4-31
The filter capacitor in the LC choke-input filter circuit is not subject to extreme voltage surges
because of the protection offered by the inductor. However, the capacitor can become open, leaky, or
shorted.
Shorted turns in the choke may reduce the value of inductance below the critical value. This will
result in excessive peak-rectifier current, accompanied by an abnormally high output voltage, excessive
ripple amplitude, and poor voltage regulation.
A choke winding that is open, or a choke winding which is shorted to the core will result in a
no-output condition. A choke winding which is shorted to the core may cause overheating of the rectifier
element(s) and blown fuses.
With the supply voltage removed from the input to the filter circuit, one terminal of the capacitor can
be disconnected from the circuit. The capacitor should be checked with a capacitance analyzer to
determine its capacitance and leakage resistance. When the capacitor is electrolytic, you must use the
correct polarity at all times. A decrease in capacitance or losses within the capacitor can decrease the
efficiency of the filter and can produce excessive ripple amplitude.
Resistor-Capacitor (RC) Filters
The RC capacitor-input filter is limited to applications in which the load current is small. This type
of filter is used in power supplies where the load current is constant and voltage regulation is not
necessary. For example, RC filters are used in high-voltage power supplies for cathode-ray tubes and in
decoupling networks for multistage amplifiers.
Figure 4-28 shows an RC capacitor-input filter and associated waveforms. Both half-wave and full-
wave rectifiers are used to provide the inputs. The waveform shown in view A of the figure represent the
unfiltered output from a typical rectifier circuit. Note that the dashed lines in view A indicate the average
value of output voltage (E
avg) for the half-wave rectifier. The average output voltage (E
avg) is less than
half (approximately 0.318) the amplitude of the voltage peaks. The average value of output voltage (Eavg)
for the full-wave rectifier is greater than half (approximately 0.637), but is still much less than, the peak
amplitude of the rectifier-output waveform. With no filter circuit connected across the output of the
rectifier circuit (unfiltered), the waveform has a large value of pulsating component (ripple) as compared
to the average (or dc) component.
