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
these holes through the junction from the base to ( 7) the collector.
This collecting of holes by the collector causes a much greater reverse current than there would
(7) be if the emitter switch were open. The large increase of reverse collector current is caused by so-called transistor action, whereby holes from (- ) the emitter pass to the collector. Instead of holes flowing through the base and back to the emitter, they flow through the collector, Ee, and Ee to the emitter; the actual base current is very small.
The sum of collector current and base current equals the emitter current. In typical transistors the collector current can be 80-99 percent of the emitter current, with the remainder flowing FIGURE 8-229. Transistor hole current flow. through the base.
flow. At first glance it may appear that the transistor cannot amplify, since there is less current in the collector than in the emitter circuit. Remember, however, that the emitter is biased in the forward direction and a small voltage causes a large current, which is equivalent to a low resistance. The collector circuit is biased in the reverse direction, and a large voltage causes a small current, which is the equivalent of a high resistance.
When both switches are closed, a phenomenon known as transistor action occurs. The emitter, biased in the forward direction, has its positive holes ejected through the junction into the "N" region of the base. (The positive battery terminal repels the holes through the junction.) The collector, being biased negatively, will now attract N Input Output FIGURE 8-230. NPN transistor circuit.
NPN Transistor In figure 8-230 an NPN transistor is connected into a circuit. Notice that the battery polarities are reversed from those for the PNP transistor.
But with the types of transistor material reversed, the emitter is still biased in the forward direction, and the collector is still biased in the reverse direction.
In this circuit a small signal applied to the input terminal causes a small change in both emitter and collector currents; however, the collector, being a high-resistance, requires only a small current change to produce large voltage changes.
Therefore, an amplified signal appears at the output terminals.
The circuit in this illustration is called a grounded base amplifier, because the base is common to input and output (emitter and collector) circuits.
Figure 8-231 shows a different type of circuit connection. This is called a grounded emitter amplifier, and is similar to a conventional triode amplifier. The emitter is like a cathode, the base like a grid, and the collector like a plate. The collector is biased for a reverse current flow.
If the input signal swings positive, as shown in figure 8-231, it will aid the bias and increase base and emitter current. This increases collector current, making the upper output terminal more negative. On the next half cycle, the signal will oppose the bias and decrease emitter and collector current. Therefore, the output will swing positive.
It is 180° out of phase with the input just as in the conventional triode tube amplifier.
Since the base current is a very small part of 372
Output FIGURE 8-231. A grounded-emitter amplifier circuit.
the total emitter current, it requires only a very small change in base current to cause a large change in collector current. Therefore, it again amplifies the signal. This circuit has the highest gain (output/input) of the various transistor amplifiers. A PIP transistor could also be used if battery polarities were reversed.
Use of Transistors Transistors can be used in all applications where vacuum tubes are used, within certain limitations imposed by their physical characteristics. The main disadvantage of transistors is their low power output and limited frequency range. However, since they are approximately one one-thousandth the physical size of a vacuum tube, they can be used in compact equipment.
Their weight, approximately one one-hundredth that of a vacuum tube, makes the equipment much lighter. Their life is approximately three times that of a vacuum tube, and their power requirement is only about one-tenth that of a vacuum tube.
Transistors can be permanently damaged by heat or by reversed polarity of the power supply.
For this reason, care must be exercised when installing them in a circuit to prevent these conditions.
Transistors can be installed in miniature tube sockets, or they can be soldered directly into the circuits. There is no maintenance to be performed on them other than to remove and replace them as necessary.
When first tracing transistor circuits, trouble may be experienced in understanding from the schematic whether a transistor is an NPN or a PNP. Refer to figure 8-232, which shows the schematic symbol for two types of transistors.
Notice an arrow in the emitter line. When this arrow is pointing away from the base, it is an NPN; if the arrow is pointing toward the base, it is a PNP transistor.
A simple rule to determine whether the transistor is a PNP or an NPN is as follows: If it is a PNP, the center letter "N" indicates a negative base or, in other words, that the base will conduct more freely on a negative charge. If the transistor is an NPN, the "P" indicates a positive base and the transistor will conduct more freely on a positive base charge.
Since there are different types of transistors based on the method used in their manufacture, there are several means of identifying the transistor in a circuit as either an NPN or a PNP.
One method used to identify the type of transistor, called the junction transistor, is illustrated in figure 8-233. In this case, the method used to determine which of the three wires connected to a transistor is the base lead, which is the collector lead, and which is the emitter lead is based on the physical spacing of the leads. Notice that there are two leads close together and one lead further apart. The center lead is always the base.
The lead closest to the base is the emitter lead, and the lead further out is the collector lead.
The schematic shown in this illustration holds true for all junction-type transistors. For detailed intormation on any transistor, the applicable manufacturer's publications should be consulted.
RECTIFIERS Many devices in an aircraft require highamperage, low-voltage d.c. for operation. This Emitter Collector Emitter Collector Base Base
NPN PNP FIGURE 8-232. Transistor schematic symbols.
Emitter Base Collector FIGURE 8-233. Junction transistor connections.
power may be furnished by d.c. engine-driven generators, motor-generator sets, vacuum tube rectifiers, or dry-disk or solid-state rectifiers.
In aircraft with a.c. systems, a special d.c.
generator is not desirable since it would be necessary for the engine accessory section to drive an additional piece of equipment. Motor-generator sets, consisting of air-cooled a.c. motors that drive d.c. generators, eliminate this objection because they operate directly off the a.c. power system. Vacuum tube or various types of solidstate rectifiers provide a simple and efficient method of obtaining high voltage d.c. at low amperage. Dry-disk and solid-state rectifiers on the other hand are an excellent source of high amperage at low voltage.
A rectifier is a device which transforms alternating current into direct current by limiting or regulating the direction of current flow. The principal types of rectifiers are dry-disk, solid-state, and vacuum tube rectifiers. Solid-state, or semiconductor, rectifiers are rapidly replacing all other types; and, since dry-disk and vacuum tube rectifiers and motor-generators are largely limited to older model aircraft, the major part of the study of rectifiers will be devoted to solid-state devices used for rectification.
Motor-Generator A motor-generator is an a.c. motor and a d.c.
generator combined in one unit. This combination is often called a converter. Converters operate directly on either single-phase or three-phase voltage. Converters used on large aircraft are often operated by a three-phase, 208-volt, a.c. system and develop a direct current of 200 amperes at 30 volts with an approximate 28-ampere current drain on the a.c. system. Units similar to those used in airplanes with d.c. systems are provided for voltage regulation and paralleling of motorgenerator sets.
A motor-generator offers a number of advantages as a source of d.c. power in an airplane.
With a motor-generator, a momentary interruption of a.c. power does not cut off the d.c. power completely, since the inertia of the armature keeps it turning during the a.c. power interruption.
Extreme temperature changes affect a motorgenerator only slightly. Failure from overheating is negligible compared to that in a vacuum tube rectifier when it is operated above a safe temperature. In addition, a motor-generator can be operated at temperatures below those required by either dry-disk or vacuum tube rectifiers.
The greatest objection to a motor-generator is that, like all rotary devices, it requires considerable maintenance and creates a noise which is especially objectionable if the set is in the cabin of the airplane. For these reasons and, because of weight, space, and cost consideration, the motorgenerator set is rapidly being replaced by various solid-state power sources.
Dry-Disk Dry-disk rectifiers operate on the principle that electric current flows through a junction of two dissimilar conducting materials more readily in one direction than it does in the opposite direction.
This is true because the resistance to current flow in one direction is low, while in the other direction it is high. Depending on the materials used, several amperes may flow in the direction of low resistance but only a few milliamperes in the direction of high resistance.
Three types of dry-disk rectifiers may be found in aircraft: the copper-oxide rectifier, the selenium rectifier, and the magnesium copper-sulfide rectifier. The copper-oxide rectifier (figure 8-234) consists of a copper disk upon which a layer of copper oxide has been formed by heating. It may also consist of a chemical copper-oxide preparation spread evenly over the copper surface. Metal plates, usually lead plates, are pressed against the two opposite faces of the disk to form a good contact. Current flow is from the copper to the copper oxide.
The selenium rectifier consists of an iron disk, similar to a washer, with one side coated with selenium. Its operation is similar to that of the copper-oxide rectifier. Current flows from the selenium to the iron.
The magnesium copper-sulfide rectifier is made of washer-shaped magnesium disks coated with a 374
Copper-oxide P-type N-type Copper - semi-conductor semi-conductor Direction of circuit flow Insulating washer External lead External lead Insulating tube Pressure plate Lead FIGURE 8-234. Copper-oxide dry-disk rectifier.
layer of copper sulfide. The disks are arranged similarly to the other types. Current flows from the magnesium to the copper sulfide.
Solid-State Rectifiers In the study of transistors it was pointed out that the solid-state diode is manufactured from semiconductor material. It consists of N-type and P-type material joined in a single crystal. The point, or junction, where the two materials are in contact is called a P-N junction. This type of semiconductor, regardless of rating or size, is called a junction diode. The first type of semiconductor used was called the point-contact diode.
It utilized a single type of semiconductor material, against which a tungsten or phosphor-bronze wire, called a "cat whisker," was pressed or fused. The point-contact diode has been largely replaced by the junction diode because of its limited currentcarrying capabilities. The most common semiconductor materials are germanium and silicon.
A typical junction diode is shown in figure 8-235.
In figure 8-236, the positive terminal of the battery is connected to the P-type semiconductor material, and the negative terminal is connected to the N-type. This arrangement constitutes forward bias. The holes in the P-type material are repelled from the positive terminal and move toward the junction. The electrons in the N-type FIGURE 8-235. Junction diode.
material are repelled from the negative terminal and likewise move toward the junction. This decreases the space charge existing at the junction, and electron current flow is maintained through the external circuit. The current in the P-type material is in the form of holes, and in the N-type material it is in the form of electrons. If the forward bias is increased, current flow will increase.
If the forward bias is increased excessively, it will cause excessive current. The excessive current will increase thermal agitation and the crystal structure will break down. One important fact worth remembering is that all solid-state devices are sensitive to heat and will be destroyed if the heat becomes too intense.
If the battery connections shown in figure 8-236 are reversed, the junction diode is reverse-biased.
Now the holes are attracted toward the negative terminal and away from the junction. The electrons are attracted toward the positive terminal, also away from the junction. This widens the depletion region, increases the space charge, and reduces current to a minimum condition. It is possible to apply too high a reverse bias. When this happens, the crystal structure will break down.
The symbol for the semiconductor diode is shown in figure 8-237. Note that this is the same FIGURE 8-236. Forward bias on a junetion diode.
High current flow N-type P-type FIGURE 8-237. Semiconductor diode symbol.
symbol used for other types of diodes, such as the copper-oxide and selenium dry-disk rectifiers.
The forward-bias, or high-current, direction is always against the arrow of the symbol.
Figure 8-238 shows a typical characteristic curve for a junction diode. As forward bias is increased a small amount, current flow is increased a considerable amount. For this reason, solid-state devices are said to be current-operated devices, since it is easier to measure the relatively large changes in current flow as compared to the small changes in applied voltage.
With forward bias applied, the diode displays a low-resistance characteristic. On the other hand, with reverse bias applied, a high-resistance state exists. The most important characteristic of a diode is that it allows current to flow in one direction only. This permits solid-state devices to be used in rectifier circuits.
+600 +500 + 400 + 300 +200
- 150 -100 -50 + 100 5.75 2
- 0.5
- 1.0 Milliamperes — 1. .5 + 10V- - - - DC + 10V Input Rectifier OV circuit Output OV — 10V FIGURE 8-239. Rectification process.
Rectification Rectification is the process of changing alternating current to direct current. When a semiconductor rectifier, such as a junction diode, is connected to an a.c. voltage source, it is alternately biased forward and reverse, in step with the a.c.
voltage, as shown in figure 8-239.
In figure 8-240 a diode is placed in series with a source of a.c. power and a load resistor. This is called a half-wave rectifier circuit.
The transformer provides the a.c. input to the circuit; the diode provides the rectification of the
a.c.; and the load resistor serves two purposes:
(1) It limits the amount of current flow in the circuit to a safe level, and (2) it develops an output signal due to the current flow through it.
Assume, in figure 8-241, that the top of the transformer secondary is positive and the bottom negative. With this polarity, the diode is forwardbiased, resistance of the diode is very low, and current flows through the circuit in the direction of the arrows. The output (voltage drop) across the load resistor follows the waveshape of the positive half of the a.c. input. When the a.c.
input goes in a negative direction, the top of the transformer secondary becomes negative and the diode becomes reverse-biased.
With reverse bias applied to the diode, the resistance of the diode becomes very great, and current flow through the diode and load resistor becomes zero. (Remember that a very small current will flow through the diode.) The output, Transformer Diode Input Output Load resistor FIGURE 8-238. Typical junction diode characteristic curve. FIGURE 8-240. Half-wave rectifier circuit.
FIGURE 8-241. Output of a half-wave rectifier.
taken across the load resistor, will be zero. If the position of the diode were reversed, the output would be negative pulses.
In a half-wave rectifier, a half cycle of power is produced across the load resistor for each full cycle of input power. To increase the output power, a full-wave rectifier can be used. Figure 8-242 shows a full-wave rectifier, which is, in effect, two half-wave rectifiers combined into one circuit. In this circuit a load resistor is used to limit current flow and develop an output voltage, two diodes to provide rectification, and a transformer to provide an a.c. input to the circuit. The transformer, used in full-wave rectifier circuits, must be center tapped to complete the path for current flow through the load resistor.
Assuming the polarities shown on the transformer, diode Di will be forward-biased and current will flow from ground through the load resistor, through diode Di, to the top of the transformer.
When the a.c. input changes direction, the transformer secondary will assume an opposite polarity.
Transformer ( Center Diode D, Tapped) Load resistor Diode D, FIGURE 8-242. Full-wave rectifier.
Diode D2 is now forward-biased and current will flow in the opposite direction, from ground through the load resistor, through diode D2, to the bottom half of the transformer.
When one diode is forward-biased, the other is reverse-biased. No matter which diode is forward biased, current will flow through the load resistor in the same direction; so the output will be a series of pulses of the same polarity. By reversing both diodes, the output polarity will be reversed.
The voltage which is felt across a rectifier when reverse bias is being applied is often referred to as "the inverse peak voltage." By definition, this is the peak value of the instantaneous voltage across the rectifier during the half-cycle in which current does not flow or that reverse bias is applied. If an inverse voltage is applied that is too large, the rectifier will be destroyed. The term "breakdown voltage" is often used instead of the term "inverse peak voltage rating," but both terms have the same meaning. Breakdown voltage is the maximum voltage that the rectifier can stand while it is not conducting (reverse-biased); the inverse peak voltage is the voltage actually being applied to the rectifier. As long as the inverse peak voltage is lower than breakdown voltage, there will be no problem of rectifier destruction.
Diode Bridge Rectifier Circuit An advantageous modification of the full-wave diode rectifier is the bridge rectifier. The bridge rectifier differs from the full-wave rectifier in that a bridge rectifier does not require a center-tapped transformer, but does require two additional diodes.
To illustrate how a bridge rectifier performs, consider a sine wave input which is on its positive alternation as denoted on the schematic of figure
8-243. With the secondary of 11 functioning as the bridge rectifier's power supply, point A is the most positive point of the bridge, while B is the most negative. Current flow will be from B to A through the forward-biased diodes. As an aid in finding the path of electron flow, consider the redrawn bridge circuit in figure 8-214. The forward-biased diodes, CR and CR, are easily recognized. Voltage is dropped across each voltage loop as indicated. Thus, on the positive half-cycle input CRs and CR4 are both forward-biased and CRI and CR are reverse-biased.
As long as diode breakdown voltage is not exceeded, current flow will be from point B up and 377