Half-wave and full-wave rectifiers

Because a diode conducts only when forward biased, an alternating voltage across it drives current only in that part of the cycle. Turning ac into one-way voltage like this is rectification, and the circuit is a rectifier.

Half-wave rectifier: one diode in series with a load resistor RL across a transformer secondary (terminals A and B). When A is positive the diode conducts; when A is negative it is reverse biased and blocks.

The reverse saturation current is so small it counts as zero, so the output is a train of positive half-cycles with gaps: still varying, but in one direction only.

The diode's reverse breakdown voltage must be well above the peak ac voltage of the secondary, or the negative half-cycle would break it down.

Full-wave rectifier: two diodes whose p-sides go to the two ends of a centre-tapped secondary. Their n-sides are joined, and the output is taken between that common point and the centre tap.

Each diode sees only half of the total secondary voltage. When A is positive with respect to the centre tap, B is negative (out of phase): D1 conducts and D2 blocks.

In the other half-cycle A is negative and B positive, so D2 conducts and D1 blocks. Current flows through RL in the same direction both times, giving output in both halves of the cycle.

Full-wave rectification uses both halves, so it is more efficient than half-wave. A second full-wave circuit, the bridge, uses four diodes and needs no centre tap.

Output pulses: half-wave gives one per input cycle and full-wave two. For 50 Hz input, the output frequency is 50 Hz for half-wave and 100 Hz for full-wave (Exercise 14.6).