron tubes due to improvedelectron mobilityin a vacuum.

Silicon transistors are much more vulnerable than electron tubes to anelectromagneticpulsegenerated by a high-altitudenuclear explosion.

LED

A light-emitting diode (LED) (pronounced/l i di/,L-E-D[1]

) is asemiconductorlight source.

LEDs are used as indicator lamps in many devices, and are increasingly used forlighting.

Introduced as a practical electronic component in 1962,[2]early LEDs emitted low-intensity red

light, but modern versions are available across thevisible,ultravioletandinfraredwavelengths,

with very high brightness. When a light-emittingdiodeis forward biased (switched

on),electronsare able torecombinewithelectron holeswithin the device, releasing energy in the

form ofphotons. This effect is calledelectroluminescenceand thecolorof the light
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(corresponding to the energy of the photon) is determined by theenergy gapof the

semiconductor. An LED is often small in area (less than 1 mm2), and integrated optical

components may be used to shape its radiation pattern.[3]

LEDs present manyadvantagesover

incandescent light sources includinglower energy consumption, longerlifetime, improved

robustness, smaller size, faster switching, and greater durability and reliability. LEDs powerful

enough for room lighting are relatively expensive and require more precise current and heat

managementthan compactfluorescent lampsources of comparable output.

Light-emitting diodes are used in applications as diverse as replacements foraviation

lighting,automotive lighting(particularly brake lamps, turn signals andindicators) as well as

intraffic signals. The compact size, the possibility of narrow bandwidth, switching speed, and

extreme reliability of LEDs has allowed new text and video displays and sensors to be

developed, while their high switching rates are also useful in advanced communications

technology.InfraredLEDs are also used in theremote controlunits of many commercial

products including televisions, DVD players, and other domestic appliances.

SWITCH
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Inelectronics, a switch is anelectrical componentthat can break anelectrical circuit,

interrupting thecurrentor diverting it from one conductor to another.[1][2]

The most familiar form

of switch is a manually operatedelectromechanicaldevice with one or more sets ofelectrical

contacts. Each set of contacts can be in one of two states: either 'closed' meaning the contacts are

touching and electricity can flow between them, or 'open', meaning the contacts are separated

and nonconducting.

A switch may be directly manipulated by a human as a control signal to a system, such as a

computer keyboard button, or to control power flow in a circuit, such as alight switch.

Automatically-operated switches can be used to control the motions of machines, for example, to

indicate that a garage door has reached its full open position or that a machine tool is in a

position to accept another workpiece. Switches may be operated by process variables such as

pressure, temperature, flow, current, voltage, and force, acting assensorsin a process and used to

automatically control a system. For example, athermostatis a temperature-operated switch used

to control a heating process. A switch that is operated by another electrical circuit is called

arelay. Large switches may be remotely operated by a motor drive mechanism. Some switches

are used to isolate electric power from a system, providing a visible point of isolation that can be

pad-locked if necessary to prevent accidental operation of a machine during maintenance, or to

prevent electric shock.

In Circuit Theory

In electronics engineering, an ideal switch describes a switch that:

has no current limit during its ON state has infinite resistance during its OFF state has no voltage drop across the switch during its ON state has no voltage limit during its OFF state has zerorise timeandfall timeduring state changes switches only once without "bouncing" between on and off positionsPractical switches have loss and limitations. The ideal switch is often used in circuit analysis as it

greatly simplifies the system of equations to be solved, however this can lead to a less accurate

solution.
http://en.wikipedia.org/wiki/Electronicshttp://en.wikipedia.org/wiki/Electronicshttp://en.wikipedia.org/wiki/Electronicshttp://en.wikipedia.org/wiki/Electrical_componenthttp://en.wikipedia.org/wiki/Electrical_componenthttp://en.wikipedia.org/wiki/Electrical_componenthttp://en.wikipedia.org/wiki/Electrical_circuithttp://en.wikipedia.org/wiki/Electrical_circuithttp://en.wikipedia.org/wiki/Electrical_circuithttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Switch#cite_note-0http://en.wikipedia.org/wiki/Switch#cite_note-0http://en.wikipedia.org/wiki/Switch#cite_note-0http://en.wikipedia.org/wiki/Electromechanicalhttp://en.wikipedia.org/wiki/Electromechanicalhttp://en.wikipedia.org/wiki/Electromechanicalhttp://en.wikipedia.org/wiki/Electrical_contacthttp://en.wikipedia.org/wiki/Electrical_contacthttp://en.wikipedia.org/wiki/Electrical_contacthttp://en.wikipedia.org/wiki/Electrical_contacthttp://en.wikipedia.org/wiki/Light_switchhttp://en.wikipedia.org/wiki/Light_switchhttp://en.wikipedia.org/wiki/Light_switchhttp://en.wikipedia.org/wiki/Sensorhttp://en.wikipedia.org/wiki/Sensorhttp://en.wikipedia.org/wiki/Sensorhttp://en.wikipedia.org/wiki/Thermostathttp://en.wikipedia.org/wiki/Thermostathttp://en.wikipedia.org/wiki/Thermostathttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Rise_timehttp://en.wikipedia.org/wiki/Rise_timehttp://en.wikipedia.org/wiki/Rise_timehttp://en.wikipedia.org/wiki/Fall_timehttp://en.wikipedia.org/wiki/Fall_timehttp://en.wikipedia.org/wiki/Fall_timehttp://en.wikipedia.org/wiki/Fall_timehttp://en.wikipedia.org/wiki/Rise_timehttp://en.wikipedia

Date post:	06-Apr-2018
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Variable Resistor

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