Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It's 100% free.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I want to create a dimmer using a Triac - it will have a duty cycle where it is on for x% of the time, and then off for Y% of the time - my question is how often should this on/off cycle happen?

I was thinking of using a BT137 - this is for a 120V / 60Hz / ~300W max circuit, however if there is an alternate/better option, I would be willing to switch.

I would like to avoid any "humming" noise if possible (which I seem to get from commercial dimmers).

share|improve this question
up vote 3 down vote accepted

Triacs do not switch 'off' except when the current drops to (almost) zero - a few mA in the case of the one you've linked to.

So, you're stuck with the mains frequency (times two, since you can turn it 'on' on either the positive or negative half-cycle). You would normally switch the triac on at some point after the zero crossing (a pulse of something like 100usec is sufficient if the load is resistive) and then wait for the next zero crossing. You should keep the timing similar on positive and negative half cycles. Note that it's significantly harder (2-3x the current) to trigger when MT2 is negative and the gate is positive.

share|improve this answer
The buzz noise is generally either from the filament of the bulb or an inductor in the dimmer. There is really no way to get rid of the former source with a triac dimmer. I think some folks have used PWM and MOSFETs but it's significantly more expensive and complex. – Spehro Pefhany Aug 26 '14 at 20:01

Normal dimmers 'cut out' the first part of the half sine wave; the TRIAC turns on when the voltage reaches a certain voltage, stays on until current drops below some value and it turns off again. So the switching frequency is twice the mains frequency. The usual circuit is like the one described here http://en.wikipedia.org/wiki/Dimmer

Once a TRIAC is on, you cannot turn it off again with some control signal like you can with a transistor. So there not much choice in the control frequency; at most you could switch at a integer fraction of the mains frequency like 30 or 15Hz but the power delivered to the lamp would be irregular and not much of dimming capability left over.

The buzzing sound is mostly generated by the inductor that's part of the dimmer, it's there to supress EMI and required to pass certification. The filament of the lamp can buzz too.

You could use some MOSFETs to PWM at a higher frequency, but that won't be a simple circuit.

share|improve this answer
I don't suppose there are any inductors that are less noisy than others? also, just for curiosity, what would one of the complex MOSFET circuits look like? – user2813274 Aug 26 '14 at 19:43
You could use a vacuum impregnated inductor, it's more rigid then. – jeroen74 Aug 26 '14 at 20:17

Do a quick Google search for "diac triac dimmer". Dimmers don't get much simpler, nor much more reliable, nor even much more effective, than that.

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.