4-33
ripple senses a voltage divider consisting of R1 and C2 between the output of the rectifier and ground.
Therefore, most of the ripple voltage is dropped across R1. Only a trace of the ripple voltage can be seen
across C2 and the load. In extreme cases where the ripple must be held to an absolute minimum, a second
stage of RC filtering can be added. In practice, the second stage is rarely required. The RC filter is
extremely popular because smaller capacitors can be used with good results.
The RC filter has some disadvantages. First, the voltage drop across R1 takes voltage away from the
load. Second, power is wasted in R1 and is dissipated in the form of unwanted heat. Finally, if the load
resistance changes, the voltage across the load will change. Even so, the advantages of the RC filter
overshadow these disadvantages in many cases.
Q24. Why is the use of large value capacitors in filter circuits discouraged?
Q25. When is a second RC filter stage used?
FAILURE ANALYSIS OF THE RESISTOR-CAPACITOR (RC) FILTER.—The shunt
capacitors (C1 and C2) are subject to an open circuit, a short circuit, or excessive leakage. The series filter
resistor (R1) is subject to changes in value and occasionally opens. Any of these troubles can be easily
detected.
The input capacitor (C1) has the greatest pulsating voltage applied to it and is the most susceptible to
voltage surges. As a result, the input capacitor is frequently subject to voltage breakdown and shorting.
The remaining shunt capacitor (C2) in the filter circuit is not subject to voltage surges because of the
protection offered by the series filter resistor (R1). However, a shunt capacitor can become open, leaky, or
shorted.
A shorted capacitor or an open filter resistor results in a no-output indication. An open filter resistor
results in an abnormally high dc voltage at the input to the filter and no voltage at the output of the filter.
Leaky capacitors or filter resistors that have lost their effectiveness, or filter resistors that have decreased
in value, result in an excessive ripple amplitude in the output of the supply.
LC Capacitor-Input Filter
The LC capacitor-input filter is one of the most commonly used filters. This type of filter is used
primarily in radio receivers, small audio amplifier power supplies, and in any type of power supply where
the output current is low and the load current is relatively constant.
Figure 4-29 shows an LC capacitor-input filter and associated waveforms. Both half-wave and
full-wave rectifier circuits are used to provide the input. The waveforms shown in view A of the figure
represent the unfiltered output from a typical rectifier circuit. Note that the average value of output
voltage (Eavg), indicated by the dashed lines, for the half-wave rectifier is less than half the amplitude of
the voltage peaks. The average value of output voltage (Eavg) for the full-wave rectifier is greater than
half, but is still much less than the peak amplitude of the rectifier-output waveform. With no filter
connected across the output of the rectifier circuit (which results in unfiltered output voltage), the
waveform has a large value of pulsating component (ripple) as compared to the average (or dc)
component.
