Rectifier diodes are available in various sizes, intended for different purposes. Most rectifier diodes are made of silicon, and are known as silicon rectifiers. Some are fabricated from selenium, and are called selenium rectifiers. Two important features of a power-supply diode are the average forward current (Io) rating and the peak inverse voltage (PIV) rating.
Average Forward Current
Electric current produces heat. If the current through a diode is too great, the heat will destroy the P-N junction. When designing a power supply, it is wise to use diodes with an Io rating of at least 1.5 times the expected average dc forward current. If this current is 4.0 A, for example, the rectifier diodes should be rated at Io = 6.0 A or more.
Note that Io flows through the diodes. The current drawn by the load is often different from this. Also, note that Io is an average figure. The instantaneous forward current is another thing, and can be 15 or 20 times the Io, depending on the nature of the filtering circuit. Some diodes have heatsinks to help carry heat away from the P-N junction. A selenium diode can be recognized by the appearance of its heatsink, which looks something like a baseboard radiator built around a steam pipe.
Diodes can be connected in parallel to increase the current rating over that of an individual diode. When this is done, small-value resistors should be placed in series with each diode in the set to equalize the current. Each resistor should have a value such that the voltage drop across it is about 1 V under normal operating conditions.
Peak Inverse Voltage
The PIV rating of a diode is the instantaneous reverse-bias voltage that it can withstand without the avalanche effect taking place. A good power supply has diodes whose PIV ratings are significantly greater than the peak ac input voltage. If the PIV rating is not great enough, the diode or diodes in a supply conduct for part of the reverse cycle. This degrades the efficiency of the supply because the reverse current bucks the forward current.
Diodes can be connected in series to get a higher PIV capacity than a single diode alone. This scheme is sometimes seen in high-voltage supplies, such as those needed for tube-type power amplifiers. High-value resistors, of about 500 Ω for each peak-inverse volt, are placed across each diode in the set to distribute the reverse bias equally among the diodes. In addition, each diode is shunted by (that is, connected in parallel with) a capacitor of 0.005 μF or 0.1 μF.