Iron Care Red Red-Yellow High-voltage winding Red Black Yellow PRIMARY Yellow-Blue 5-volt FIGURE 8-200. Power supply transformer. Yellow winding When the turns ratio and the input voltage are known, the output voltage can be determined as follows:
Where E is the voltage of the primary, E, is the output voltage of the secondary, and Ni and N2 are the number of turns of the primary and secondary, respectively.
Transposing the equation to find the output voltage gives:
The most commonly used types of voltage transformers are as follows:
(1) Power transformers are used to step up or step down voltages and current in many types of power supplies. They range in size from the small power transformer shown in figure 8-200 used in a radio receiver to the large transformers used to step down highpower line voltage to the 110-120 volt level used in homes.
In figure 8-201, the schematic symbol for an iron-core transformer is shown. In this case the secondary is made up of three separate windings. Each winding supplies a different circuit with a specific voltage, which saves the weight, space, and expense of three separate transformers. Each secondary has a midpoint connection, called a "center tap," which provides a selection of half the voltage across the whole winding.
The leads from the various windings are color-coded by the manufacturer, as labeled in figure 8-201. This is a standard color code, but other codes or numbers may be used.
Black Green Green-Yellow 6 volt Green winding
SECONDARY
WINDINGS FIGURE 8-201. Schematic symbol for an iron-core power transformer.
(2) Audio transformers resemble power transformers. They have only one secondary and are designed to operate over the range of audio frequencies (20 to 20,000 c.p.s.).
(3) RF transformers are designed to operate in equipment that functions in the radio range ot frequencies. The symbol for the RF transformer is the same as for an RF choke coil.
It has an air core as shown in figure 8-202.
(4) Autotransformers are normally used in power circuits; however, they may be designed for other uses. Two different symbols for autotransformers used in power or audio circuits are shown in figure 8-203.
If used in an RF communication or navigation circuit (B of figure 8-203), it is the same, except there is no symbol for an iron core. The autotransformer uses part of a winding as a primary; and, depending on whether it is step-up or step-down, it uses all or part of the same winding as the secondary. For example, the autotransformer shown in A of figure 8-203 could use the following possible choices for primary and secondary terminals.
FIGURE 8-202. An air-core transformer.
Current Transformers Current transformers are used in a.c. power supply systems to sense generator line current and to provide a current, proportional to the line current, for circuit protection and control devices.
The current transformer is a ring-type transformer using a current-carrying power lead as a primary (either the power lead or the ground lead of the a.c. generator). The current in the primary induces a current in the secondary by magnetic induction.
The sides of all current transformers are marked "H1" and "H2" on the unit base. The transformers must be installed with the "HI" side toward the generator in the circuit in order to have proper polarity. The secondary of the transformer should never be left open while the system is being operated; to do so could cause dangerously high voltages, and could overheat the transformer.
Therefore, the transformer output connections should always be connected with a jumper when the transformer is not being used but is left in the system.
Transformer Losses In addition to the power loss caused by imperfect coupling, transformers are subject to "copper" and "iron" losses. Copper loss is caused by the resistance of the conductor comprising the turns of the coil. The iron losses are of two types called hysteresis loss and eddy current loss. Hysteresis loss is the electrical energy required to magnetize the transformer core, first in one direction and then in the other, in step with the applied alternating voltage. Eddy current loss is caused by electric currents (eddy currents) induced in the transformer core by the varying magnetic fields. To reduce eddy current losses, cores are made of laminations coated with an insulation, which reduces the circulation of in- Power in Transformers Since a transformer does not add any electricity to the circuit but merely changes or transforms the electricity that already exists in the circuit from one voltage to another, the total amount of energy in a circuit must remain the same. If it were possible to construct a perfect transformer, there would be no loss of power in it; power would be transferred undiminished from one voltage to another.
Since power is the product of volts times amperes, an increase in voltage by the transformer must result in a decrease in current and vice versa.
There cannot be more power in the secondary side of a transformer than there is in the primary. The product of amperes times volts remains the same.
The transmission of power over long distances is accomplished by using transformers. At the power source the voltage is stepped up in order to reduce the line loss during transmission. At the point of utilization, the voltage is stepped down, since it is not feasible to use high voltage to operate motors, lights, or other electrical appliances.
Connecting Transformers in A. C. Circuits Before studying the various means of connecting transformers in a.c. circuits, the differences between single-phase and three-phase circuits must be clearly understood. In a single-phase circuit the voltage is generated by one alternator coil. This -3 single-phase voltage may be taken from a singlephase alternator, or from one phase of a threephase alternator, as explained later in the study of a.c. generators.
In a three-phase circuit three voltages are Input generated by an alternator with three coils so spaced within the alternator that the three volt- Output ages generated are equal but reach their maximum values at different times. In each phase of a 400- B cycle, three-phase generator, a cycle is generated every 7400 second.
FIGURE 8-203. Autotransformers. In its rotation, the magnetic pole passes one 361
Phase A 3 Phase motor Phase B Phase C FIGURE 8-204. Three-phase generator using a three conductors.
coil and generates a maximum voltage; one-third cycle (1200 second) later, this same pole passes another coil and generates a maximum voltage in it; and the next one-third cycle later, it passes still another coil and generates a maximum voltage in it. This causes the maximum voltages generated in the three coils always to be one-third cycle (1200 second) apart.
The early three-phase generators were connected to their loads with six wires and all six leads in the circuit carried the current. Later, experiments proved that the generator would furnish as much power with the coils connected so that only three wires were needed for all three phases as shown in figure 8-204. The use of three wires is standard for the transmission of threephase power today. The return current from any one alternator coil always flows back through the other two wires in the three-phase circuit.
Three-phase motors and other three-phase loads are connected with their coils or load elements arranged so that three transmission lines are required for delivery of power. Transformers that are used for stepping the voltage up or down in a three-phase circuit are electrically connected so 60V 90V 120V 30V FIGURE 8-206. Tapped transformer secondary.
that power is delivered to the primary and taken from the secondary by the standard three-wire system.
However, single-phase transformers and singlephase lights and motors may be connected across any one phase of a three-phase circuit, as shown in figure 8-205. When single-phase loads are connected to three-phase circuits, the loads are distributed equally among the three phases in order to balance the loads on the three generator coils.
Another use of the transformer is the singlephase transformer with several taps in the secondary. With this type of transformer, the voltage can be lowered to provide several working voltages, as shown in figure 8-206.
A center tapped transformer, powering a motor requiring 220 volts along with four lights requiring 110 volts, is shown in figure 8-207. The motor is connected across the entire transformer output, and the lights are connected from the center tap to one end of the transformer. With this connection only half of the secondary output is used.
This type of transformer connection is used FICURE 8-205. Step-down transformer using FIGURE 8-207. Step-down transformer using two-wire system. a three-wire system.
3 Phase C AC Voltage Primaries Secondaries B FIGURE 8-208. Wye-to-wye connection.
extensively on aircraft because of the combinations of voltages that can be taken from one transformer. Various voltages can be picked off the secondary winding of the transformer by inserting taps (during manufacture) at various points along the secondary windings.
The various amounts of voltage are obtained by connecting to any two taps or to one tap and either end.
Transformers for three-phase circuits can be connected in any one of several combinations of the wye (y) and delta (4) connections. The connection used depends on the requirements for the transformer.
When the wye connection is used in three-phase B 3 Phase AC Voltage Primaries Secondaries transformers, a fourth or neutral wire may be used. The neutral wire connects single-phase equipment to the transformer. Voltages (115v) between any one of the three-phase lines and the neutral wire can be used for power for devices such as lights or single-phase motors.
In combination, all four wires can furnish power at 208 volts, three-phase, for operating threephase equipment, such as three-phase motors or rectifiers. When only three-phase equipment is used, the ground wire may be omitted. This leaves a three-phase, three-wire system as illustrated in figure 8-208.
Figure 8-209 shows the primary and secondary with a delta connection. With this type of connection the transformer has the same voltage output as the line voltage. Between any two phases the voltage is 240 volts. In this type of connection, wires A, B, and C can furnish 240-volt, three-phase power for the operation of three-phase equipment.
The type of connection used for the primary coils may or may not be the same as the type of connection used for the secondary coils. For example, the primary may be a delta connection and the secondary a wye connection. This is called a delta-wye-connected transformer. Other combinations are delta-delta, wye-delta, and wyewye.
Troubleshooting Transformers There are occasions when a transformer must be checked for opens or shorts, and it is often necessary to determine that a transformer is a step-up or step-down transformer.
An open winding in a transformer can be located by connecting an ohmmeter as shown in figure
8-210. Connected as shown, the ohmmeter would read infinity. If there were no open in the coil, the ohmmeter would indicate the resistance of wire in the coil. Both primary and secondary can be checked in the same manner.
open FIGURE 8-210. Checking for an open in a FIGURE 8-209. Delta-to-delta connection. transformer winding.
All transformers discussed in this section are designed with one primary winding. They operate on a single source of a.c. Transformers which operate from three voltages from an alternator, or a.c. generator, are called three-phase or polyphase transformers. These transformers will be FIGURE 8-211. Checking for shorted transformer windings. discussed in the study of generators and motors.
The ohmmeter may also be used to check for shorted windings, as shown in figure 8-211, however, this method is not always accurate. If, for example, the transformer had 500 turns and a resistance of 2 ohms, and 5 turns were shorted out, the resistance would be reduced to approximately 1.98 ohms, which is not enough of a change to be read on the ohmmeter. In this case, the rated input voltage can be applied to the primary to permit measurement of the secondary output voltage. If the secondary voltage is low, it can be assumed that the transformer has some shorted windings, and the transformer should be replaced. If the output voltage is then normal, the original transformer can be considered defective.
An ohmmeter can be used to determine whether a transformer is a step-up or step-down transformer. In a step-down transformer, the resistance of the secondary will be less than that of the primary, and the opposite will be true in the case of a step-up transformer. Still another method involves applying a voltage to the primary and 1t. the voltages measuring the secondary output. The voltages used should not exceed the rated input voltage of the primary.
If a winding is completely shorted, it usually becomes overheated because of the high value of current flow. In many cases, the high heat will melt the wax in the transformer, and this can be detected by the resulting odor. Also, a voltmeter reading across the secondary will read zero. If the circuit contains a fuse, the heavy current may cause the fuse to blow before the transformer is heavily damaged In figure 8-212 one point on a transformer winding is shown connected to ground. If the external circuit of the transformer circuit is grounded, a part of the winding is effectively shorted. A megger connected between one side of the winding and the transformer case (ground) will verify this condition with a low or zero reading. In such a case, the transformer must be replaced.
MAGNETIC AMPLIFIERS The magnetic amplifier is a control device being employed at an increasing rate in many aircraft electrical and electronic systems. This is because of its ruggedness, stability, and safety in comparison to vacuum tubes.
The principles on which the magnetic amplifier operates can best be explained by reviewing the operation of a simple transformer. If an a.c. voltage is applied to the primary of an iron core transformer, the iron core will be magnetized and demagnetized at the same frequency as that of the applied voltage. This, in turn, will induce a voltage in the transformer secondary. The output voltage across the terminals of the secondary will depend on the relationship of the number of turns in the primary and the secondary of the transformer.
The iron core of the transformer has a saturation point after which the application of a greater magnetic force will produce no change in the intensity of magnetization. Hence, there will be no change in transformer output, even if the input is greatly increased.
The magnetic amplifier circuit in figure 8-213 will be used to explain how a simple magnetic amplifier functions. Assume that there is 1 ampere of current in coil A, which has 10 turns of wire.
If coil B has 10 turns of wire, an output of 1 ampere will be obtained if coil B is properly loaded. By applying direct current to coil C, the core of the magnetic amplifier coil can be further magnetized. Assume that coil C has the proper number of turns and, upon the application of 30 milliamperes, that the core is magnetized to the Input FIGURE 8-212. Part of a transformer winding grounded.
Load A.C. Input A.C. Output D.C.
FIGURE 8-213. Magnetic amplifier circuit.
point where 1 ampere on coil A results in only
0.24 ampere output from coil B.
By making the d.c. input to coil C continuously variable from 0 to 30 milliamperes and maintaining an input of 1 ampere on coil A, it is possible to control the output of coil B to any point between 0.24 ampere and 1 ampere in this example. The term "amplifier" is used for this arrangement because, by use of a few milliamperes, control of an output of 1 or more amperes is obtained.
The same procedure can be used with the circuit shown in figure 8-214.
By controlling the extent of magnetization of the iron ring, it is possible to control the amount of current flowing to the load, since the amount of magnetization controls the impedance of the
a.c. input winding. This type of magnetic amplifier is called a simple saturable reactor circuit.
Adding a rectifier to such a circuit would remove half the cycle of the a.c. input and permit a direct current to flow to the load. The amount of A.C.
FIGURE 8-215. Self-saturating, full-wave magnetic amplifier.
d.c. flowing in the load circuit is controlled by a
d.c. control winding (sometimes referred to as bias). This type of magnetic amplifier is referred to as being self-saturating.
In order to use the full a.c. input power, a circuit such as that shown in figure 8-215 may be used. This circuit uses a full-wave bridge rectifier. The load will receive a controlled direct current by using the full a.c. input. This type of circuit is known as a self-saturating, full-wave magnetic amplifier.
In figure 8-216 it is assumed that the d.c.
control winding is supplied by a variable source, such as a sensing circuit. In order to control such a source and use its variations to control the a.c.
output, it is necessary to include another d.c.
winding that has a constant value. This winding, referred to as the reference winding, magnetizes the magnetic core in one direction.
D.C.
Control Load Load D.C.
A.C.
A.C. Input FIGURE 8-214. Saturable reactor circuit. FIGURE 8-216. Basic preamplifier circuit.
Electrons -0.
- -0- Filament -o- Plate FIGURE 8-217. Principle of vacuum tube operation.
The d.c. control winding, acting in opposition to the reference winding, either increases (degenerative) or decreases (regenerative) the magnetization of the core to change the amount of current flowing through the load. This is essentially a basic preamplifier.
VACUUM TUBES The use of vacuum tubes in aircraft electrical and electronic systems is rapidly declining because of the many advantages of using transistors. On the other hand, some systems still employ vacuum tubes in special applications, and a large number of older model aircraft still in service are equipped with devices that use vacuum tubes. For these reasons a general study of vacuum tubes is still considered a necessary part of the aviation maintenance program.
Originally, vacuum tubes were developed for radio work. They are used in radio transmitters as amplifiers for controlling voltage and current, as oscillators for generating audio and radio frequency signals, and as rectifiers for converting alternating current into direct current. Radio tubes are used for similar purposes in many electrical devices in aircraft, such as the automatic pilot and the turbosupercharger regulator.
When a piece of metal is heated, the speed of the electrons in the metal is increased. If the metal is heated to a high enough temperature, the electrons are accelerated to the point where some of them actually leave the surface of the metal, as shown in figure 8-217. In a vacuum tube, electrons are supplied by a piece of metal, called a cathode, which is heated by an electric current. Within limits, the hotter the cathode the greater the number of electrons it will give off or emit.
To increase the number of electrons emitted, the cathode is usually coated with special chemical compounds. If the emitted electrons are not drawn away by an external field, they form about the cathode into a negatively charged cloud called the space charge. The accumulation of negative electrons near the emitter repels others coming from the emitter. The emitter, if insulated, becomes positive because of the loss of electrons.
This establishes an electrostatic field between the cloud of negative electrons and the now positive cathode. A balance is reached when only enough electrons flow from the cathode to the area surrounding it to supply the loss caused by diffusion of the space charge.
Types of Vacuum Tubes There are many different types of vacuum tubes, most of which fall into four general types: (1) The diode, (2) the triode, (3) the tetrode, and (4) the pentode. Of these, the diode is used almost exclusively for changing a.c. current to d.c. current.
In some vacuum tubes, the cathode is heated by d.c. and is both the electron emitter and current carrying member, while in others the cathode is heated by a.c. Tubes designed for a.c.
operation employ a special heating element which heats the electron emitter (cathode) indirectly.
When a d.c. potential is applied between the cathode and another element in the tube called a plate, with the positive side of the voltage connected to the plate, the electrons emitted by the cathode are attracted to the plate. These two elements constitute the simplest form of vacuum tube, which is the diode. In the diode, electrons are attracted to the plate, when it is more positive than the cathode, and are repelled when the plate is less positive than the cathode.
Current flows through the tube when it is connected in a circuit only when the plate is positive with respect to the cathode. Current does not flow when the plate is negative (less positive) with respect to the cathode as illustrated in figure
8-218. This characteristic gives the diode its principle use, that of rectification, or the changing of alternating current into direct current.
Diode rectifiers are used in aircraft electrical systems, especially when high voltage d.c. is desired for light loads. They may be used as either half-wave or full-wave rectifiers; they may be 366
Current flows No current flows Plate
• Grid cỏỏổ đ88ỏ
- Plate
• Cathode Cathode Cathode FIGURE 8-218. Diode tube operation.
used singly, in parallel, or in bridge circuits. As shown in figure 8-219, a half-wave rectifier contains two tube elements (plate and cathode). A full-wave rectifier contains three elements (two plates and a cathode).
In the half-wave circuit, current flows only during the positive half of the cycle of the applied voltage (plate positive, cathode negative for electron flow). It flows from the cathode to the plate and then through the load back to the cathode.
On the negative cycle of the applied voltage, no current flows through the tube. As a result, the rectified output voltage is d.c., but it consists of pulses, or half cycles, of current.
In a vacuum tube connected as a full-wave rectifier, current flows to the load on both half cycles of the alternating voltage. In the full-wave rectifier, current flows from the top plate through the d.c. load on one alternation, and on the next A.C. input voltage 0000 D.C. output voltage Plate coil D.C.
load Cathode or filament Load current Plate Filament coil FIGURE 8-219. Half-wave vacuum tube rectifier circuit.
FIGURE 8-220. Triode tube.
alternation, current flows to the lower plate and through the load in the same direction.
Vacuum tube rectifiers have been replaced to a great degree in aircraft systems by dry-disk or semiconductor diodes. In the study of solid state devices the process of rectification is treated in greater detail.
The triode tube is a three-element tube. In addition to the plate and cathode, there is a third element, called the grid, located between the cathode and the plate as shown in figure 8-220.
The grid is a fine-wire mesh or screen. It serves to control the electron flow between the cathode and the plate. Whenever the grid is made more positive than the cathode, there is an increase in the number of electrons attracted to the plate, resulting in an increase in plate current flow. If the grid is made negative with respect to the cathode, electron movement to the plate is retarded and plate current flow decreases.
Usually the grid is negative with reference to the cathode. One method of making the grid negative is to use a small battery connected in series with the grid circuit. This negative voltage applied to the grid is called bias.
The most important use of a triode is as an amplifier tube. When a resistance or impedance is connected in series in the plate circuit, the voltage 367
Plate Plate Screen grid Suppressor grid Screen grid Control grid Cathode Control grid -Cathode
- Heater • Heater FIGURE 8-221. Tetrode tube schematic.
FIGURE 8-222. Pentode schematic.
drop across it, which depends upon the current flowing through it, can be changed by varying the grid voltage. A small change in grid voltage will cause a large change in the voltage drop across the plate impedance. Thus, the voltage applied to the grid is amplified in the plate circuit of the tube.
A tetrode tube is a four-element tube, the additional element being the screen grid (figure 8-221). This grid is located between the control grid and the plate. The screen grid is operated at a positive voltage somewhat lower than the plate voltage. It reduces the sometimes undesirable effect in tube operation caused by energy fed from the output of a tube back into the input (grid) circuit.
Under certain operating conditions, this feedback action is very pronounced in a triode and causes the tube to act as an oscillator instead of an amplifier. The chief advantages of tetrodes over triodes are greater amplification for smaller input voltages, and less feedback from the plate to the grid circuit.
An undesirable characteristic of the tetrode tube is secondary emission. Secondary emission is the term applied to the condition where electrons are knocked out of the plate into the space between the elements of a tube by rapidly moving electrons striking the plate. In triode tubes, since the grid is negative with respect to the cathode, it repels the secondary electrons and tube operation is undisturbed. In the tetrode, the effect of secondary emission is especially noticeable since the screen grid, which is positive with respect to the cathode, attracts the secondary electrons and causes a reverse current to flow between the screen and plate.
The effects of secondary emission are overcome by adding a third grid, called the suppressor grid, between the screen grid and the plate. This grid repels the secondary electrons toward the plate.
A tube with three grids is called a pentode, which has a high amplification factor and is used to amplify weak signals. The schematic of a pentode is shown in figure 8-222.
Another type of vacuum tube is the gas tube.
Gas-filled tubes are primarily diodes and are used mostly as rectifiers. In tubes of this type the gas should be one ten-thousandth as dense as air under normal atmospheric pressure. When an electron meets a gas molecule, the energy imparted by the impact can cause the molecule (or atom) to lose or gain one or more electrons.
Consequently, ionization takes place.
Any gas or vapor having no ions is practically a perfect insulator. If two electrodes are placed in such a medium, no current will flow between them. However, gases always have some residual ionization because of cosmic rays, radioactive materials in the walls of the containers, or the action of light. If a potential is applied between two elements in such a gas, the ions migrate between them and give the effect of current flow.
This is called the dark current because no visible light is associated with it.
If the voltage on the electrodes is increased, the current starts to rise. At a certain point, known as the threshold, the current suddenly begins to go up without any increase in applied voltage. If there is enough resistance in the external circuit to prevent the current from rising quickly, the voltage immediately drops to a lower value and breakdown occurs. This abrupt change takes place as a result of the ionization of the gas by electron collision.
The electrons released by the ionized gas join the stream and liberate other electrons. The process, then, is cumulative. Breakdown voltage is determined primarily by the type of gas, the 368
materials used for the electrodes, and their size and spacing. Once ionization takes place, the current can rise to 50 milliamperes (ma.), or more, with little change in the voltage applied.
If the voltage is increased, the current increases and the cathode is heated by the bombardment of the ions which strike it. When the tube gets hot enough, thermionic emission results.
This emission reduces the voltage loss in the tube, which, in turn, causes more current to flow and increases the rate of emission and ionization.
This cumulative action causes a sudden decrease in the voltage drop across the tube and an extremely high rise in current flow. Unless the tube is designed to operate in this manner, it can be damaged by heavy current flow. This is basic to the formation of an arc; therefore, tubes that operate at these high currents are called arc tubes.
For currents up to 50 milliamperes, the unit usually is small and is termed a glow tube because of the colored light it emits. An example of such a tube is the familiar neon light.
The principle of grid control can be applied to almost any gas tube, but it is used specifically with cold cathode, hot cathode, and are types of triodes and tetrodes. The hot cathode type of three-element gas tube is given the general name of thyratron.
The phototube is another special type of vacuum tube. It is basically the same as the simple diode discussed earlier. It has an evacuated glass bulb, a cathode which emits electrons when light is allowed to fall upon it, and a plate which Twin triode Twin-diode-triode Diode pentode Triode pentode FIGURE 8-223. Multiunit tubes.
attracts electrons when a voltage is applied. The sensitivity of the tube depends on the frequency or color of the light used to excite it and is specified in these terms.
For example, some tubes are sensitive to red light, others to blue light. In most phototubes, the cathode resembles a half cylinder. It is covered with multiple layers of the rare metal, cesium, overlaid on cesium oxide, which, in turn, lies on a layer of silver. The plate is shaped like a small rod and is located in the center of the cathode.
Other types of vacuum tubes include those with the characteristics of several tubes incorporated into one, as shown in figure 8-223. Among these, for example, are twin triode tubes containing two triode sections in a single tube envelope, and diode-triode tubes with a rectifier diode and an amplifier triode in the same envelope. There are many other tube combinations.
TRANSISTORS The transistor is an electronic device that is capable of performing most of the functions of vacuum tubes. It is very small, light in weight, and requires no heater. It is also mechanically rugged and does not pick up stray signals. Transistors have been in general use for more than a decade, but compared to some of the components they are replacing they are relatively new. As research progresses, new discoveries often cause some elements of transistor theory to be modified.
A transistor is a semiconductor device consisting of two types of materials each of which exhibits electrical properties. Semiconductors are materials whose resistive characteristics fall approximately midway between those of good conductors and insulators. The interface between the parts is called a junction. Selenium and germanium diodes (rectifiers) are examples of such devices and are called junction diodes. Most transistors are made of germanium to which certain impurities are added to impart certain characteristics. The impurities used are generally arsenic or indium.
The type of transistor which may be used in some applications in place of the triode tube is the "junction" transistor, which actually has two junctions. It has an emitter, base, and collector which correspond to the cathode, grid, and plate, respectively, in the triode tube. Junction transistors are of two types, the NPN type and the PIP type (see figure 8-224).
Emitter Collector Emitter Base Collector N Emitter Collector Emitter Base Collector P B FIGURE 8-224. NPIN and PNP transistors.
Theory of Transistor Operation Before transistor operation and the meaning of "N" and "P" can be explained, it is necessary to consider the theory of transistor action.
An electron is a negatively charged particle.
In any material, there are electrons separated from each other by some minute distance. Whenever there is an electron, there is a negative charge.
An atom of the semiconductor material has a specified number of electrons, depending on the type of material. If one of the electrons is removed, the hole from which the electrons moved is more positive than the electron that was removed.
To hole is considered to have a positive charge.
if an electron from a neighboring atom moves into the hole, the hole apparently moves to the place from which the electron came. The hole does not really move; it is filled in one place and formed in another. In A of figure 8-225, the electrons are represented as black dots and the holes as dotted circles.
In B of figure 8-225, the electrons have moved one space to the left of their position occupied in A of figure 8-225. In effect, the holes have, therefore, moved one space to the right.
The movement of the electrons is current. In the same sense, the movement of holes is current also. Electron current moves in one direction;
hole current travels in the opposite direction. The movement of the charge is a current. In transistors both electrons and holes act as carriers of current.
In transistors, materials referred to as N-materials and P-materials are used. The N-materials are rich in electrons and, therefore, electrons act as the carriers. The P-material is lacking in electrons and, therefore, has holes as carriers.
An NPN transistor is not interchangeable with a PINP transistor and vice versa. However, if all power supplies are reversed, they may be interchanged.
Since temperature is critical in a transistor circuit, there must be sufficient cooling for the transistors. Another precaution to observe which applies to any circuit is: Power should never knowingly be applied to an open circuit.
Diodes Figure 8-226 illustrates a germanium diode and consists of two different types of semiconductor materials. With the battery connected as shown, positive holes and electrons are repelled by the battery toward the junction, causing an interaction between the holes and electrons. This results in electrons flowing through the junction to the holes and to the positive terminal of the battery. The holes move toward the negative terminal of the battery. This is called the forward direction and is a "high" current.
Connecting the battery as shown in figure 8-227 causes the holes and electrons to be pulled away from the junction, and little interaction between holes and electrons occurs at the junction. This results in very little current flow, called reverse current.
Holes Electrons -
- Electron current B Hole current - FIGURE 8-225. Electrons and holes in transistors.
junction Holes Electrons FIGURE 8-226. Electron and hole flow in a diode with forward bias.
The potential on the electrodes of the transistor diodes applied from the battery is called bias. It may be either forward or reverse bias, that is, in a high-current or a low-current direction.
The N-germanium is manufactured with an impurity, such as arsenic, added to give it an excess of electrons. Arsenic gives up its electrons readily and can be used as a carrier. The P-germanium has an impurity, such as indium, added.
This takes the electrons from the germanium and leaves holes, or positive carriers.
Zener Diodes Zener diodes (sometimes called "breakdown diodes") are used primarily for voltage regulation.
They are designed so that they will break down (allow current to pass) when the circuit potential is equal to or in excess of the desired voltage.
Below the desired voltage the zener blocks the circuit like any other diode biased in the reverse direction. Because the zener diode allows free flow in one direction when it is used in an a.c. circuit, Holes Electrons FIGURE 8-227. Electron and hole flow in a diode with reverse bias.
FIGURE 8-228. Transistor electron flow.
two diodes connected in opposite directions must be used. This takes care of both alternations of current.
The zener may be used in many places where a gas-filled vacuum tube cannot be used, because it is smaller in size and can be used in low voltage circuits. The gas-filled tube is used in circuits above 75 volts, but the zener diode may be used in regulating voltages as low as 3.5 volts.
PNP Transistor Figure 8-228 shows a transistor circuit powered by batteries. The emitter circuit is biased by the battery Ee in the forward or high-current-flow direction. The collector circuit is biased by battery E, in the reverse or low-current-flow direction.
If the switch in the emitter circuit were closed (collector switch open), a high emitter current would flow since it is biased in the forward direction. If the collector switch were closed (emitter switch open), a low current would flow since it is biased in the reverse direction.
At the same time, a hole current is flowing in the opposite direction in the same circuit, as shown in figure 8-229. Hole current flows from the positive terminal of the battery, whereas electron current originates at the negative terminal.
The operation with both switches closed is the same as with a PNP transistor, except that the emitter now ejects electrons instead of holes into the base, and the collector, being positive, will collect the electrons. There is again a large increase in collector current with the emitter switch closed.
With the emitter switch open, the collector current will be small, since it is biased for reverse 371