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SCR vs Triac Heat dissaption?

Discussion in 'Electronic Design' started by Adam Aglionby, Jun 18, 2004.

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  1. In a 10A lighting dimmer, will a triac generate less heat than an inverse
    parallel pair of SCRs if all other things are equal?

    Have in my mind that SCRs run hotter than triacs but this may be just be
    nonsense, would appreciate clarification.


  2. Similarly rated (eg. triacs rated at double the SCR rating) will have
    fairly similar power dissipations (around 1W/A), maybe a bit higher
    for the triacs than for the SCRs, but the SCR dissipation is split
    between two devices so they may be easier to cool.

    This info is in the data sheets, BTW.

    Best regards,
    Spehro Pefhany
  3. Terry Given

    Terry Given Guest

    also it depends on how you drive the SCRs - applying gate pulses to a
    reverse-biased SCR greatly increases the leakage current, and as the SCR is
    reverse-biased this can be significant. I watched a guy do this with a
    3-phase half-controlled bridge rectifier for a 600kW drive - the SCRs got
    HOT, so much so that our first thermal test showed the problem quite
    clearly. The fix is simple - if its reverse-biased by more than a few volts,
    block the gate pulses.

  4. I didn't know that. I suppose it's only a problem with 3-phase AC
    circuits because in single phase designs the other SCR will turn on
    and remove the reverse bias. Thanks.

    Best regards,
    Spehro Pefhany
  5. Terry Given

    Terry Given Guest

    Hi Spehro,

    neither did I until I saw them break. IIRC the reverse leakage increased
    about 1000x, but I imagine that is a strong function of the SCR. Mistakes
    are a great way of learning things, especially if you get to observe others
    making the mistakes :)

    I hadnt thought about the single-phase arrangement but yeah, you're right.

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