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Triac snubber

Discussion in 'Electronic Components' started by I.F., Apr 5, 2006.

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  1. I.F.

    I.F. Guest

    What I'm trying to do is use a triac to switch an assortment of loads on/off
    using a low voltage control via an opto-diac. The problem is that once the
    unit has been running and is then switched off - the triac keeps firing but
    not at full duty-cycle! The triac switch works fine with just a transformer
    load (wall wart adapter) but when combined with loads that include a
    switching PSU, I think the SMPSU pulses cause high Dv/Dt transients across
    the triac.

    What I think is needed is a snubber network to limit the Dv/Dt, but I do not
    know how to calculate the values of the capacitor & resistor. Having tried a
    couple of searches for triac application notes, I'm coming up a little thin
    on articles about how to handle the math for this - so I'm obviously not
    wording the search string well enough!
  2. I'd try a 0.1uF and 100R for a start, you can get them in one package
    with the cap being an X class type. If all else fails use a relay, you
    can really small ones these days with low operating powers.
    Most triac application notes would have been written in the seventies
    and the companies will have changed their names several times by now.
    There's a lot of stuff on the STmicroelectronics site.
    I used to have a good book on the subject from IR.
    Dv/dt is a problem in some applications. An odd half-cycle in a heating
    load is not much of a problem but can be a real pain in a big (kW)
  3. I.F.

    I.F. Guest

    Thanks for reply,

    After a couple more searches I got loads of hits and downloaded hundreds of
    triac related pdfs, now I have to search through them to find the info I
    want. In the interim I tried a 0.22uF cap in series with a pair of parallel
    470 Ohm resistors (235 Ohm), this made matters worse!
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