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1500w 120vac variable power control

  1. Feb 15, 2012 #1

    I've got an extremely limited background in electronics, so I'd like to ask for some advice regarding this circuit:


    I found the diagram online, It's supposed to allow variable control of a load up to 1500w at 120v, using an ordinary light dimmer and a 600V / 40A triac.

    After doing some research on the subject, I found a few warnings that boiled down to "do not control a large load with a light dimmer because regardless of rating, dimmers use phase switching and introduce direct current into the neutral line and multiple harmonics into the power supply frequency"

    A few people have told me this is not a concern, but I'd feel better if someone with some electrical engineering in their background could advise one way or the other.

  2. jcsd
  3. Feb 15, 2012 #2


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    Welcome to PhysicsForums!

    Your question is a little general: what is it that you're trying to control, and in what environment? (e.g. a trough heater in a barn vs. a peristaltic pump in a hospital)
  4. Feb 15, 2012 #3


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    Light dimmers use triacs and so give a symmetrical output to a lamp. So, they do not produce DC in the neutral line.

    It will certainly introduce harmonics, though. Light dimmers are notorious for producing radio interference.
    I had a couple and got rid of them for this reason.

    That circuit looks like it might work OK. I keep wanting to put a resistor from the gate to MT1.
    I guess the worst that could happen is you could blow up a triac and the load would get power all the time.

    The gate current is a bit of a worry, though. It would be about an amp. So, you could check for the maximum rating of your triac.

    As a safety warning, you probably realize that the voltages in this circuit are very dangerous and you should not even turn it on unless all metal parts are covered.
  5. Feb 18, 2012 #4
    I've seen Triac based dimmers used in theatre work that have a heat sink about as long as my arm. So, there'll always be someone willing to take it to insanety. Then again, when it comes to driving 10,000 watt Xenon lamps, maybe that's the right answer.

    If you're looking to control a heater, I'd simply buy a zero-crossing module and pucky it to a heat sink about the size of your hand. With a thermal time constant in the minutes, your controller won't even realize that your PWM rate is only .5Hz...
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