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The Capacitor-Input Filter

Author: N.H. Crowhurst

Capacitor-input filter using a choke

The most common circuit used for smoothing is the capacitor-input filter. It produces a starting voltage in the same way as a capacitor connected directly across the load. The load current is then passed through a choke and another capacitor is connected after the choke. Because the fluctuation at the input end of the choke is now quite small, the choke can do much more toward stabilizing the current passing through it. Any residual fluctuations in voltage that still might appear at its output end are "soaked up" by the second capacitor.

For low-current supplies, or even moderately larger current supplies (up to 100 or 200 milliamps in modern amplifiers), a resistor is sometimes used to replace the choke; this is an economy measure. (Resistors are considerably cheaper than chokes, and modern electrolytic-type capacitors can get very large values of capacitance into quite a small space at low cost.) The disadvantage of the resistor is that it produces a voltage drop so that the rectified voltage needed is appreciably higher than the required output voltage.

Capacitor-input filter using a resistor

A problem that arises with a capacitor-input filter is that the rectified output voltage always changes with load current. The reason for this is that the load current determines how much the voltage drops between charging pulses. The output voltage is averaged between these peaks and the amount that the voltage drops between them. A larger load current produces a bigger drop, and the average output voltage drops as well. Some kinds of audio circuits require considerable fluctuation in plate current of the output tubes. If the capacitor-input filter is used, the supply voltage also fluctuates with the current. This is where the choke-input filter has an advantage.

In a capacitor-input filter the rectified output voltage always changes with the load current

Last Update: 2010-11-03