A diode rectifier is often used to convert the AC waveform at the secondary, to a DC waveform. A very common general purpose rectifier diode is the 1N4001 series.

Diodes

As a first order approximation, diodes act like a voltage polarity (or current direction) switch. They will tolerate only a reverse (or negative polarity) voltage and a forward current.

This characteristic curve suggests that the diode does not consume power since the there is no voltage drop across it when it is conducting current.

Real diodes are not perfect rectifiers. They exhibit both junction drop and bulk resistance. In fact, there is a voltage drop across the diode when it conducts. This drop is approximately 0.7 volts for most silicon diodes.

If the circuit draws significant current, the power rating of the diode must be considered. However, an actual diode exhibits yet another characteristic called bulk resistance.

Under heavy load current conditions, the voltage drop across the diode can increase by as much as 2.5 volts.

An even more detailed examination of the diode reveals that it is even more complex than this third approximation suggests, since a real diode does not have such an abrupt change in its characteristic curve. Furthermore, if the reverse voltage exceeds the maximum reverse bias diode rating, the diode will breakdown and likely self-destruct.

When a power supply if first turned on, there is an initial surge current which charges the filter capacitors. This current is often at least twice the rated power supply load current.

Some general rules for selecting rectifier diodes:

• Only use diodes specifically designed for power supply applications.
• Always calculate the diode power dissipation by multiplying the maximum peak current by 0.7 volts and then double the result as a safety margin.
• The minimum reverse bias breakdown voltage should be at least 1.4 x the peak input voltage for a full-wave rectifier and 2.8 x for a half-wave rectifier circuit.
• The surge current is often twice the maximum load current.

There are three basic types of rectifier circuits:

• Half wave rectifier
• Full-wave bridge rectifier
• Center-tapped full wave rectifier

Half-Wave Rectifier	Approximation Formulas

Full-Wave Bridge Rectifier	Approximation Formulas

Center-Tapped Full-Wave Rectifier	Approximation Formulas

The center-tapped circuit has less loss than the bridge rectifier since the current on each half-cycle passed through only one diode. However, it does require a larger transformer since the two secondary windings only conduct current on alternate half-cycles.

A center tapped bridge circuit is used to create a dual polarity supply.