4-72. The single capacitor filter is suitable for many noncritical, low-current
applications. However, when the load resistance is very low or when the percent of ripple
must be held to an absolute minimum, the capacitor value required must be extremely
large. While electrolytic capacitors are available in sizes up to 10,000 microfarads or
greater, the large 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
4-73. Figure 4-28, views (A), (B), and (C) shows the output waveforms of a half-wave
and a full-wave rectifier. Each waveform is shown with an RC filter connected across the
output. The following explanation of how a filter works will show you that an RC filter of
this type does a much better job than the single capacitor filter.
4-74. C1 performs exactly the same function as it did in the single capacitor filter. It is
used to reduce the percentage of ripple to a relatively low value. Therefore, the voltage
across C1 might consist of an average DC value of +100 volts with a ripple voltage of
10 volts peak-to-peak. This voltage is passed on to the R1-C2 network, which reduces the
ripple even further.
4-75. C2 offers an infinite impedance (resistance) to the DC component of the output
voltage. Therefore, the DC voltage is passed to the load, but reduced in value by the
amount of the voltage drop across R1. However, R1 is generally small compared to the
load resistance. Therefore, the drop in the DC voltage by R1 is not a drawback.
4-76. Component values are designed so that the resistance of R1 is much greater than
ripple frequency. Since the AC ripple senses a voltage divider consisting of R1 and C2
between the output of the rectifier and ground, 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 following are some disadvantages of the RC filter:
The voltage drop across R1 takes voltage away from the load.
Power is wasted in R1 and is dissipated in the form of unwanted heat.
If the load resistance changes, the voltage across the load will change.
However, in many cases, the advantages of the RC filter outweigh the disadvantages.
4-78. 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.
4-79. 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.
4-80. 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
23 June 2005