The half-wave rectifier

Chapter The half-wave rectifier

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

  • Diodes can be connected in parallel to increase the current rating. When this isdone, small-value resistors are placed in series with each diode in the set to equalize thecurrent burden among the diodes (Fig. 21-3). Each resistor should have a voltage dropof about 1 V.386 Power supplies21-3When diodes areconnected in parallel,resistors help equalizethe current load.Peak inverse voltageThe PIV rating of a diode is the instantaneous inverse, or reverse-bias, voltage that itcan withstand without avalanche taking place. A good power supply has diodes whosePIV ratings are significantly greater than the peak voltage of the ac at the input.If the PIV rating is not great enough, the diode or diodes in a supply will conduct forpart of the reverse cycle. This will degrade the efficiency of the supply; the reverse cur-rent will “buck” the forward current. It would be like having a team of rowers in a longboat, with one or two rowers trying to propel the boat backwards instead of forwards.Diodes can be connected in series to get a higher PIV capacity than a single diodealone. This scheme is sometimes seen in high-voltage supplies, such as those needed fortube-type ham radio power amplifiers. High-value resistors, of about 500 Ω for eachpeak-inverse volt, are placed across each diode in the set to distribute the reverse biasequally among the diodes (Fig. 21-4). Also, each diode is shunted by a capacitor of0.005 µF or 0.1 µF.21-4Diodes in seriesshould be shunted byresistors andcapacitors.The half-wave rectifierThe simplest rectifier circuit uses just one diode (or a series or parallel combination) to“chop off” half of the ac input cycle. You saw this circuit in the previous chapter, dia-grammed in Fig. 20-1.In a half-wave circuit, the average output voltage is approximately 45 percent of therms ac input voltage. But the PIV across the diode can be as much as 2.8 times the rmsac input voltage. It’s a good idea to use diodes whose PIV ratings are at least 1.5 times the