A rectifier is a device that transforms AC into DC 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 handbook Figure 12-215 illustrates the basic concept of a half-wave rectifier. When an AC signal is on a positive swing as shown in Figure 12-215A, the polarities across the diode and the load resistor are also positive. In this case, the diode is forward biased and can be replaced with a short circuit as shown in the figure. The positive portion of the input signal appears across the load resistor with no loss in potential across the series diode.

Figure 12-215B now shows the input signal being reversed.

minus the negative voltage swing of the wave. For this reason, this type of rectifier is called a half-wave rectifier.

rectifier. The term “full-wave” indicates that the output is a continuous sequence of pulses rather than having gaps that appear in the half-wave rectifier.

Figure 12-216C shows the initial condition, during which, a positive portion of the input signal is applied to the network. Note the polarities across the diodes. Diodes D2 and D4 are reverse biased and can be replaced with an open circuit. Diodes D1 and D3 are forward biased and act as an open circuit. The current path through the diodes is clear to see, and the resulting waveform is developed across the load resistor.

During the negative portion of the applied signal, the diodes reverse their polarity and bias states. The result is a network shown in Figure 12-216D. Current now passes through diodes D4 and D2, which are forward biased, while diodes D1 and D3 are essentially open circuits due to being reverse biased.

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 the copper oxide rectifier, the selenium rectifier, and the magnesium copper-sulfide rectifier. The copper oxide rectifier consists of a copper disk upon which a layer of copper oxide has been formed by heating. [Figure 12-217] 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.

Today, there are many varieties of diodes that can be grouped into one of several basic categories.