3-31
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 as
part of decoupling networks for multistage amplifiers.
Figure 3-37 shows an RC capacitor-input filter and its associated waveforms. Both half-wave and
full-wave rectifiers are used to provide the inputs.
Figure 3-37.—RC filter and waveforms.
The RC filter in figure 3-37 consists of an input filter capacitor (C1), a series resistor (R1), and an
output filter capacitor (C2). Although not part of the RC filter, RL is shown to help explain the circuit.
This filter is sometimes referred to as an RC pi-section filter because its schematic symbol resembles the
Greek letter !.
Although the single capacitor filter is suitable for many noncritical, low-current applications, when
the load resistance is very low or when the percent of ripple must be held to an absolute minimum, the
capacitor must have an extremely large value. While electrolytic capacitors are available in sizes up to
10,000 µF or greater, the larger sizes are quite expensive. A more practical approach is to use a more
sophisticated filter that can do the same job but that has lower capacitor values, such as the RC filter.
The waveforms shown in the figure represent the unfiltered output from a typical rectifier circuit.
Note that the dashed line, which indicates the average value of output voltage (Eavg) for the half-wave
rectifier, is less than half the amplitude of the voltage peaks (approximately 0.318). 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.
