Protection

What is an RC Filter?

Written by Galco | Aug 17, 2025

What is an RC Filter?

RC Filters are easily selectable electronic components designed to prevent or substantially minimize the occurrence of arcing and noise generation in relay and switch contacts.

RC Filters consist of specially designed capacitors and resistors connected in series. Spark discharges and induced noise are absorbed over a wide range by the accumulation characteristic and impedance of the capacitor, while the RC time constant delays and averages surge voltage and oscillations.

RC Filters must have the capacity to store surge voltages and current energy and afford protection against inductively induced potentials. The dielectric material in capacitors, used in RC Filters, affords a very high degree of voltage withstand strength. All resistors are non-inductive types to ensure a high degree of protection against pulse potentials.

 

RC Filters Benefits

RC Filters can be used effectively in many situations such as arc suppression, damping oscillation, back electromotive force suppression, and DV/DT suppression. Here is a quick overview of the benefits of using RC Filters in each application.

  • Arc Suppression - at the moment of switch opening, the RC combination absorbs and suppresses the energy of the arc by letting it bypass the switch
  • Damping Oscillation - the RC combination absorbs the high frequency oscillations caused by mechanical vibrations such as relay contact chattering. Similarly, the oscillations created by arcing are also averaged and suppressed by the RC combination regardless of their origin.
  • Back Electromotive Force Suppression - with back electromotive force due to inductance, the surge voltage peak is suppressed by conducting it through the RC circuit on the low impedance side. The peak is absorbed by the capacitance of RC. The waveform is averaged and smoothed by the time constant of the RC; thus generated noise is eliminated or substantially minimized.
  • DV/DT Suppression - the RC combination allows the dv/dt of the "on" and "off" operation of thyristors or similar devices to decrease; thus surge voltages are suppressed and semiconductor elements are protected. Even in the case of zero crossing circuits, such as AC circuits, protection is necessary since harmonic noise occurs when there is a gap between phases of current and voltage of the load circuit.

Apply RC Filters

Applications for RC Filters include:

  • Protection for contacts and from noise during switching operations of equipment such as radio, TV, copiers, mixers, coffee grinders, washing machines, dryers, tool machine equipment, and packaging machinery

  • Protection of electronic instruments during operation of relays, solenoids, and motors

  • Electrical noise protection of semiconductor devices during control of triacs, thyristors, motors, welders, and illumination equipment

In general, the calculated RC value is difficult to determine using the formula listed below. This is due to contributing factors such as equipment wiring and component locations that can vary from machine to machine.

The combination that is used for 90% of the applications is 0.47-.50 Mfd @ 220 ohms. The voltage should be selected for the normal DC or AC voltages; however, the designer must take into consideration the peak voltages involved.

The resistor wattage depends upon the number of times per minute the circuit is activated. As a general rule of thumb, the following chart should be considered.



RC Filters are available in a variety of AC and DC voltage combinations from 125 VAC/600 VDC through 480 VAC/2000 VDC, and 22 through 470 ohms. Coils are available in 50-133 and 110-415.
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