The Choke-Input Filter

Author: N.H. Crowhurst

The choke-input filter. The output of a choke filter is always 0.637 times the peak alternating voltage or 0.9 times its RMS value.

The voltage at the output end of the choke (provided its resistance is reasonably low) is constant because the choke averages out a voltage fluctuation at its input end that is always the same - from zero to the peak of the alternating voltage. According to each half-wave of the rectifier waveform, the output of a choke input filter is always 0.637 times the peak alternating voltage which is (0.637/0.707) or 0.9 times its rms value.

In practice, the output will be slightly lower than this figure, owing to the voltage drop in the resistance of the choke. Further smoothing is achieved by means of a capacitor connected at the output end of the choke. It does not act as a reservoir capacitor as in the case of capacitor input, but merely serves to minimize voltage fluctuation by soaking up the slight current fluctuation in the choke.