Electronic – Snubber design for switching an inductive mains load with a relay


I am envisaging a relay which switches an inductive load consisting of a couple of switched-mode power supplies (standard ATX computer power supplies) in addition to several wall-wart style power supplies powering other equipment. The total power consumption is about 600 W at 220 VAC. The exact nature of the load may vary over time as different equipment is substituted, but what I have described is the typical size and character of the load.

I've been reading about snubber designs and it seems that some combination of a RC snubber and/or a bidirectional TVS (Transorb/Tranzorb) diode in parallel with the load would be required. I'm not comfortable with the alternative design of snubbing in parallel with the contacts because I want zero current flow when the relay contacts are open. Something like the diagram in this
application note for Driving and Relay and Contact Snubbing [PDF] is what I imagine.

First of all, what combination of snubber components is appropriate for my situation – RC network, TVS diode or maybe a MOV? Secondly how can I conservatively estimate the values and ratings of the required components to ensure that my relay contacts don't erode or weld?

Best Answer

The RC network's purpose is to provide a dampened path for the charge stored in the inductor. The MOV and TVS diode purposes' is to limit the high voltage that may appear. So they're complementary. Use both a RC network plus either the MOV or TVS.

I used 100 nF + 100 ohm for my loads for years and never had any problems so that's a couple of starting values.