Rectifier - Linear Power Supply

# The Rectifier

The rectifier converts the AC from the power grid to pulsing DC. Here is a diagram of the alternating current as it comes from the power grid.

 Original sine wave from the power grid.

This waveform shows the voltage going from 0 volts to +16 volts then to -16 volts and back repeatedly. This is much like the piston in an air compressor. When the piston goes up it increases the pressure, let's say to 100 psi. When the piston goes down it creates a vacuum. This is like a negative voltage. Once the compressed air has been pushed out of the cylinder by the piston, a check valve prevents air from returning to the cylinder. Now, instead of getting alternating positive and negative pressure, you only get positive pressure. A rectifier does the same thing with electricity. Once the electricity has been pushed past the rectifier it cannot return, so we only get positive voltages after going through the rectifier.

There are two types of rectifiers: half wave and full wave.

## Half-wave rectifier

A half-wave rectifier is simply a diode in series with the load. This is most like the check valve in a compressed air system. Since the diode will only conduct current in one direction, only the positive halves of the sine wave cycles make it past the rectifier.

 A half-wave rectifier

The half-wave rectifier essentially chops off the negative half of each cycle leaving a series of positive pulses.

 Sine wave after passing through a half-wave rectifier.

If all you need is some pulsing DC and don't mind losing half of the energy in the AC power this is adequate. In parts of the world where the frequency standard is 60 Hz the pulses occur every 16.66 mS. The frequency of the pulses remains at 60 Hz. In parts of the world where the frequency standard is 50 Hz the frequency remains at 50 Hz

## Full-wave Rectifier

There are two main types of full wave rectifier. The following one uses two diodes and requires a center-tapped transformer. Only half of the transformer secondary is used during each half cycle. This means the transformer needs twice as many turns on the secondary winding as a half-wave rectifier. For a 12 volt power supply you need a 24 volt center-tapped secondary.

 Full-wave rectifier

During the first half of the cycle the current goes through the top diode. During the second half it goes through the bottom diode.

 Current flow during the first half cycle. Current flow during the second half cycle.

Either way, the current travels the same direction through the load. The result is that the negative half of the cycle is inverted to become positive in relation to the load.

 Sine wave after passing through a full-wave rectifier.

This results in a series of DC pulses at a rate of 120 per second (100 per second where the frequency standard is 50 Hz). All of the energy contained in the AC waveform is available. This also requires less filtering (see Filter, the next section).

## Full-wave Bridge Rectifier

This circuit uses four diodes arranged in a bridge configuration.

 Full-wave Bridge Rectifier

The full-wave bridge rectifier has the advantage that it doesn't require a center-tapped transformer. For a 12 volt power supply you only need a 12 volt non-tapped secondary. The full-wave bridge rectifier operates as shown below.

 Current flow during the first half cycle. Current flow during the second half cycle.

As in the previous full-wave rectifier, the current always flows in the same direction through the load.

One slight disadvantage of the full-wave bridge rectifier is that there are always two diodes in series with the load. This means there is a 1.4 volt loss from the transformer to the load.

Full-wave Bridge Rectifier