3-32
An RC filter, such as a pi-section filter, does a much better job than a single capacitor filter.
Figure 3-37 illustrates an RC filter connected across the output of a rectifier. C1 performs the same
function that it did in the single capacitor filter. It is used to reduce the percentage of ripple to a relatively
low value. Thus, the voltage across C1 might consist of an average dc value of +100 volts with a ripple
voltage of 10 volts. This voltage is passed on to the R1-C2 network, which reduces the ripple even further
(view C).
C2 offers an infinite impedance (resistance) to the dc component of the output voltage. Thus, 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.
Component values are designed so that the resistance of R1 is much greater than the reactance of C2
at the ripple frequency. C2 offers a very low impedance to the ac ripple frequency. Thus, the ac 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, however. 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.
Q27. Is an RC filter used when a large current or a small current demand is required?
Q28. Why is the use of large value capacitors in filter circuits discouraged?
Q29. When is a second RC filter stage used?
The resistor-capacitor (RC) filter is also subject to problems that can cause it to fail. 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, it 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.
