### Full-Wave Center-Tap Circuit

At A, the output of a half-wave rectifier. At B, the output of a fullwave rectifier. Note the difference in how the effective (eff ) voltages compare with the peak voltages.
A better scheme for changing ac to dc takes advantage of both halves of the ac cycle. A full-wave center-tap rectifier has a transformer with a tapped secondary (above figure B). The center tap is connected to electrical ground, also called chassis ground. This produces voltages and currents at the ends of the winding that are in phase opposition with respect to each other. These two ac waves can be individually half-wave rectified, cutting off one half of the cycle and then the other, over and over. The effective output voltage from a power supply that uses a full-wave center-tap rectifier is greater, relative to the peak voltage, than is the case with the half-wave rectifier (above figure B). The PIV across the diodes can, nevertheless, be as much as 2.8 times the applied rms ac voltage. Therefore, the diodes should have a PIV rating of at least 4.2 times the applied rms ac voltage to ensure that they won’t break down.

The output of a full-wave center-tap rectifier is easier to filter than that of a half-wave rectifier because the frequency of the pulsations in the dc (known as the ripple frequency) from a full-wave rectifier is twice the ripple frequency of the pulsating dc from a half-wave rectifier, assuming identical ac input frequency in either situation. If you compare above figure B with above figure A, you will see that the full-wave-rectifier output is closer to pure dc than the half-wave rectifier output.

Another advantage of a full-wave center-tap rectifier is the fact that it’s gentler with the transformer and diodes than a half-wave rectifier. Yet another asset: When a load is applied to the output of a power supply that uses a full-wave center-tap rectifier circuit, the voltage drops less than is the case with a half-wave supply. But because the transformer is more sophisticated, the full-wave center-tap circuit costs more than a half-wave circuit that delivers the same output voltage at the same rated maximum current.