Direct Current Theory

8/7/2019 Direct Current Theory

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Direct Current Theory

If we take a light bulb and connect it to abattery, the bulb will light up. The lamp lights

up because current flows through it.

The current leaves the battery at the negative

terminal, flows through the bulb, and returns to

the positive terminal of the battery. The

electrons flow in one direction. This is known in

electronics as DIRECT CURRENT flow because

the electrons flow only in one direction.

The arrows in the figure show the direction that

the current would flow in this circuit. As long as

we can follow the current from the negative terminal of the battery throughout

the entire circuit, and back to the positive terminal, we have a COMPLETE

CIRCUIT PATH . It is very important to remember that current will ONLY

flow if the circuit path is complete. If we were to remove the light bulb from the

circuit, the circuit path would not be complete, and while voltage would still exist

on the battery, no current would flow through the circuit.

In order to have any complete circuit, you are required to have at least 3 parts:

(1) The SOURCE or SUPPLY of Voltage.

(2) The LOAD which uses the source Voltage.

(3) A complete path of connecting wires.

Schematic Symbols

Sometime over the years, some bright soul determined that it would be difficult

to draw a picture of every component that you decided to put into a circuit.

However, they needed a way to tell their colleagues about discoveries and

accomplishments. So a system was developed that was a sort of "electrical

shorthand". They call it a SCHEMATIC DIAGRAM and the individual


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component representations are called SCHEMATIC SYMBOLS .

Throughout the course, I will be introducing you to the various SCHEMATIC

SYMBOLS one by one. This lesson will take you through the first two symbols,

and describe how they are used in a circuit.

The first three SCHEMATIC SYMBOLS you will be introduced to are the lamp,

battery and resistor.

Remember that a

resistor is any device

which causes

electrical friction. In

electronics, the

resistor can be

substituted for any

current load. The

schematic symbol

for a battery can likewise be substituted for any direct current supply voltage.

So, in essence, we could theoretically use our battery and resistor to represent

our light bulb circuit.


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You will notice that the picture on the left is the same one we just looked at. The

one on the right actually has two schematic diagrams. The schematic on the left

is an exact representation of the picture on the left. The schematic on the right

we say is an ELECTRICAL EQUIVILANT circuit for the one on the left. Any

circuit, no matter how complex, can be broken down to being a source and a

load. The resistor represents the light bulb, which is the load of the circuit.

Anytime you are having a problem figuring out how a circuit works, it can be

helpful to break it down to an ELECTRICAL EQUIVILANT circuit.

The SCHEMATIC SYMBOL for the light bulb is pretty self explanatory. The

Schematic for the resistor looks like a series of sharp turns. Just remember that

on a road, you have to slow down at sharp turns, and electrical flow (current) has

to slow down at a resistor. The battery needs a little explanation. The lines

represent the electrodes of a battery. Note that the SHORT line is always the

NEGATIVE terminal, and the longer line is always the POSITIVE terminal.

Also along the way, I will try to give you an idea

of what certain types of electronic components

look like, although there are so many shapes

out there, I can not possibly cover them all.

I am fairly certain you already know what a

battery and a light bulb look like, but you may

never have seen a resistor. There are manytypes of resistors, but some of the most common

types are shown in the picture to the left.

The top one is a ceramic coated " wirewound ", which, as its name implies,

consists of a winding of wire, cut to a certain length to create a certain amount of

resistance. The second is a carbon composite, and the third is a metal film or

metal oxide, which has very tight resistance tolerances.

Note that on wirewound resistors, the values are printed on the side, whereas the

carbon and metal types have their values painted on as color coded bands

around the resistor.

EXTRA CREDIT

Not a required part of the course, but if you wish to pursue electronics, you

should probably memorize the resistor color code. It will be used throughout

your career.


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THE RESISTOR COLOR CODE

Many resistors that are produced are very small. In addition, resistors can get

extremely hot with use. So hot, in fact, that they will often burn off any small

lettering that may be printed on them. For this reason, resistors have been made

with colored bands painted onto them. These bands conform to a universal colorcode, which identifies the value and tolerance of the resistor. Each of the colors

below, correspond to a particular number.

For the purpose of memorization, I was taught a MNEMONIC to remember the

colors and their related numbers. However, for reasons of political correctness, I

can not teach you the same mnemonic. The mnemonic procedure, though, is still

valid, so I will present you with a new - more politically correct one. If you

memorize this phrase, you will never forget the resistor color code:

Black Bunnies Run Over Your Greens But People Get Wise - Ripe GoldenSquash Now

If you remember this mnemonic, you will not only know the values of resistors on

sight, but also their tolerances. Here's how it works:

Using the above phrase, it will indicate the following numbers:

BLAC

K

BROW

N REDORANG

E

YELLO

W

GREE

N

BLU

E

PURPL

E

GRE

Y WHITE

0 1 2 3 4 5 6 7 8 9

Resistors may have anywhere from 3 to 6 colored bands on them. As a rule, the

first two bands are the "value bands", so the color directly corresponds to the

value. In the example, we are using a 27,000 Ohm ( or 27K Ohm ) resistor. The

first two colors are RED and PURPLE, indicating the numbers 2 and 7. This is

where things get tricky. On the 3 an 4 band resistors, the third band, called the

"MULTIPLIER" - in this case being Orange, indicates that the 27 is followed by

THREE zeros ( 000 ). So in this case, we have 27 followed by 000 or a 27,000

Ohm resistor.

If there are only 3 bands, then we are done. 2 Questions arise though:


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1. Why did we memorize the "Ripe Golden Squash Now" portion of the

mnemonic? We already have all 10 numbers!

2. What about the 4th band?

Very good questions. They are both answered at the same time. The "Ripe

Golden Squash Now" portion of the mnemonic refers to the 4th band,

which is known as the "TOLERANCE" band. It has a very important job.

(Note that the LAST band is ALWAYS the Tolerance Band. It usually has

a wider separation than the other bands have from each other ( it is

farther away ).

We as people, are not perfect. Because of this, we make imperfect

products. No resister is perfect. They are, however, all close to the value

listed on them, plus or minus a certain amount. The amount of difference


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between their actual value, and the value listed on them should always fall

within a certain tolerance. That tolerance is listed on the resistor, and is

also designated by a colored band. Ripe Golden Squash Now (RGSN)

corresponds with Red, Gold, Silver, None - the order of resistor tolerances

in ascending order. Red = 2%, Gold=5%, Silver=10%, and No Band

(None) = 20%.

Let's assume that you have a 1000 Ohm resistor. If it has a 10% tolerance,

it can be off by 100 Ohms, and still be good (1000 +/- 100). So it will be

allowed to be anywhere from 1100 to 900 Ohms, and still be considered

good. If a 1000 Ohm resistor has a SILVER tolerance band, and is only

920 Ohms, it is considered to be within tolerance, and is a good resistor.

However, if a 1000 Ohm resistor has a GOLD or RED tolerance band, and

is only 920 Ohms, it is OUT of acceptable tolerance, and is considered to be

a bad resistor.

Now in the second example, we also have a 27,000 Ohm resistor, but the

color code scheme is a little different. We still have a RED and a PURPLE

as our first two colors, indicating the number 27, but the third band,

instead of being orange, is BLACK, indicating Zero. The FOURTH band

is the multiplier, and being RED indicates 2 zeros. Here is how this resistor

is read: 27 0 00, or 27,000.

The Relay Races

Knowing that magnetism and electronics are related is a very important lesson.

Just how important will become evident in the next few lessons, as we will be

discussing the interaction of electricity and magnetism in greater detail. Let's

review some of the things we have learned:

We know that when two magnets

are brought close enough to eachother, they will have one of two

reactions. If their poles are the

same polarity, they repel, or push

away from each other. If, on the

other hand, their poles are

opposite, they attract, or pull


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toward each other. This is called the LAW OF POLES and it applies (to an

extent) to electronics as well as with magnetics.

Note that the electrons from the negative side of a battery will attract

toward the positive side, if the two are brought electrically close enough to

be allowed to do so. This typically happens by connecting a wire, lamp, or

some other electrical device between the two electrical poles.

If we think of electronics from this standpoint, the questions soon arises:

Does electricity move from positive to negative, or from negative to positive?

This is a good time to discuss the fact that because we can not truly see the

electrons in motion, but can only study their effects, there are 3 differing schools

of thought on this subject, all of which have some merit.

1).According to the CONVENTIONAL THEORY of electron flow, also

known as the FRANKLIN THEORY, or the POSITIVE CURRENT

FLOW theory, electricity flows FROM POSITIVE TO NEGATIVE.

2).According to the EDISON THEORY , or the NEGATIVE CURRENT

FLOW theory, electricity flows FROM NEGATIVE TO POSITIVE

3). According to the ELECTROMAGNETIC CURRENT FLOW theory,

electricity, like magnetic lines of force, are free floating in space, and

PUSH OR PULL WITH EQUAL FORCE IN BOTH DIRECTIONS . This

theory, depending on the amounts of negative and positive energy, and the

electrical proximity of the components between them, gives merit to either

of the two above theories.

Which of the 3 theories you choose to believe is totally up to you, but it wouldbehoove you to remember the fact that there are 3 differing theories. Some

writers write books based upon positive flow. Most modern authors choose to

assume negative. But there are times when it is convenient to switch sides of the

fence, in order to figure out exactly what is going on inside a circuit. The third

theory is rare to find in books, however it does have its merits as well. The

important point here is to make sure you know which theory your author is

using, and try not to get too utterly confused.

Another fact we know is that we can control the polarity of anelectromagnet, by controlling the polarity of the voltage being

fed into it. The North pole of the electromagnet is ALWAYS

on the positive side of the battery.


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With this thought in mind,

we can control the physical

movement of a permanent

magnet, by controlling the

voltage going through a

given electromagnet. If we

attach a battery to an

electromagnet in such a way

that it has the opposite

polarity of a nearby

permanent magnet, it will pull the permanent magnet closer to it. If we then

swap the wires going to the battery, the electromagnet will change its polarity,

and the permanent magnet will be pushed away from it.

If we physically attach the permanent magnet to a plunger, we can control the

movement of the plunger in and out using electrical current. In this way, we use

electric current to push a button, pull a lever, open

or close a valve, or any number of other tasks.

Because magnets attract ferrite based metals, we

can also use electricity to control the physical

movement of iron. In the examples given to the

right, we are using electric current to move a type

of reed switch. These are handy for allowing us to use a small amount of currentto, for example, turn on a motor which needs a very large amount of current.

In the case of the break contact relay, the reed switch inside the relay is

constantly CLOSED (meaning connected), allowing current to flow

through it. The motor is on all the time. When we connect the battery to

our circuit via the switch, it will cause the magnet to pull at the iron reed,

opening the switch, and turning the motor off.

In the case of the make contact relay, the

reed switch inside the relay is constantly

OPEN (meaning disconnected), so no

current is allowed to flow through it. Themotor is normally turned off. When we

connect the battery to our circuit via the

switch, it will cause the magnet to push at

the iron reed, closing the switch, and turning the motor on.

Now would be a good time to show a


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schematic diagram and picture of a relay. The diagram to the left is an

exact duplicate of the make contact relay circuit represented by the above

picture. The break contact relay schematic symbol would be similar,

except the contacts would be connected. Keep in mind, that not all

schematic symbols are standard. You may see variations of schematic

symbols over the years, but they will all be understandable and descriptive

of the function of the component.

Below is a picture of a relay

A C Theory

Earlier we discussed that there are various ways to

produce electricity. We can produce electricity

chemically with a battery. We just learned that

electricity can be produced mechanically by a

generator. What we did not discuss in detail,

though, was the difference between electricity

produced by a battery, and electricity produced by

a generator.

In the case of a battery, electricity flows in one

direction, from positive to negative. Everything is

straightforward. In the case of a generator,

however, things get a bit more complicated. It is


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possible to generate electricity by spinning a coil within a magnetic field. The coil is

in constant motion within the magnetic field, and thus is transformed into electricity

via the magnets. The electricity exits by way of the brushes and slip rings, but it is not

exactly like the electricity which is produced by a battery.

If we look at the current leaving the battery, it is constantly moving in the same

direction. We call this DIRECT CURRENT . But if we attach a generator instead of a

battery in the same circuit, we notice a major change. The meter would swing back

and forth from negative to positive. This seems strange until we examine what is

going on inside the generator.

As the wire coil rotates, it first passes

the north pole of the magnet,

producing an electric current flowing

in a given direction. As the coil

continues in its circular path, it passes

the north pole, moving toward the

south. As it approaches the south

pole, the electric current begins to flow in the OPPOSITE direction from which

it was originally moving. It continues to move in this direction until, once

again, it approaches the north pole. We say, then that the electrical current is

ALTERNATING between positive and negative. We call this type of current

ALTERNATING CURRENT .

If we were to plot this swing from positive to negative on a graph, and compare it to

the time it takes the motor to turn, we would come up with something like the chart tothe left. Notice, that if we begin with the coil positioned directly in the center,

between the permanent magnets, the current output is 0.

However as the coil begins to turn, one side

of the coil moves toward the north pole. This

end of the wire would become positive. At

the same time, the other side of the coil

moves toward the south pole. This side of

the coil becomes negative. At this time,

current begins to flow from the positive tothe negative. Current continues to flow in

this direction and reaches a peak in its cycle. This Maximum amount of current flow

is reached when the coil is pointing exactly north and south. We call this the 90 o

point, and say that the signal has reached its positive peak. After it passes this point,

the voltage begins to drop, but doesn't reach 0 until once again the coil is positioned

directly between the permanent magnets. This is the 180 o point.


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Now comes the switch up. As the coil continues to turn, the end that was positive now

moves toward the south pole of the magnet. Because it is passing by the south pole,

this end of the coil swings negative. At the same time, the side of the coil that was

negative, is now swinging positive. Thus, the direction of current flow within the wire

is switched. The current flow continues in this direction until it again reaches a (this

time negative) peak at 270 o . Finally, as the coil approaches its original position, it

swings positive until current flow again reaches 0.

By graphing the current vs. time, we end up with a pattern known as a SINUSOIDAL

WAVE , orSINE WAVE for short. We say that the sine wave has positive and

negative peeks at 90 o and 270 o respectively.

Capacitor - a new component

So far, we have studied the effects of electricity flowing through wires, and have

discussed resistors, coils, and metering devices. Both resistors and coils, as we have

found, have a restricting effect on the flow of current. We also discussed how a coil

has more resistance to AC than it does to DC. You will learn later just how important

these effects are, but first we must discuss a few more electrical components. Another

component which has a restricting effect on current flow, but in a different way. This

component is called the CAPACITOR.

Once again, we will resort to

our water examples to describe

the function of a capacitor, as it

is easier to see fluid in motion,

when it is water, than when it is

electricity. Examine the

example on the left. Here we

have 2 tanks of water, equally

full. The two tanks are

connected in the middle by a pipe or piece of tubing. Let us say now that we

have, in the middle of the tubing, a thin rubber membrane. The membrane

would keep the liquid in the two tanks from ever coming into contact with each

other. We could further illustrate this by adding food coloring to one of the

tanks of water.


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If we now take a plunger, and

apply pressure to the tank on the

left, it will push the water

downward, and try to push it out

the tube and into the other tank.However, the membrane will

not allow the water to actually

exit the tank, and enter the

second tank. While the two

systems are sealed off from one another, the rubber membrane would flex, and

allow the EFFECT of movement, in that it would push the water level of the

second tank higher, in direct proportion to the movement in the first tank. For

instance, assuming both tanks are of equal diameter, if the first tank went down

2 inches, the second tank would rise 2 inches.

Now, if we should reverse the

action, and push the plunger

down in the second tank, it

would move the membrane in

the opposite direction, also

moving the water within the

tanks in the opposite direction,

but AT NO TIME would the

water flow from one tank intothe other tank. It would have the effect of movement from one tank to the other,

without actually having done so.

This is basically the same operating principle behind another of electronics most

important components - the capacitor. The capacitor appears to have the effect of

passing alternating current, while actually not passing anything. At the same time, it

blocks the flow of direct current. Just as in the water circuit, the water flow in either

given direction is blocked by the membrane, if we should push the water pressure, and

hence the membrane back and forth, it would appear as if the membrane weren't there

at all, except that the food coloring would not pass from one container to the other.

In its most basic form, a capacitor is

made up of 2 plates of conducting

material (for instance copper,

aluminium, iron), divided by a piece of


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of the battery. There is good reason for this. In a battery, we have 2 (or more)

conductive plates divided by some kind of dialectric material (usually an acid). In a

capacitor, we have 2 (or more) plates divided by some kind of dialectric material - an

insulator. A battery has the ability to generate electricity chemically, and can store

energy for long periods of time. While a capacitor does not "generate" electricity, it

does have some amazing "storage" capabilities, as we will discuss now.

Recall that when we applied power to the capacitor/lamp circuit, electric current

flowed for an instant from one side of the battery and lit the lamp for a moment, but

then the light went out? What took place, was while the electric current was flowing, a

potential was being built up on the surface of the plates of the capacitor. As long as

the potential kept building, current continued to flow, and the light remained lit. At

some point, however, the capacitor reaches its maximum CAPACITY to hold an

electric potential. In other words, it reaches its peak voltage limit, and we say the

capacitor is fully charged. If at this time, we were to remove the battery from the

circuit, the capacitor, in theory, would remain

at full charge indefinately.

If at this time, we shorted the wires

between the capacitor and lamp, such

that it formed a complete circuit, the

lamp would light for just a second.....

WITHOUT THE BATTERY. Where

does the energy come from to light the

lamp if the battery is not connected? The

answer lies in one of the magicalproperties of the capacitor....it can

STORE energy! When energy is stored in a capacitor, we say it is charged.

When a capacitor releases its energy, we say it is discharging.

Passive vs. Active Components

As you have figured out by now, there are many different types of electronic

components, and you must be familiar with all of these. They all act differently with

reference to voltage, current, temperature, pressure, and other outside influences. In

order to make learning electronic components easier, they have been divided into two

categories:

PASSIVE COMPONENTS

and

ACTIVE COMPONENTS.


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While possibly not the best definition, the key difference between active and passive

components, is that active components have the ability to produce gain, or amplify a

signal, and passive components do not. Some would argue that a component's ability

to switch a signal makes it an active component, but I don't see a toggle switch as

being active. I may modify this definition later, but for now, this one is enough for

you to grasp the concept.

So far, all the components we have discussed are resistors, capacitors, and coils.

These are passive components. Now we are going to begin learning about active

components.

Some examples of Active components include Vacuum Tubes, Transistors, Integrated

Circuits, etc. We will first study Vacuum tubes, as they are a fundamental building

block in the understanding of other active components.

Many "modern" schools today are skipping right over tubes. I plan onEMPHASIZING them, as I see them as still a very viable and cutting edge

technology. There are new tubes being developed and used every day, because up 'till

now, we simply haven't found a device which is more capable of linear amplification

at high power and high frequency levels. Some examples would be the klystron,

magnetron, Inductive Output Tube (IOT), Traveling Wave Tube (TWT) et al. There

have also been leaps and bounds in nanotube technology, and lasers still use tubes as

well.

I'll be willing to bet that you have at LEAST 1 vacuum tube devices that you use on a

regular basis in your home right now! You probably cook meals in a MicrowaveOven, which uses a magnetron. In some cases, your TV or possibly your computer

monitor may also have a Cathode Ray Tube (CRT). And should we, someday, find a

way to replicate food or transport people as in "Star Trek", I believe it will be first

developed using technology similar in nature to vacuum tubes.

Now some might say that these are the exception, not the rule - that the majority of


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electronics jobs will never require tube knowledge. I concur, but submit that if you

want to make the big bucks - you have to be a specialist, and specialists deal in cutting

edge technology - many of which require tube knowledge. Satellites going into orbit

still typically use Traveling Wave Tubes, not transistors for their main power

amplifier stages.

Lesson 44 - Circuits Circuits Everywhere!

n the last section, we saw how a very simple transmitter worked. It was made up of several different ty

f electronic components, including capacitors, transistors, resistors, etc. When we assemble several typ

f electronic components in a configuration that serves some purpose, we call it a CIRCUIT.

ome common electronic components are:

WireResistors

Capacitors

Coils

Transformers

Tubes

Transistors

Diodes

All circuits are combinations of individual electronic components assembled to perform a function. The

type" of circuit it is, depends on the function of the circuit. We have already discussed some simpleircuits, called "filters", and have also gone through "power supplies". Now we have introduced you to

amplifiers".

Here is a list of some of the major types of circuits we will discuss and explain how they operate:

ower Supplies

ilters

Amplifiers

Oscillators

Mixers

Logic Circuits

Almost any device or type of electrical equipment is made up of a COMBINATION of these circuits.


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Our transmitter, for instance, is an Audio Amplifier, which drives an RF Mixer, which has a second inprom the RF Oscillator (High Frequency Generator). The Mixer combines the inputs from the Audio Am

nd the Oscillator to create a radio signal that is modulated by the audio signal, which then goes out to t

ntenna.

Of course this is an over simplification of what really has to happen, but the point is, that it is ALL done

asic circuits, and that if you learn and understand the simple circuits, you can look at VERY complex

evices, and understand how they work!

We need to discuss each of the basic types of circuits in greater detail until you fully comprehend the

heory behind how they work. Then you will have a firm grasp on electronics, and can begin combininghem to create useful circuits.

irst, let's cover the two electronic components we haven't covered yet:

Transistors and Diodes.

Lesson 47 - Transistors


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Recall that in a semiconductor diode, we have 2 regions of

DOPED semiconductive material. One region is doped positiv

and the other region is doped negative. There is also a junction

where the two regions are joined.

When a diode is forward biased, it conducts electricity easily,

like a ball rolling down a hill. When it is reverse biased, it is

extremely resistive to current flow, as the ball is rolling uphill

and is much harder to get over the hump.

Remember also, that we had diode tubes, which operated in a

similar manner. They would conduct electricity in one directio

easily, but would not conduct in the opposite direction.

When we added another element to a tube, we created a triode, which would not only allow electricity

flow, but could also amplify the signal. Reason tells us that if we add another element to a semiconduc

diode, that a similar effect should take place. In

December of 1947, Scientists at Bell Laboratories

would prove this theory correct.

With the addition of a 3rd semiconductive layer,

joined at a second P-N junction, W. H. Brattain

made the world famous comment, " We've Got

Gain! " implying that this 3 layer device couldamplify!

With proper bias applied, there is a small hill to

overcome at the first P-N junction

(approximately 0.7 Volts for Silicon, 0.3 Volts Germanium), which is the normal characteristic for any

semiconductor diode.

But then the electrons reach the peak of the hill at the second P-N junction, and have a fast run

downward. There is an increase in flow downhill, and the electrons act like a waterfall, pouring into th

collector. It may seem at first, that a transistor is like 2 diodes placed back to back, and in resistance

checks will even resemble this. Actually though, 2 diodes back to back will not operate like a transisto

in circuit.

A diode only has 2 semiconductive regions, and therefore has 2 leads. A transistor, on the other hand, h

3 regions, and must have 3 leads. To the left is a photo of a small signal transistor. Just as you must kn

which end is which on a diode, a transistor has markings which identify which lead is which. The three

leads are called the Emitter, Base, and Collector. The Emitter is the lead that current enters into. It can


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compared to the Cathode of a tube. The Collector is the lead that current exits from. It can be compare

to the Plate of a tube. Finally, the Base is the controlling lead, and is comparable to the Control Grid o

tube. It might help to remember that electrons are emitted at the Emitter, collected at the Collector, and

controlled by the Base.

Transistors come in many different packages, and while they are NOT always

marked so that you will know which lead is which, they are by no means

standard either! One transistor may have the emitter on the left, another may

have it in the middle. Transistors often do, however have identifying marks, and

can be referenced to find out which lead is which. In any case - it is ALWAYS

best to check the specification sheet for any given transistor before using it.

Doing so will save you a load of heartache.

Of the many kinds of transistors there are, probably the most commonly used is the Small Signal, Bipo

transistor, as pictured above.

Bipolar transistors come in two flavors: PNP and NPN. This is because the semiconductive material ca

be laid out in ( basically ) two different ways.

If we look closely at how a bipolar

transistor is made, we can understand

more easily how this can be. The

illustration to the left is a cutaway of a

semiconductor transistor. Try tovisualize this as being circular ( button

shaped ) from the top view, with 3

layers, one upon another.

Transistors are built in layers by very

precise machines. Each layer is added

the layer below it. We begin with a

single layer ( or substrate ), and add layers on top of it. If we begin with a layer of N type semiconduct

( on the bottom ), the second layer would be P type, followed by another N type. We say that transistor

and other semiconductive devices, are "grown" in this manner. The second layer ( in this case a P typeis very thin, along the order of 800 micrometers ( M ) or less.

As shown by the blue line, electric current enters via the N type emitter substrate layer, passes through

the ( red ) P type base substrate layer, until it reaches the N type collector substrate layer. The gold

colored lines represent the leads that connect the transistor to outside circuitry.


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If we were to reverse the N and P layers, we would have a PNP transistor, with the base being N type,

and the emitter and collector being P type material.

The schematic diagram symbol for a bipolar

transistor is shown to the right. Notice that the

only difference between an NPN and PNP type

transistor, is the direction of the arrow. To

remember which is which, just keep in mind

that the NPN is Not Pointing to the base.

( NP = Not Pointing ) Otherwise, the two

symbols are identical. The EMITTER is

ALWAYS the ARROW, the base is always

the line ( think baseline ), and the collector is

the one left over.

Rules for Bias Connections


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So far so good! Now comes the tricky part. I

a NPN transistor, the Base is P type doping,

while the collector is N type. But we want it

be REVERSE biased ( Positive to N &

Negative to P ), so we want the N doped

collector to be MORE POSITIVE than the P

doped Base, which is at 3.7 Volts. So any

voltage above 3.7 would work. Let's say, 5

Volts. ( We plot that on the number line ).

So in order to turn on this NPN transistor, we would need the following voltages:

Emitter = 3.0 Volts

Base = 3.7 Volts

Collector = 5.0 Volts.

This is why, in the picture above, I have a minus sign ( - ) next to the base, a plus sign ( + ) next to the

base, and TWO plus signs ( ++ ) next to the collector. It demonstrates the relative polarity of each

terminal.

We have a pattern then, that while it is an NPN transistor, it is biased N-P-PP, with the COLLECTOR

being the MOST POSITIVE point.

If we go through the same logic for the PNP transistor, we would find that it needs to be biased P-N-N

with the COLLECTOR being the MOST NEGATIVE point.

Lesson 47 - Transistors

Recall that in a semiconductor diode, we have 2regions of DOPED semiconductive material. Oneregion is doped positive, and the other region isdoped negative. There is also a junction, where thtwo regions are joined.

When a diode is forward biased, it conductselectricity easily, like a ball rolling down a hill.When it is reverse biased, it is extremely resistiveto current flow, as the ball is rolling uphill, and ismuch harder to get over the hump.

Remember also, that we had diode tubes, which


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operated in a similar manner. They would conduct electricity in one direction easibut would not conduct in the opposite direction.

When we added another element to a tube, we created a triode, which would notonly allow electricity to flow, but could also amplify the signal. Reason tells us thaif we add another element to a semiconductor diode, that a similar effect shouldtake place. In December of 1947, Scientists at Bell Laboratories would prove thistheory correct.

With the addition of a 3rd semiconductivelayer, joined at a second P-N junction, W.H. Brattain made the world famouscomment, " We've Got Gain! " implyingthat this 3 layer device could amplify!

With proper bias applied, there is a smallhill to overcome at the first P-N junction(approximately 0.7 Volts for Silicon, 0.3Volts Germanium), which is the normalcharacteristic for any semiconductordiode.

But then the electrons reach the peak of the hill at the second P-N junction, andhave a fast run downward. There is an increase in flow downhill, and the electron

act like a waterfall, pouring into the collector. It may seem at first, that a transistois like 2 diodes placed back to back, and in resistance checks will even resemblethis. Actually though, 2 diodes back to back will not operate like a transistor incircuit.

A diode only has 2 semiconductive regions, and therefore has 2 leads. Atransistor, on the other hand, has 3 regions, and must have 3 leads. To the lef

is a photo of a small signal transistor. Just as you must know which end is which oa diode, a transistor has markings which identify which lead is which. The threeleads are called the Emitter, Base, and Collector. The Emitter is the lead that

current enters into. It can be compared to the Cathode of a tube. The Collector isthe lead that current exits from. It can be compared to the Plate of a tube. Finallythe Base is the controlling lead, and is comparable to the Control Grid of a tube. Imight help to remember that electrons are emitted at the Emitter, collected at thCollector, and controlled by the Base.


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Transistors come in many different packages, and while theyare NOT always marked so that you will know which lead iswhich, they are by no means standard either! One transistormay have the emitter on the left, another may have it in themiddle. Transistors often do, however have identifying marks,and can be referenced to find out which lead is which. In anycase - it is ALWAYS best to check the specification sheet for anygiven transistor before using it. Doing so will save you a load ofheartache.

Of the many kinds of transistors there are, probably the most commonly used is tSmall Signal, Bipolar transistor, as pictured above.

Bipolar transistors come in two flavors: PNP and NPN. This is because the

semiconductive material can be laid out in ( basically ) two different ways.

If we look closely at how abipolar transistor is made, wecan understand more easily hothis can be. The illustration tothe left is a cutaway of asemiconductor transistor. Try tvisualize this as being circular( button shaped ) from the top

view, with 3 layers, one uponanother.

Transistors are built in layers bvery precise machines. Each layer is added to the layer below it. We begin with asingle layer ( or substrate ), and add layers on top of it. If we begin with a layer oftype semiconductor ( on the bottom ), the second layer would be P type, followedby another N type. We say that transistors, and other semiconductive devices, ar"grown" in this manner. The second layer ( in this case a P type ), is very thin,along the order of 800 micrometers ( M ) or less.

As shown by the blue line, electric current enters via the N type emitter substratelayer, passes through the ( red ) P type base substrate layer, until it reaches the Ntype collector substrate layer. The gold colored lines represent the leads thatconnect the transistor to outside circuitry.

If we were to reverse the N and P layers, we would have a PNP transistor, with the


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base being N type, and the emitter and collector being P type material.

The schematic diagram symbol for abipolar transistor is shown to theright. Notice that the only differencebetween an NPN and PNP typetransistor, is the direction of thearrow. To remember which is which,just keep in mind that the NPN is NotPointing to the base.( NP = Not Pointing ) Otherwise, thetwo symbols are identical. The EMITTER is ALWAYS the ARROW, the base is alwaythe line ( think baseline ), and the collector is the one left over.

Rules for Bias Connections


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This is important! Pay Attention!!

The Emitter - Base connection is always FORWARD biased. This means morePositive voltage goes to P type & more Negative to N type. Also, for a Silicontransistor, there must be at least a 0.7 Volt DC bias across the emitter-basejunction in order for the transistor to be active. Many times, when without aschematic, I have been able to repair a circuit simply by looking for 0.7 VDC acrosthe E-B of every transistor in the circuit. If it doesn't have at least 0.7 V across it, isn't turned on! Of course, the bias is 0.3 Volts DC for Germanium Transistors, soyou must also know a little about the transistor itself. When in doubt - look up thenumber on the transistor, and read its specification sheet.

The Collector - Base connection is always REVERSE biased. This means more

Positive goes to N type & more Negative goes to P type. You must be wonderingnow, how the Collector-Base junction can be reverse biased while the Emitter -Base junction is forward biased?

The answer lies in the words "More Positive" and "More Negative". You see,electronics is more of a relative science than an exact science. Is 5 Volts D.C.positive or negative? Well, it's more positive than 2 Volts D.C., but less positivethan 9 Volts D.C. Did I lose you yet?

It's simple.

Let's try plotting it out on a number line:

Assuming a Silicon NPN transistor:We know that the the Emitter-Basejunction must be FORWARD biased(Positive to P type doping & Negative to Ntype doping). So the Emitter must bemore Negative and the Base must bemore Positive. We know then, that the

Base must be 0.7 Volts ( minimum )Higher than the emitter.

So if, say, the Emitter is at 3 Volts, ( I just picked that number at random). We plo3 Volts on the number line.

If the Base has to be 0.7 volts Higher than the Emitter, then the base has to be at


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least 3.7 Volts.( 3 + .7 = 3.7 ) So we plot that on the number line.

So far so good! Now comes the tricpart. In a NPN transistor, the Base iP type doping, while the collector isN type. But we want it to beREVERSE biased ( Positive to N &Negative to P ), so we want the Ndoped collector to be MORE POSITIVthan the P doped Base, which is at3.7 Volts. So any voltage above 3.7

would work. Let's say, 5 Volts. ( We plot that on the number line ).

So in order to turn on this NPN transistor, we would need the following voltages:Emitter = 3.0 VoltsBase = 3.7 VoltsCollector = 5.0 Volts.

This is why, in the picture above, I have a minus sign ( - ) next to the base, a plussign ( + ) next to the base, and TWO plus signs ( ++ ) next to the collector. Itdemonstrates the relative polarity of each terminal.

We have a pattern then, that while it is an NPN transistor, it is biased N-P-PP, with

the COLLECTOR being the MOST POSITIVE point.

If we go through the same logic for the PNP transistor, we would find that it needsto be biased P-N-NN, with the COLLECTOR being the MOST NEGATIVE point.

Direct Current (DC)

Direct Current" is produced when Electrons flow constantly in one direction. It is abbreviated as"DC." Since Direct Current flows in one direction only, its electrical pressure orvoltage is

always oriented in one direction (or "polarity").


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Static Electricity

"Static Electricity" is the electrical charge associated with Lightning; the "shock" you

experience when you touch the Doorknob; or when your clothes stick together when they have

just come out of the Dryer.

Static is caused by the buildup ofelectric charges when two objects trade someof their Electrons from one to the other. The object with the greatest number of

Electrons has a greater negative charge. Since this occurs without the flow ofcurrent, it is called "static."

When the negative charge becomes high enough, any contact with a less negatively-orpositively-charged body will cause an extremely rapid, high-currentelectrical "discharge." This

is what happens when you cross the room in your socks and touch the Doorknob. Your body is

negatively-charged, and the Doorknob is positively-charged. The negative charge dischargesrapidly to the positive charge, bringing the two items back in to electrical balance.

Because static discharge can be damaging to sensitive electronics and disastrous around volatile

substances (i.e. industrial solvents and fuels) preventive devices are commonly used.

"Grounding Straps" on vehicles, aircraft, and Computer Operators' wrists -plus anti-staticflooring and carpeting- are all means of providing a continual path forcurrent flow and

preventing the buildup ofStatic Electricity.

Direct Current Flow

Interestingly, the first commercial electrical systems set up by Thomas Edison and others weredirect current systems. But, for economic reasons, these were later changed to Alternating

Current or "AC" systems, and are described in the Alternating Current Section of this course.

Today, Batteries, Solar Panels, Fuel Cells and special "DC" Generators (i.e. Wind Turbines)

produce Direct Current.

Heat

A small amount ofelectricity can be generated from Heat by connecting two dissimilar metalsand heating the spot where they are joined. Metals such as Copper and Constantan (a

Copper/Nickel Alloy) or Iron and Nickel are typical pairs.

Static Shock


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Each metal reacts to the Heat differently, causing a different movement ofElectrons between

the two. This device is called a "Thermocouple" and the spot where the two metals are connected

is called the "Junction."

Iron-Nickel Thermocouple

In an Iron-nickelThermocouple, applying Heat to the Junction force the Electrons to move fromthe Iron to the Nickel, resulting in a small but measurable voltage. These voltages are typically

in the thousandths of a volt or "millivolts."

This thermoelectric process is frequently used in Furnaces to sense the presence ofHeat, hold

the Gas Valve open as long as the Heat is present, and to allow it to close if the "flame" goes out.It is a simple way to measure "temperature" but not a very efficient way to generate any

significant quantities ofelectricity.

Photovoltaic Cells

Photovoltaic or "solar cells" are made of Silicon and can turn sunlightdirectly intoDCelectricity. Each Cell produces a small amount ofcurrent. By connecting manyCells together and placing them on larger Panels, the electric currentproduced canbe significant. This can be used directly in a DC Appliance, stored in Batteries, orconverted to Alternating Current to operate AC Appliances using an "inverter."

While extremely simple, Photovoltaic Cells are expensive compared to othergenerating sources. While the Cells themselves are fairly reliable, the Sun's raysare not a very predictable resource in most areas. As a result, other equipment suchas battery storage systems and an "inverter" to convert the DCcurrentto AC areoften needed. "Solar" or "PV Power" has consequently been used primarily forspecialized situations (i.e. Satellites, Portable Electronic Equipment, and Power) inremote locations.

Piezoelectric Principle
http://www.study-center.com/femp/content/demo/graphics/bdheat.jpg


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Crystalline materials produce small amounts ofelectricity when a

force is applied that changes their shape in some way. These are

called "piezoelectric" materials. Quartz is an example of apiezoelectric substance. When small amounts of pressure are applied

to a Quartz Crystal, a small voltage is produced from the changing

charge created by the moving Electrons. Phonographs using a CrystalCartridge utilize the Piezoelectric Principle to convert the movement

of the Needle to an electrical signal which is later amplified and

played through Speakers. Microphones and Barbecue Lighters alsouse this Principle.

Primary Cell Batteries

Combinations of certain metals (i.e. Copper and Zinc) will produce electrical activity when

placed in special solutions called "electrolytes." The two metals form the "electrodes." Theelectrolyte creates a chemical action that causes the Zinc to form positive "ions" and the Copperto form negative "ions." These ions are freely flowing in the electrolyte. No current flow can

occur until the "Electrode Terminals" are connected to a Circuit (i.e. a Light Bulb). The

Electrons then flow from the Zinc Electrode through the external Circuit to the CopperElectrode. The chemical reaction between the Zinc and the electrolyte continues, and the Zinc is

eventually used up in the process.

Common Dry Cell Batteries work on a similar principle with a "paste-like" electrolyte and aCarbon Electrode rather than Copper.

Storage Cell Batteries

"Storage Batteries" produce electricity from chemical action somewhat similar to the Primary

Cells. However, applying an external source ofelectricity in a charging process can also reversethe process. This is normally done with a Battery Charger or with a Car or Truck's Alternator.

During "recharging," some-but not all-of the Electrons are moved back on to their original

Electrode. Each time the Battery is "recharged," fewer Electrons are moved back to their originalElectrode -and eventually- the Battery can no longer be "recharged."

Quartz Crystal


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There are several common types ofStorage Batteries including:

lead-acid batteries (used in Cars, Trucks, and Boats),

alkaline batteries,

nickel-cadmium batteries,

lithium and

others.

The Nickel-Cadmium Battery is commonly used in Tools where the "rechargeable" feature isdesired. There is also significant ongoing storage cell research to support Electric Vehicle

development, since battery performance is critical to these Vehicles.

Fuel Cells

A "Fuel Cell" consists of a container in which fuels react in the presence of an Electrolyte. In

this reaction, Electrons are made available at the Negative Electrode Terminal. Energy is

provided by the continuous supply offuels. Two fuels must be used to provide the necessaryreaction. Oxygen and Hydrogen are two of the fuels that can be used. Fuel Cells are used as a

source ofelectricity in Space Vehicles.

DC Generator

A single "loop" of Wire in a Magnetic Field can be used as a DC Generator. When the "loop" is

stationery, it is not cutting any "magnetic lines of force," and the currentand voltage are "zero."As the "loop" of Wire is rotated through the Magnetic Field, it starts to break the "magnetic lines

of force"; and currentand voltage are induced in the Wire Loop.


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The "magnetic lines of force" induce currentinto the Wire Loop in the same direction of flow as

the "loop" moves in a circle, so the electricity produced is DC since current flow is always the

same direction

Alternating Current (AC)

Alternating Current, or "AC" (as it is often called) is the kind ofpowerwith which we are all

familiar. We rely on this kind ofpowerin our homes, businesses, and industries. That is because

ACpoweris much more economical to produce and use than DCpower.

George Westinghouse set up the first commercial ACpowerin 1886. At that time, Edison was

still providing DCcurrentto homes, but the range ofpower transmission was about one mile

from his Plant in New Jersey. Because ACpowerwas found to be much cheaper to distribute, it

became the obvious preference.

The primary characteristic ofACpower(that makes it so economical) is the ability to change the

voltage levels by using Transformers. The voltage can be "stepped up" (or "down") as the needarises. This allows the powerto be distributed as widely as needed.

Unlike DC voltage and current(which remain steady), AC voltage and currentchanges -orcycles- 60 times per second in North America. ACpowerin Europe cycles 50 times per second.

This "cycling" has many advantages that we shall see in the next sections.

Sine Wave Characteristics

A "sinusoidal" or "sine wave form" represents ACpowergraphically -called sine wave for short.

As you look at this sine wave, remember that this apparently stable picture changes 60 timesevery second. In doing so, we think in terms of averages ofcurrent, voltage and any changes in

Frequency. There are five characteristics ofACpower: Amplitude, Cycles, Frequency, Peak-to-

Peak, and RMS.

A Sine Wave Form

Amplitude

The first characteristic ofACpoweris its "amplitude." Amplitude is the maximumvalue ofcurrentor voltage. It is represented by either of the two "peaks" of thesine wave. This voltage level is also referred to as the "Peak Voltage," and it canbe either positive or negative. Positive and negative refer only to the direction of


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current flow. A negative number does not mean that the voltage or current floware less than "zero," only that the currentflows in the opposite direction.

Cycles

A "cycle" is one complete repetition of the sine wavepattern. It is produced by one completerevolution (360 degrees) of the AC Generator.

Since the sine wave begins at "zero," goes positive through the Positive Peak, then negative

through "zero," reaches the Negative Peakand to "zero," we say a fullcycle has been completed.

Frequency

The number of times the Sine Wave Pattern Cycle occurs in a second is called the "frequency."

Frequency was originally measured in "cycles per second" (CPS). Today, the unit of

measurement forFrequency is called "Hertz," in honor of the German Scientist Heinrich Rudolf

Hertz (1857-1894

Peak-to-Peak Voltage

There are two values ofvoltage with which we must be familiar. The first is "peak-to-peak" voltage. This is the voltage measured between the maximum Positiveand Negative Amplitudes on the sine wave. (It is twice the Amplitude.) This value isthe maximum voltage available, but it is not all useable in practical applications

Root Mean Square (RMS)

The second value ofvoltage is the actual useful voltage that is available and is called RMS. This

stands for "Root Mean Square," and it is the standard way of measuring and reporting effective

Alternating Current and voltage. It is not the "peak"; it is the "average".


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Useful Voltage

The RMS is found by multiplying the Peak Amplitude by the Square Root of 2 (approximately

0.707). This yields the actual, useable voltage. It is typically represented by a dotted line drawnacross each "peak" near the 70 percent point.


The second value ofvoltage is the actual useful voltage that is available and is called RMS. Thisstands for "Root Mean Square," and it is the standard way of measuring and reporting effectiveAlternating Current and voltage. It is not the "peak"; it is the "average".

Useful Voltage

The RMS is found by multiplying the Peak Amplitude by the Square Root of 2 (approximately0.707). This yields the actual, useable voltage. It is typically represented by a dotted line drawn

across each "peak" near the 70 percent point.


The second value ofvoltage is the actual useful voltage that is available and is called RMS. Thisstands for "Root Mean Square," and it is the standard way of measuring and reporting effective

Alternating Current and voltage. It is not the "peak"; it is the "average".


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Useful Voltage

The RMS is found by multiplying the Peak Amplitude by the Square Root of 2 (approximately

0.707). This yields the actual, useable voltage. It is typically represented by a dotted line drawnacross each "peak" near the 70 percent point.

AC Generator

One of the easiest ways to think about AC or electric power generation is to thinkabout it as the opposite ofelectric power use (like a Motor running backwards).Motors convert electricityinto powerand motion. Generators convert motion andpowerinto electricity.

A typical Generator has a large Electromagnet spinning inside a "stationary coil" of Wire. As the

Magnetic Field produced by the ends of the Magnet moves across the turns of Wire in the

"stationary coil", an electric currentis set up in the Wire. Increasing the number of turns of Wirein a "ring" ("doughnut") configuration increases the additive currentin the Wire.

There are two types ofAlternating Current commonly in use today:

Single-Phase

Three-Phase

Single-Phase AC

Single-phase Alternating Current" is most often used in homes, small businessesand on farms. In large commercial buildings and industrial locations where largerMotors are used, single-phase poweris not usually adequate.

The production ofSingle-phase Alternating Current is best described by thinking of the

Generator as a simple Bar Magnet rotating inside a single coil-shaped loop of Wire. When the

Magnet rotates, the "magnetic lines of force" cut through the coiled Wires. The strength of thefield created depends on the number of these "lines" that are cut each second. At a constant

speed, more "coils" of Wire will be cut per second as the loop approaches the one-fourth

revolution point and the generated-voltage reaches a maximum at this point. As the north pole

moves from the one-fourth revolution point to the one-half revolution point, fewer Wire Coilsare being cut per second. The voltage decreases and goes to "zero" at the one-half revolution

point where the Magnetic Field is parallel to the "coils" of Wire.


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As the Magnet continues to rotate, the south pole's Magnetic Field cuts the coiled Wires in the

opposite direction, producing an opposing voltage which again builds up to a maximum at the

three-fourths revolution point. As the north pole moves from the three-fourths turn to one fullrevolution, the voltage then decreases to "zero."

One complete revolution of the Magnetic Field is called a "cycle." If there was only one "coil" ofWire in the outer portion of the Generator, this would be a "single-phase" device. By adding two

additional "coils" of Wire to the Generator, we could then generate currentin three individual"coils" (phases) orthree-phase power.

Three-Phase Power

"Three-phase Power" is designed especially for large electrical loads where the total electrical

loadis divided among the three separate "phases." As a result, the Wire and Transformers will beless expensive than if these large loads were carried on a single-phase system.

Three-phase Generators usually have three separate "windings," each producing their ownseparate single-phase voltage. Since these "windings" are staggered around the Generatorcircumference, each of the single-phase voltages is "out of phase" with one another.

That is, each of the three reaches the maximum and

minimum points in the ACcycle at different times.

Electricity is generated at Power Companies in these threephases. But, ifthree-phase poweris better than single-

phase, why not four-, five- orsix-phase? Theoretically,these would be even better, but equipment manufacturers

would have to build Motors to use it; and that just wouldnot be cost-effective (given the installed base of Three-phase Equipment that must continue to be

powered).

The word "phase" is often abbreviated using the Greek letter "phi" and is written as a "zero" with

a slash mark through it ( ).

Three-Phase Power

Sine Wave Pattern


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Direct Current Theory

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