specifying a TVS or MOV for relay contacts

Jun 15, 2007 2 Replies

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Matt 19 years ago

My design calls for a relay with silver-tin-oxide contacts rated at

30A@277VAC. The actual load would be something like a 120VAC heater or motor.

I would like something to suppress arcs at the contacts when the load is switched off. I have some vague ideas of the design issues ...

longevity/reliability
whether it needs a fuse
cost

I am leaning toward a bidirectional zener/transil TVS device, but I need some help specifying it. Can I just choose a stand-off voltage somewhat above the peaks of a 120VAC wave? If the line voltage is sometimes as much as 125VAC and I allow for 15% error in the manufacture of the TVS, I would get a stand-off of sqrt(2)*125V*1.15 = 203V. That would bring the clamping voltage to something like 324V (Littelfuse SMCJ200CA, cf.

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The other important parameter seems to be Peak Pulse Power Dissipation on 10/1000usec waveform. Some common values for this are from 600W to

1500W. I don't know how to choose from them. Would I have to know the load inductance and arc duration to estimate this power rating?

I guess I would consider a MOV, but I don't know whether it is going to wear out before the relay.

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Marra 19 years ago

Bang a capacitor across the contacts.

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Rich Grise 19 years ago

Well, there's a lot of difference between snubbing a heater and snubbing a motor - the heater typically isn't inductive, all you need to deal with is the switchoff arc, no inductive kick.

These look almost like a second source for Transorbs

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But for a 120VRMS circuit, you shouldn't need a standoff voltage more than about the 203 as you've mentioned above. I'd go 210 or 220 - you'd have to check the charts for max/min for a particular TVS.

Probably not - a 1500W TVS should handle it OK, but all disclaimers apply. :-)

MOVs are crap. Avoid them like the plague. They're for one-time emergency use only, albeit there are folks who will say they only deteriorate a little bit per hit - Transzorbs (and presumably your Littelfuse devices) are actual silicon "zener" diodes, and don't wear out.

Cheers! Rich

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