Filtering

Most dc-powered devices need something better—more pure—than the rough, pulsating dc that comes right out of a rectifier circuit. The pulsations (ripple) in the rectifier output can be eliminated by a filter.

Capacitors Alone

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A large-value capacitor can be used all by itself as a power-supply filter.
The simplest power-supply filter consists of one or more large-value capacitors, connected in parallel with the rectifier output (above figure). A good component for this purpose is known as an electrolytic capacitor. This type of capacitor is polarized, meaning that it must be connected in the correct direction in the circuit. Each capacitor is also rated for a certain maximum voltage. Pay attention to these ratings if you ever work with electrolytic capacitors! Filter capacitors work by trying to maintain the dc voltage at its peak level, as shown in below figure. This is easier to do with the output of a full-wave rectifier (drawing A) than with the output of a half-wave rectifier (drawing B). With a full-wave rectifier receiving a 60-Hz ac electrical input, the ripple frequency is 120 Hz, but with a half-wave rectifier it is 60 Hz. The filter capacitors are thus recharged twice as often with a full-wave rectifier, as compared with a half-wave rectifier. This is why full-wave rectifier circuits produce more pure dc than half-wave rectifier circuits.

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Filtering of ripple from a full-wave rectifier (A) and from a half-wave rectifier (B).

Capacitors and Chokes

Another way to smooth out the dc from a rectifier is to place a large-value inductor in series with the output, and a large-value capacitor in parallel. The inductor is called a filter choke.

In a filter that uses a capacitor and an inductor, the capacitor can be placed on the rectifier side of the choke. This is a capacitor-input filter (Below Figure A). If the filter choke is placed on the rectifier side of the capacitor, the circuit is a choke-input filter (below figure B). Capacitor-input filtering can be used when a power supply is not required to deliver much current. The output voltage, when the load is light (not much current is drawn), is higher with a capacitor-input filter than with a choke input filter having identical input. 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.
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At A, a capacitor-input filter. At B, a choke input filter.
If the output of a power supply must have an absolute minimum of ripple, two or three capacitor/ choke pairs can be connected in cascade (following figure). Each pair constitutes a section of the filter. Multisection filters can consist of capacitor-input or choke-input sections, but the two types are never
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Two choke-input filter sections in cascade.
mixed. In the example of above figure, both capacitor/choke pairs are called L sections. If the second capacitor is omitted, the filter becomes a T section. If the second capacitor is moved to the input and the second choke is omitted, the filter becomes a pi section. These sections are named because their schematic diagrams look something like the uppercase English L, the uppercase English T, and the uppercase Greek Π,respectively.