______________________________________________________________ Solid State Power Supplies
4-68. With the supply voltage removed from the input to the filter circuit, one terminal
of the capacitor can be disconnected from the circuit. Check the capacitor with a
capacitance analyzer to determine its capacitance and leakage resistance. You must use the
correct polarity at all times when the capacitor is electrolytic. A decrease in capacitance or
losses within the capacitor can decrease the efficiency of the filter and can produce
excessive ripple amplitude.
Resistor-Capacitor Filters
4-69. 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 CRTs and in decoupling networks for multistage amplifiers.
4-70. Figure 4-28 shows an RC capacitor-input filter and associated waveforms. Half-
wave and full-wave rectifiers are used to provide the inputs. The waveforms shown in view
(A) represent the unfiltered output from a typical rectifier circuit. Notice that the dashed
lines in view (A) indicate the average value of output voltage (Eavg) for the half-wave
rectifier. The average output voltage (Eavg) 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.
Figure 4-28. RC Filter and Waveforms
4-71. The RC filter in Figure 4-28 consists of an input filter capacitor (C1), a series
resistor (R1), and an output filter capacitor (C2). This filter is sometimes referred to as an
RC pi-section filter because its schematic symbol resembles the Greek letter π.
23 June 2005
TC 9-62
4-25
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