Electronics/Rectifier

Rectifiers are used to make DC out of AC. Usually equipment that uses semiconductors, such as TV sets or computers, operate on DC, yet they obtain their power from the AC socket, which is commonly found in buildings.

While the voltage supplied is usually either about 115 or 240 Volt, the equipment connected to this AC supply may use different DC voltages, and there is a need for suitable transformers to change the line/mains voltage to whatever is needed. This lower voltage is then fed into a set of rectifier diodes, often in the form of a "rectifier bridge" that consists of 4 rectifier diodes, all 4 often contained in a single plastic package with 4 connections: 2 for the applied AC voltage, and the other 2 for the + and - DC output voltage. Different rectifier ratings exist to provide a suitable range of maximum available output current. The frequency (usually 50 or 60 Hz) is important only for the transformer rating, not the rectifiers.

Normally there is a comparatively large capacitor connected across the DC terminals to "smooth" the DC output voltage, the capacitor's size depending on not only the acceptable maximum "ripple" of the DC output voltage, (AC superimposed on the DC voltage) but also on the maximum current rating of the rectifier arrangement.

example

Reference: "Silicon Zener Diode and Rectifier Handbook, theory, design characteristics and applications" published in 1961 by Motorola Inc.

• from page 119 "The Diffused-Junction Silicon Rectifier":

The Forward Current (Amps) versus Forward Voltage (Volts) graph shows that there are slight differences depending on the rectifier's temperature, but only a very small current flows unless the voltage is at least about 0.6 V. The current is about 2 A, with no resistance in the circuit, when the voltage is about 1 V.

• Note that rectifier power ratings must not be exceeded in order to prevent damage, and any graph should also show the proper curve according to the maximum permitted current versus voltage relationship.
• from page 120: The typical static current versus reverse voltage characteristic curves, again depending on the temperature of the rectifier, show that, with a reverse voltage of about 500 V, only about 10 microamperes flow. There are one million microamperes in an Ampere. A breakdown happens if the reverse voltage is too high.