The function of the RC network is to:
(1) Keep the voltage across the contacts below 300 volts. C> (I
(2) Keep the rate of voltage change below 1V/
s. C> I
(3) Keep the current below that described in Table
ELECTRONIC FILM CAPACITORS, INC.
Reidville Industrial Park * 41 Interstate Lane
WATERBURY, CONNECTICUT 06705
PHONE (203) 755-5629 FAX (203) 755-0659
Noise suppression and contact protection
Inductors do not instantly "let go" of a current. At the instant a contact opens, the current through the contact
does not want to change. Using Ohms Law (E = IR), we can see that when the contact is closed, the resistance
is essentially zero, making the voltage zero, as well. As the contact opens the resistance begins to increase.
This leads to the generation of a high voltage which is generally destructive to contact life.
Capacitors will restrict an instantaneous change in voltage. A capacitor, therefore is useful to counter the
destructive voltages generated at the switch contacts. When the contacts are closed, the capacitor is out of the
circuit. As the contacts are opened any change in voltage is restricted by the capacitor if its value is large enough
to restrict the voltage change to less than 1 V/
A capacitor alone is not an ideal solution. When the contacts are open, the capacitor charges up to the supply
voltage. As the contact is "made", an inrush of current results limited only by a residual resistance and damage
may still result. For this reason, a resistor is configured in series with the capacitor. The higher the resistance
value the smaller the inrush current and the less effective is the capacitor. The voltage across the contacts when
opened is equal to the load current x resistance. (V = IR) It is recommended to keep V =< supply voltage.
In this case, the maximum resistance of the network (R
) will be equal to the load resistance (R
The above formulas are recommended for determining the RC network parameters. They do not identify all
of the subtleties but should serve as a guideline for effective results.
There are two types of discharges that can damage switching contacts and generate noise:
(1) "Glow Discharges" caused by the ignition of gases between contacts. These develop at about 320 volts at
about a 0.0003 inch gap range and can be sustained at a much wider gap range.
(2) "Arc Discharges" occur at much lower voltages, about 0.5 MV/cm. Minimum voltages and currents are
necessary for "Arc Discharges" to be sustained. Contact material also plays a role in sustained "Arc Discharges".
(see Table 1)
Arcing Current mA
The circuit to be used is dependent upon particular situations. Either circuit may be enough protection for
your needs. For extreme situations, both "Load" and "Contact" protection may be required.
Fig. 1 (across the load)
Fig. 2 (across the contacts)