Electronic – TRIAC dimmer, circuit design help (resistive load)

acdimmingopto-isolatortriaczero crossing

Okay guys, I've been tasked with making a light-dimming control system for a 240V heat lamp dissipating around 250W. I need to adjust the heat output on the lamp by control from a microprocessor.

I'm developing on an 8051-based SoC with RF circuitry and some sensors and actuators. Basically it's a node in a wireless sensor network. I've left all other components out.

I'm mostly a software guy, so I might need some help here. Pardon my lingo and if I raise fundamental questions, I don't have a lot of experience with this.

I am thinking of a design like this for the dimmer circuit:

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.. and I have some questions 🙂 I've marked red boxes with numbers matching the questions:


  • Is this a sensible way to drive a triac ? The resistor on pin 6 should have a higher power rating. Like 5W or something I would guess.
  • The GPIO leg can supply up to 500mA, so it should be plenty to drive the gate for the transistor right?
  • And mostly any transistor will do I take it.


I don't know how inductive the heat lamp is, or how its resistance changes with temperature. I want to do an estimate of how the triac should be spec'd.

  • 240VAC(rms) would be ~340VAC(peak2peak), so 400V max isolation voltage should suffice, no?
  • A max surge-current rating of 4A would leave some overhead I would assume, but I'm not sure how much I need. I am assuming a switch-on from cold could draw a lot of current – should I instrument to be sure? I don't have any data on the heat lamp other than its rated for 250W max.


  • Am I understanding correctly that this is the snubber?

I've found a couple of topics that helped me, but I'd appreciate a comment. I read these:

TL;DR: Am I way off with the schematic? Some help on guestimating the rating of the components would be nice as well 🙂

Best Answer

First off if you are controlling a heating element you don't need a snubber.

Second, safe is a relative term. If by safe you are asking if it will explode and catch fire then build it an find out. If by safe you mean can I or anyone here tell you that you have built a safe circuit, meaning that you won't hurt someone.... well good luck getting a commitment there. 240 VAC is generally considered unsafe voltages to mess around with, so its not wise for anyone to give you the go ahead.

Here is a link that shows an application note that will help you design the circuit.


Safety advice: One more comment, remember that you need to handle the peak voltage when specifying a part, and handling surges and overvoltage situations that can arise from the power company. So 240 VAC is the RMS value (average), and the peak is times the sqrt of 2, or 340 volts. So make sure you use a triac that can handle 600V for a 240VAC circuit.