Bridge rectifier consists of four diodes connected in a bridge circuit that provides the output voltage of same polarity for any polarity of the input voltage. Bridge rectifier is used for conversion of alternating current (AC) input into direct current (DC) output.
There are two simplest type of rectifier circuit used namely half wave rectifier and full wave rectifier. Half-wave rectifier only allows one half of an AC waveform to pass through to the load while full wave rectifier use both half-cycles of the sine wave. Both use components called diodes to convert AC into DC.
A simple half-wave rectifier can be constructed by using a single diode in a one phase supply. A diode is a device, which only allows current to flow through it in one direction. Thus, when the AC input is positive, the diode is forward-biased, allowing the current to pass through it. But when the AC input is negative, the diode is reverse-biased due to which current passing through the diode is obstructed.
A half wave rectifier acts like a clipper. In half wave rectification process, the positive or negative half cycle of the AC wave is passed while the other half of the AC wave cycle is chopped. Usually, chopping of the wave depends on the polarity of the rectifier.
Since half wave rectifier produces positive DC output only by using half cycle, it couldn`t be used as a power supply for a circuit. In addition, the output voltage continually varies between 0V and Vs-0.7V, and for half the time there is no output at all, which further makes it unsuitable for making a reliable power supply.
Full wave rectifier
A full wave rectifier is constructed using four diodes. The four diodes in full wave rectifier are arranged in such a manner that both the positive and negative parts of the AC waveform are converted to DC.
A full wave rectifier uses both half-cycles of the sine wave to produce a DC output consisting of both negative and positive cycle. This is achieved by reversing the negative (or positive) portions of the alternating current waveform. Thus, the positive (negative) portions gets added to the reversed negative (positive) portions to produce a complete positive(negative) voltage/current waveform.
· Since full-wave rectifier uses two diodes at a time, there is a voltage loss of 1.4V across the diodes.
· Although full wave rectifier uses two diodes out of four diodes, it still cannot be used for a power supply, as there is a loss of 0V to 1.4V at the output.
The loss of 0.7V, which is called threshold voltage, due to a diode results in a loss of peak input voltage to the peak output voltage. Thus in a diode bridge circuit, the peak output will be lower than the peak input by an amount equal to twice this value. Also, since the diodes does not conduct below the threshold voltage, the circuit cannot pass current instantly when required during changing of states, causing short segments of zero voltage to appear between each "hump"