The full-wave, center-tap rectifier
A much better scheme for changing ac to dc is to use both halves of the ac cycle. Suppose
you want to convert an ac wave to dc with positive polarity. Then you can allow the
positive half of the ac cycle to pass unchanged, and flip the negative portion of the wave
upside-down, making it positive instead. This is the principle behind full-wave
rectification.
One common full-wave circuit uses a transformer with a center-tapped secondary,
as shown in Fig. 21-5A. The center tap, a wire coming out of the exact middle of the secondary
winding, is connected to common ground. This produces out-of-phase waves at
the ends of the winding. These two waves can be individually half-wave rectified, cutting
off the negative half of the cycle. Because the waves are 180 degrees (half a cycle)
out of phase, the output of the circuit has positive pulses for both halves of the cycle
(Fig. 21-5B).
In this rectifier circuit, the average dc output voltage is about 90 percent of the rms
ac input voltage. The PIV across the diodes can be as much as 2.8 times the rms input
voltage. Therefore, the diodes should have a PIV rating of at least 4.2 times the rms ac
input.
Compare Fig. 21-5B with Fig. 20-1B . Can you see that the
waveform of the full-wave rectifier ought to be easier to smooth out? In addition to this
advantage, the full-wave, center-tap rectifier is kinder to the transformer and diodes
than a half-wave circuit. Furthermore, if a load is applied to the output of the full-wave
circuit, the voltage will drop much less than it would with a half-wave supply, because
the output has more “substance.”
outlawstc
