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specifying a TVS or MOV for relay contacts

Discussion in 'Electronic Design' started by Matt, Jun 15, 2007.

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  1. Matt

    Matt Guest

    My design calls for a relay with silver-tin-oxide contacts rated at
    [email protected] 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. http://www.littelfuse.com/data/en/Data_Sheets/smcj.pdf ).

    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.
     
  2. Marra

    Marra Guest

    Bang a capacitor across the contacts.
     
  3. Rich Grise

    Rich Grise Guest


    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
    http://www.siliconix.co.uk/diodes/protection-tvs-esd/trans-zorb/

    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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