18
•
Appdenix A. Realy Contact Protection Circuits
C : 0.5 to 1
µ
F per 1A contact current
Value 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
Device Selection
Use a zener diode with a zener voltage about the same as the power
supply voltage.