The full-wave, center-tap rectifier

Chapter The full-wave, center-tap rectifier

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • maximum expected PIV; therefore, with a half-wave supply, the diodes should be ratedfor at least 4.2 times the rms ac input voltage.Half-wave rectification has some shortcomings. First, the output is hard to smoothout, because the waveform is so irregular. Second, the voltage output tends to dropwhen the supply is connected to a load. (This can be overcome to some extent bymeans of a good voltage regulator. Voltage regulation is discussed later in this chapter.)Third, half-wave rectification puts a disproportionate strain on the power transformerand the diodes.Half-wave rectification is useful in supplies that don’t have to deliver much current,or that don’t need to be especially well regulated. The main advantage of using ahalf-wave circuit in these situations is that it costs a little less than full-wave or bridgecircuits.The full-wave, center-tap rectifierA much better scheme for changing ac to dc is to use both halves of the ac cycle. Sup-pose you want to convert an ac wave to dc with positive polarity. Then you can allow thepositive half of the ac cycle to pass unchanged, and flip the negative portion of the waveupside-down, making it positive instead. This is the principle behind full-waverectification.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 sec-ondary winding, is connected to common ground. This produces out-of-phase waves atthe ends of the winding. These two waves can be individually half-wave rectified, cut-ting 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 rmsac input voltage. The PIV across the diodes can be as much as 2.8 times the rms inputvoltage. Therefore, the diodes should have a PIV rating of at least 4.2 times the rms acinput.Compare Fig. 21-5B with Fig. 20-1B from the last chapter. Can you see that thewaveform of the full-wave rectifier ought to be easier to smooth out? In addition to thisadvantage, the full-wave, center-tap rectifier is kinder to the transformer and diodesthan a half-wave circuit. Furthermore, if a load is applied to the output of the full-wavecircuit, the voltage will drop much less than it would with a half-wave supply, becausethe output has more “substance.”The bridge rectifierAnother way to get full-wave rectification is the bridge rectifier. It is diagrammed inFig. 21-6. The output waveform is just like that of the full-wave, center-tap circuit.The average dc output voltage in the bridge circuit is 90 percent of the rms ac in-put voltage, just as is the case with center-tap rectification. The PIV across the diodesis 1.4 times the rms ac input voltage. Therefore, each diode needs to have a PIV ratingof at least 2.1 times the rms ac input voltage.The bridge rectifier387