ADLINK PCI-7256 - Manual (Page 45)

Appendix A Relay Contact Protection Circuits • 37
Device Selection:
As a guide in selecting R and C,
R : 0.5 to 1 Ω per 1V contact voltage
C : 0.5 to 1 µF per 1A contact current
Values vary depending on the properties of the capacity C acts to suppress
the discharge the moment the contacts open. Resistor R acts to limit the
current when the power is turned on the next time. Test to confirm. Use a
capacitor with a breakdown voltage of 200 to 300V. Use AC type capacitors
(non-polarized) for AC circuits.
2. Diode Circuit
This circuit is suitable for DC application. The diode connected in parallel
causes the energy stored in the coil to flow to the coil in the form of current
and dissipates it as joule heat at the resistance component of the inductive
load. This circuit further delays the release time compared to the RC circuit.
Contact
Inductive
Load
Diode
Device Selection:
Use a diode with a reverse breakdown voltage at least 10 times the circuit
voltage and a forward current at least as large as the load current. In
electronic circuits where the circuit voltages reverse breakdown voltage of
above 2 to 3 times the power supply voltage.
3. Diode & Zener diode Circuit
This circuit is also suitable for DC application. Effective when the release
time i the diode circuit is too long.
Contact
Inductive
Load
Diode