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Triac replacement

Discussion in 'General Electronics Discussion' started by bigone5500, Aug 21, 2017.

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  1. bigone5500

    bigone5500

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    Apr 9, 2014
    I have a dimmer for a lamp that has a bad triac. I was wondering if there was something else that could be used in its place. The part in question is a MAC97A6. It's a logic level triac. Could a MOSFET be used instead?
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Sure, but the dimmer won't work any more and the failure mode might be interesting.
     
  3. kellys_eye

    kellys_eye

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    No. A triac conducts on both halves of the AC cycle - a MOSFET will only switch on one half briefly.

    Use the parameter search option at www.alldatasheet.com to list other logic level triacs that can be used in its place.
     
  4. bigone5500

    bigone5500

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    Apr 9, 2014
    I was just hoping that a MOSFET would work. I have lots of those around. I can get triacs really cheap but if I don't have to buy something I won't.

    Thanks!!
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Actually the body diode in the MOSFET will make it conduct in the reverse direction.

    The impedance of the gate of a MOSFET is much higher than the gate of a TRIAC, so it's possible that you'd exceed Vgs(max) and destroy the gate. It would be hard to predict what happens next.

    When all you have is a hammer, the superficial similarity between nails, screws, golf tees, and sewing needles should not lead you think the tool you have is the right one for all of them.
     
    Alec_t likes this.
  6. bigone5500

    bigone5500

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    I went to aliexpress and bought 100 for around $3. It will be fixed in a month or so...LOL!
     
  7. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    With a little bit of ingenuity and a diode bridge rectifier, you can use a MOSFET to serve in lieu of an SCR or TRIAC. Connect the n-channel MOSFET drain to the positive terminal of the bridge rectifier, connect the source to negative terminal. Connect the AC bridge input in series with your load and power line feed.

    Drive the MOSFET between source and gate with an isolated circuit that provides square pulses synchronized to the line voltage zero-crossings. It is important to drive the gate hard enough to force the MOSFET into its minimum Rds(on) condition to minimize self-heating. Connect the bridge rectifier, with MOSFET attached, in series with the AC line and the load. Vary the zero-crossing synchronized pulse-width to control conduction angle in each half of the power line sinusoidal wave form. Or purchase a hundred, possibly correct, replacement parts from China for a few bux.
     
  8. Terry01

    Terry01

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    Jul 5, 2017
    That's exactly what I was thinking hevans 1944! I was just about to type it out but you beat me to it! My first big correct answer on the forum too! :p
     
  9. bigone5500

    bigone5500

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    Apr 9, 2014
    I got the triacs in and the dimmer is now functional.

    Cost: 2¢
     
    hevans1944 likes this.
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