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Filtering

Author: N.H. Crowhurst

 A choke is used to smooth out rectifier output

One method of smoothing is to put a choke in series with the feed to the amplifier. If we regard the amplifier as a resistance taking a constant current at a constant voltage, a choke of sufficiently large inductance will pass almost constant current and allow a considerable voltage fluctuation across itself. (If an inductance is large, a very large voltage is necessary to produce only a small change in current.) In this case, the voltage at the input to the choke is the same fluctuating voltage that comes out from the rectifier, whereas the output of the choke is an smooth d-c because of the almost constant current in the resistance "load."

The other element of a smoothing arrangement is a capacitor. If we connect a capacitor in parallel with the resistance load that represents the amplifier, the rectifier will charge this capacitor up to the peak value of the alternating voltage. As the wave dips back toward zero, the capacitor will maintain the output current by discharging into the load, thereby keeping the voltage nearly constant between peaks. If only a small current is taken by the load relative to the charge contained in the capacitor at this voltage, the output of a rectifier, using a capacitor in this way, will come very close to the peak voltage of the a-c waveform.

 An input capacitor may also be used to smooth out output

If the capacitor is not large enough to maintain this high a charge over the interval between consecutive half-cycle pulses, the voltage will drop away more during this interval, and the average output voltage will not be quite so high as the peak of the alternating waveform.

A single choke or a single capacitor does not smooth out the ripple completely. The choke has to have a fluctuating current, however small, to produce the fluctuating voltage across its terminals. The voltage across the capacitor drops by some amount, however small, between charges, before the next pulse comes along to restore the charge. Hence we need further smoothing action to get an adequately smooth or steady d-c and to avoid producing hum in the amplifier.

Last Update: 2010-11-03