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Electronic equipment doesn t like the pulsating dc that comes straight from a rectifier. The ripple in the waveform must be smoothed out, so that pure, battery-like dc is supplied. The filter does this.
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Capacitors alone
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The simplest filter is one or more large-value capacitors, connected in parallel with the rectifier output (Fig. 21-8). Electrolytic capacitors are almost always used. They are polarized; they must be hooked up in the right direction. Typical values range in the hundreds or thousands of microfarads.
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The more current drawn, the more capacitance is needed for good filtering. This is because the load resistance decreases as the current increases. The lower the load resistance, the faster the filter capacitors will discharge. Larger capacitances hold charge for a longer time with a given load. Filter capacitors work by trying to keep the dc voltage at its peak level (Fig. 21-9). This is easier to do with the output of a full-wave rectifier (shown at A) as compared with a half-wave circuit (at B). The remaining waveform bumps are the ripple. With a half-wave rectifier, this ripple has the same frequency as the ac, or 60 Hz. With a full-wave supply, the ripple is 120 Hz. The capacitor gets recharged twice as often with a full-wave rectifier, as compared with a half-wave rectifier. This is why the ripple is less severe, for a given capacitance, with full-wave circuits.
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21-8 A simple filter. The capacitor, C, should have a large capacitance.
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21-9 Filtered output for full-wave rectification (A) and half-wave rectification (B).
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Capacitors and chokes
Another way to smooth out the dc from a rectifier is to use an extremely large inductance in series with the output. This is always done in conjunction with parallel capacitance. The inductance, called a filter choke, is on the order of several henrys. If the coil must carry a lot of current, it will be physically bulky. Sometimes the capacitor is placed ahead of the choke. This circuit is a capacitor-input filter (Fig. 21-10A). If the coil comes ahead of the capacitor, the circuit is a choke-input filter (Fig. 21-10B).
21-10 Capacitor-input (A) and choke-input (B) filtering.
Engineers might use capacitor-input filtering when the load is not expected to be very great. The output voltage is higher with a capacitor-input circuit than with a choke-input circuit. If the supply needs to deliver large or variable amounts of current, a choke-input filter is a better choice, because the output voltage is more stable.
392 Power supplies If a supply must have a minimum of ripple, two or three capacitor/choke pairs might be cascaded, or connected one after the other (Fig. 21-11). Each pair is called a section. Multisection filters can consist of either capacitor-input or choke-input sections, but the two types are never mixed.
21-11 Two choke-input filter sections in cascade.
Voltage regulation
A full-wave rectifier, followed by a choke-input filter, offers fairly stable voltage under varying load conditions. But voltage regulator circuitry is needed for electronic devices that are finicky about the voltage they get.
Zener diodes
You learned about Zener diodes in the last chapter. If a reverse-biased Zener diode is connected across the output of a power supply, as shown back in Fig. 20-7, the diode will limit the output voltage of the supply by brute force as long as it has a high enough power rating.
Zener/transistor regulation
A Zener-diode voltage regulator is not very efficient if the load is heavy. When a supply must deliver high current, a power transistor is used along with the Zener diode to obtain regulation (Fig. 21-12). This greatly reduces the strain on the Zener diode, so that a lower-power (and therefore less costly) diode can be used.
Integrated circuits
In recent years, voltage regulators have become available in integrated-circuit (IC) form. You just connect the IC, perhaps along with some external components, at the output of the filter. This method provides the best possible regulation at low and moderate voltages. Even if the output current changes from zero to maximum, the output voltage stays exactly the same, for all practical purposes.
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