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Reverse Biased Diode When the battery is turned around as shown in Figure 10-199, then the diode is reverse biased and current will not flow. The most noticeable effect seen in this illustration is the widened depletion zone.
The applied battery voltage is in the same direction as the depletion zone field. Because of this, holes and electrons tend to move away from the junction. Simply stated, the negative terminal attracts the holes away from the junction, and the positive terminal attracts the electrons away from the barrier. Therefore, the result is a wider depletion zone. This action increases the barrier width because there are more negative ions on the P-side of the junction and more positive ions on the N-side of the junction. This increase in the number of ions at the junction prevents current flow across the barrier by the majority carriers. To summarize, the important thing to remember is that these PN junction diodes will offer very little resistance to current in a forward biased diode. Maximum resistance will happen when the diode is reversed biased. Figure 10-200 shows a graph of the current characteristics of a diode that is biased in both directions.
Rectifiers 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 aircooled AC motors that drive DC generators, eliminate this objection because they operate directly off the AC power system. Vacuum tube or various types of solidstate 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 that transforms alternating current into direct current by limiting or regulating the direction of current flow. The principal types of rectifiers are dry disk and solid state. Solid-state, or semiconductor, rectifiers have replaced virtually all other types; and, since dry disk and motor generators are largely limited to older model aircraft, the major part of the study of rectifiers is devoted to solid-state devices used for rectification. The two methods discussed in this text are the halfwave rectifier and the full-wave rectifier. |
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