For AC devices a resistor/capacitor in series combination is used across the coil, the values depend on the size of the device, also on AC a varistor (voltage dependant resistor) can be used. On DC a diode should be wired across the coil in reverse bias to the DC supply i.e. anode to DC+. The diode will delay the opening slightly of the relay, but this is not usually a problem. |
Or you can use a MOV ( ?Momentarily? Over Voltage) in parelell with the relay. These will pass a voltage that is over its rated value only. ie spikes. They are commponly use as surge protecting devices on computers.