_________________________________________________________________ Semiconductor Diodes
current flows through the diode. As indicated by the broken lines (see Figure 1-21), there is
no output developed across the load resistor during the negative alternation of the input
signal. Although only one cycle of input is shown, you should realize that the action
described above continually repeats itself, as long as there is an input. Therefore, since
only the positive half-cycles appear at the output, then this circuit converted the AC input
into a positive pulsating DC voltage. The frequency of the output voltage is equal to the
frequency of the applied AC signal since there is one pulse out for each cycle of the AC
input. For example, if the AC input is 60 Hz then the input frequency would be 60 cycles
per second and the output frequency would be 60 pulses per second.
1-84. However, if the diode were reversed (see Figure 1-21, view (B)), a negative output
voltage would be obtained. This is because the current would be flowing from the top of
RL toward the bottom, making the output at the top of RL negative in respect to the bottom
or ground. Since current flows in this circuit only during half of the input cycle, it is called
a half-wave rectifier.
1-85. The semiconductor diode shown in the figure can be replaced by a metallic
rectifier and still achieve the same results. The metallic rectifier, sometimes referred to as a
dry-disc rectifier, is a metal-to-semiconductor, large-area contact device. Its construction is
distinctive; a semiconductor is sandwiched between two metal plates, or electrodes (see
Figure 1-22). Notice that a barrier, with a resistance many times greater than that of the
semiconductor material, is constructed on one of the metal electrodes. The contact having
the barrier is a rectifying contact; the other contact is nonrectifying. Metallic rectifiers act
just like the diodes previously covered in that they permit current to flow more readily in
one direction than the other. However, the metallic rectifier is fairly large compared to the
crystal diode shown in Figure 1-23. The reasons for this is that metallic rectifier units are
stacked (to prevent inverse voltage breakdown), have large area plates (to handle high
currents), and usually have cooling fins (to prevent overheating).
1-86. There are many known metal-semiconductor combinations that can be used for
contact rectification. Copper oxide and selenium devices are by far the most popular.
Copper oxide and selenium are often used over other types of metallic rectifiers because
they have a large forward current per unit contact area, low forward voltage drop, good
stability, and a lower aging rate. In practical applications, the selenium rectifier is used
where a relatively large amount of power is required. However, copper-oxide rectifiers are
generally used in small-current applications such as AC meter movements or for delivering
DC to circuits requiring not more than 10 amperes.
1-87. Since metallic rectifiers are affected by temperature, atmospheric conditions, and
aging (in the case of copper oxide and selenium), they are being replaced by the improved
silicon crystal rectifier. The silicon rectifier replaces the bulky selenium rectifier as to
current and voltage rating and can operate at higher ambient (surrounding) temperatures.
23 June 2005