full-wave, center-tap rectifier:

A much better scheme for changing alternating current to direct current is to use both halves of the alternating current cycle. Assume that you want to convert an alternating current wave to direct current with positive polarity. Then you can allow the positive 1/2 of the alternating current cycle to pass unchanged, and flip negative portion of wave upside down, making it positive. This is principle behind the full-wave rectification.

One common full wave circuit uses a transformer with the center tapped secondary, as shown in the Figure given below. The center tap, a wire coming out of the middle of the secondary winding, is connected to common ground. This generates out of phase waves at the ends of winding. These 2 waves can be individually 1/2-wave rectified, cutting off the negative 1/2 of the cycle. As the waves are 180 °(half a cycle) out of phase, the output of circuit has positive pulses for both halves of cycle.

In this rectifier circuit, average direct current output voltage is around 90 % of the RMS alternating current input voltage. The PIV across diodes can be 2.8 times the RMS input voltage. Thus, the diodes should have a PIV rating of 4.2 times the RMS alternating current input. The waveform of full-wave rectifier ought to be easier to smooth out? In addition to this benefit, the full-wave, center-tap rectifier is helpful to the transformer and diodes than a 1/2-wave circuit. Further, if a load is applied to output of the full wave circuit, the voltage will drop less than it would with a 1/2-wave supply, as the output has more substance.