Figure 10-6.-Capacitive filtering of a three-phase attenuator.
frequency must be considered in setting capacitance
limits and in filtering the return lead. Normal values
of capacitance for filtering 400-hertz leads vary from
0.05 to 0.1 µF.
Capacitive Filtering of Switching Devices
Normally, a capacitor should not be used by itself
as a filter on a switch in a dc system. In the open
position, the capacitor bridging the switch assumes a
charge equal to the line voltage. When the switch
closes, the capacitor discharges at such a rapid rate
that it generates a transient energy, whose interference
value exceeds that caused by the opening of the
unfiltered circuit. The capacitor across a switch
should have enough series resistance to provide a
slow discharge when the capacitor is shorted by the
A resistive-capacitive (RC) filter is an effective
arc and transient absorber. The RC filter reduces
interference in two waysby changing the waveform
of transients and by dissipating transient energy.
Figure 10-8 shows how an RC filter is connected
across a switch.
Without the RC filter, the voltage appearing
across the switch at the instant the switch is opened
is equal to the sum of the line voltage and an
inductive voltage of the same polarity. The
amplitude of the inductive surge depends upon the
inductance of the line and the amplitude of the
When the sum of the voltages appearing across
the switch is great enough, arcing occurs. When the
capacitance is large enough, the capacitor absorbs
sufficient transient energy to reduce the voltage to
below arcing value. During the charging time of the
capacitor, the resistor is passing current and
dissipating some of the transient energy.
For maximum absorption of the circuit opening
transients, resistance should be small and capacitance
should be large.
Good representative values are
R = 1/5 load resistance and C = 0.25 µF.
Figure 10-7.-Capacitive filtering of a servomotor.
Figure 10-8.-An RC filter connected across a switch.