Inductive Surge Suppressors

[Guest Carl]

Hi, I am having some trouble with sticking output relay contacts on an A-B Micro 1000. The load device is a 120 vac solenoid valve. I have replaced the entire PLC but would like to install surge protection to keep the problem from happening again. I know Allen-Bradley recommends a IN4004 diode for dc load devices. I have 120 VAC solenoid coils (not sure of the specs at this time.) What do I use? Thanks.

[JimRowell]

Most relay/contactor manufacturers sell ac suppressors for use across the coils of their products. Just ask your supplier. Or, if you wish, you can go in to your local electronics supplier and pick up some zeners or varistors. If your are not sure what you are doing then I'd recommend the varistors. You need to choose a voltage rating which will be the point at which the varistors will conduct and create a short circuit across your solenoid. The exact rating is not too important. It must be higher than will be experienced during normal operation. For 120 vac, you should choose something like maybe 300 volt varistors. They come with 2 leads. Wire 1 lead to each solenoid terminal. I'm assuming you are not exceeding the specs of the relays with the current these solenoids are drawing. If you are then you should place intermediate relays in there. Edited May 25, 2004 by JimRowell