Figure Appendix B-3 Contact Noise Suppression - Honeywell VPR100 User Manual

Contacts
When a contact interrupts an inductive load, a certain amount of energy is stored in the load. An MOV or
RC circuit in parallel with the load provides a place where this energy may be dissipated. However, if there
is no MOV or RC circuit, the energy may create a visible electrical arc across the open contacts. This, in
turn, results in electrical noise as well as damage to the contacts.
One way to eliminate this arc is to connect a resistor and capacitor across the contacts (see Figure Appendix
B-3). A combination of 47 ohms and 0.1 microfarads (1000 Vdc) is recommended for circuits up to 3 amps
and 300 Vac. For voltages above 2000 Vac, an MOV across the contact may be added for extra protection.
Figure Appendix B-3 is an illustration of a resistor and capacitor connected across a contact to eliminate
electrical noise.
For large load currents, a rule of thumb is to size the capacitor so that the number of microfarads equals the
number of amperes in the load current, and the resistor has the same resistance value as the load. The
objective is to eliminate the visible arc.
Either discrete resistors and capacitors or packaged RC networks may be used. An RC network (47 ohms
and 0.1 microfarad) is available from Honeywell as part number 30371852-001. Similar RC networks are
available from Electrocube Inc. (part number RG1782-3) and from Industrial Condensor Corporation.
12/00
MOV
R
C
A.C.
supply

Figure Appendix B-3 Contact Noise Suppression

VPR & VRX – User Manual
Appendix B
Inductive
load
20781
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