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21-2 A selenium rectifier can be recognized by its heatsink.
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386 Power supplies Diodes can be connected in parallel to increase the current rating. When this is done, small-value resistors are placed in series with each diode in the set to equalize the current burden among the diodes (Fig. 21-3). Each resistor should have a voltage drop of about 1 V.
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21-3 When diodes are connected in parallel, resistors help equalize the current load.
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Peak inverse voltage
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The PIV rating of a diode is the instantaneous inverse, or reverse-bias, voltage that it can withstand without avalanche taking place. A good power supply has diodes whose PIV 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 for part of the reverse cycle. This will degrade the efficiency of the supply; the reverse current will buck the forward current. It would be like having a team of rowers in a long boat, 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 diode alone. This scheme is sometimes seen in high-voltage supplies, such as those needed for tube-type ham radio power amplifiers. High-value resistors, of about 500 for each peak-inverse volt, are placed across each diode in the set to distribute the reverse bias equally among the diodes (Fig. 21-4). Also, each diode is shunted by a capacitor of 0.005 F or 0.1 F.
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21-4 Diodes in series should be shunted by resistors and capacitors.
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The half-wave rectifier
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The 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, diagrammed in Fig. 20-1. In a half-wave circuit, the average output voltage is approximately 45 percent of the rms ac input voltage. But the PIV across the diode can be as much as 2.8 times the rms ac input voltage. It s a good idea to use diodes whose PIV ratings are at least 1.5 times the
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The bridge rectifier 387 maximum expected PIV; therefore, with a half-wave supply, the diodes should be rated for at least 4.2 times the rms ac input voltage. Half-wave rectification has some shortcomings. First, the output is hard to smooth out, because the waveform is so irregular. Second, the voltage output tends to drop when the supply is connected to a load. (This can be overcome to some extent by means of a good voltage regulator. Voltage regulation is discussed later in this chapter.) Third, half-wave rectification puts a disproportionate strain on the power transformer and 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 a half-wave circuit in these situations is that it costs a little less than full-wave or bridge circuits.
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The full-wave, center-tap rectifier
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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 from the last chapter. 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.
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