Many devices in an aircraft require high amperage, low voltage dc for operation. This power may be furnished by dc engine driven generators, motor generator sets, vacuum tube rectifiers, or dry disk or solid state rectifiers.
In aircraft with ac systems, a special dc 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 ac motors that drive dc generators, eliminate this objection because they operate directly off the ac power system. Vacuum tube or various types of solid state rectifiers provide a simple and efficient method of obtaining high voltage dc 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.
A motor generator is an ac motor and a dc 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, ac system and develop a direct current of 200 amperes at 30 volts with an approximate 28 ampere current drain on the ac system. Units similar to those used in airplanes with dc systems are provided for voltage regulation and paralleling of motor generator sets.
A motor generator offers a number of advantages as a source of dc power in an airplane. With a motor generator, a momentary interruption of ac power does not cut off the dc power completely, since the inertia of the armature keeps it turning during the ac power interruption. Extreme temperature changes affect a motor generator 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 motor generator set is rapidly being replaced by various solid state power sources.
|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 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 current carrying 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 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 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.
Rectification is the process of changing alternating current to direct current. When a semiconductor rectifier, such as a junction diode, is connected to an ac voltage source, it is alternately biased forward and reverse, in step with the ac voltage, as shown in figure 8-239.
In figure 8-240 a diode is placed in series with a source of ac power and a load resistor. This is called a halfwave rectifier circuit.
The transformer provides the ac input to the circuit; the diode provides
the rectification of the ac; 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 forward biased, 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 ac input. When the ac input goes in a negative direction, the top
of the transformer secondary becomes negative and the diode becomes reverse
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, taken across the load resistor, will be zero. If the position of the diode were reversed, the output would be negative pulses.
In a halfwave 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 fullwave rectifier can be used. Figure 8-242 shows a fullwave rectifier, which is, in effect, two halfwave 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 ac input to the circuit. The transformer, used in fullwave 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 D1 will be forward biased and current will flow from ground through the load resistor, through diode D1, to the top of the transformer.
When the ac input changes direction, the transformer secondary will assume an opposite polarity. 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 fullwave diode rectifier is the bridge rectifier. The bridge rectifier differs from the fullwave 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 T1 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 8-244. The forward biased diodes, CR3 and CR4 are easily recognized. Voltage is dropped across each voltage loop as indicated. Thus, on the positive half cycle input CR3 and CR4 are both forward biased and CR1 and CR2 are reverse biased.||
As long as diode breakdown voltage is not exceeded, current flow will be from point B up and across CR4 down and to the left across RL. After current crosses RL, it will flow to point A through CR3. Notice that current flow across RL is from right to left, or in respect to polarity, a negative half cycle output for positive half cycle input.
Remember that, when tracing current flow for the negative half cycle, electron flow through a diode is against the symbolic arrow and from negative to a less negative or positive point. Therefore, no confusion should arise when tracing electron flow up to and away from the common point between CR3 and CR1. Although it may appear that CR1 as well as CR4 is forward biased, such is not the case. The collector of CR1 is more negative than its emitter; therefore, it is reverse biased.
Since, on the negative half cycle, CR1 and CR2 are forward biased, the output signal on the negative half cycle is negative.
Since both half cycles of the input signal result in negative output pulses, the bridge rectifier has accomplished the same goal as the fullwave diode rectifier.