Aviation Electronics Technician 1 - Historic Naval Ships Association

Figure 10-6.-Capacitive filtering of a three-phase attenuator.frequency must be considered in setting capacitancelimits and in filtering the return lead. Normal valuesof capacitance for filtering 400-hertz leads vary from0.05 to 0.1 µF.Capacitive Filtering of Switching DevicesNormally, a capacitor should not be used by itselfas a filter on a switch in a dc system. In the openposition, the capacitor bridging the switch assumes acharge equal to the line voltage. When the switchcloses, the capacitor discharges at such a rapid ratethat it generates a transient energy, whose interferencevalue exceeds that caused by the opening of theunfiltered circuit. The capacitor across a switchshould have enough series resistance to provide aslow discharge when the capacitor is shorted by theswitch.RESISTIVE-CAPACITIVE FILTERSA resistive-capacitive (RC) filter is an effectivearc and transient absorber. The RC filter reducesinterference in two ways—by changing the waveformof transients and by dissipating transient energy.Figure 10-8 shows how an RC filter is connectedacross a switch.Without the RC filter, the voltage appearingacross the switch at the instant the switch is openedis equal to the sum of the line voltage and aninductive voltage of the same polarity. Theamplitude of the inductive surge depends upon theinductance of the line and the amplitude of theclosed-circuit current.When the sum of the voltages appearing acrossthe switch is great enough, arcing occurs. When thecapacitance is large enough, the capacitor absorbssufficient transient energy to reduce the voltage tobelow arcing value. During the charging time of thecapacitor, the resistor is passing current anddissipating some of the transient energy.For maximum absorption of the circuit openingtransients, resistance should be small and capacitanceshould be large. Good representative values areR = 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.10-11