Relay Protection

[teej]

Default Relay Protection
I've designed a circuit which controls 3 relays that switch inductive loads. Specifically, the loads are these solenoid valves: http://www.orbitonline.com/products/spr ... ow-control. I couldn't find much in the way of electrical specs for these valves, so I plugged the AC/DC transformer supplying them into a Kill A Watt, and measured about 0.05 A of current draw (at 120 Vrms) for each solenoid. At the 28 VAC on which these operate, that should mean a RMS current of about 0.2 A each.

With that in mind, I selected the Spartan 9007-05-01 relay to switch the solenoids: http://media.digikey.com/pdf/Data%20She ... Series.pdf

It has a rated voltage of 200V, and a rated current of 0.5 A.

After acquiring all the components for my circuit, I soldered them all on to my PCB, and tested for correct operation with about 1000 cycles of all of the relays. They worked flawlessly. However, today I connected the loads to the relays for the first time, and all three failed within the first few switches. (The switched contacts are now permanently connected.)

I suspect this is due to the contact arcing and voltage spike which accompany the magnetic field breakdown in the solenoids. I was not ignorant to this principle when I was designing the circuit, so I included a varistor (specifically the MOV-14D151K http://www.bourns.com/data/global/pdfs/MOV14D.pdf) across the switched contacts of the relay, with the intent of protecting the switched contacts of the relays from voltage spikes and arcing. The varistor is rated for a maximum continuous RMS voltage of 95 V, a maximum voltage of 165 V at 1mA, and a maximum clamping voltage of 250 V.


Can anyone explain why my relays have failed under load, and how I could have better protected the relays in my circuit?

[I_Daniel]

This is probably a clueless answer but do your contacts have the proper rating and I keep thinking on the line of points in a motor car where the flashback is very high and a capacitor is used across the "points" to limit it and thus also the burning of the "points"
On a relay you normally also have a diode across the coil to limit the flashback