Large AC Loads with Opto Triac and Triac

Hi All,

I am new in the field of large loads (well actually quite newbie on anything electronics) but I would like to ask if someone can take a look at this schematic and see if it works, what elements are not needed or needed that are not in the schematic.

Quick Questions:

  1. Why Use an opto-triac that controls another triac? Why not only the opto triac?
  2. Would the zero crossing be automatic? Meaning that every pulse I send from the control the opto triac will wait for the zero crossing to turn it on for the given duty cycle?
  3. How would you control from the Spark how much of the AC phase will be allowed thru the triac? with an analogWrite (PWM) or do we need a more specific PWM command to set the duty cycle?

Sorry for all the questions :blush:

Hi @frlobo,

Here Are the answers to your q’s

1 the power handling capacity of opto triacs is generally to low for real loads. There are components available from eg Sharp that combine the the two in to one chip. Another solution might be a Solid State Relay. But that is basically the same what you wnat to make, only more expensive. An opto triac is in fact an opto diac, a diode that trips both ways (no pun intended) at a certain threshold. This means that it will fire the triac when it is the right moment.
2 Yes. the on chip circuit takes care of the switch timing.
3 Okay phase control. zerocrossing devices don’t allow that. They switch at… zero. Phase control works like this: you need a zero crossing detector circuit like this one or something like that. feed the output to an interrupable input pin of the spark. Attach a function to the pin that counts between 0 and 30 or 25 milliseconds. At 0 write a high to the opto pin and feed this function also with a number at which you want to stop firing the triac. When this number is reached, write a low to the opto pin. Voila you have made a light dimmer.

Thanks Marcus.

About my schematic. Will this schematic work to turn on/off and turn on with limited power a heater element? Or is there any change you can suggest?

thanks for all the information you wrote!

Hi @frlobo,

If you want to switch heaters with triacs, the only thing you have to think of that you choose a triac that can handle the current.
Let’s assume the heater produces 2000 watt of heat. This means it consumes 2000 / 230 = 9 ampere. If you live in a 110 country the calculation is: 2000 / 110 = 18 ampere.
The triac you must choose has to be able to withstand 4 times the current. That’s safe.
So the conclusion is: a TIC206d is to weak to switch a heater.
I suggest this one:
for a 230 volt application.

And be careful ! Don’t electrocute yourself :stuck_out_tongue_winking_eye:


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Thanks for the answers. Still have some questions! I appreciate it, and of course I take full responsibility for anything with this circuit, since it might not even see the light of day, it’s just for educational porpoises.

  1. Why an opto triac behind a triac? Why not a transistor behind a triac since the triac is what switches the hight load?

  2. Zero crossing. If the opto-triac was handling the load directly and since the opto triac has zero crossing detector, will the zero crossing detection be automatic? Meaning that I don’t have to time the sine wave and trigger it from the spark? Or why would I want an opto TRIAC with zero crossing if I still have to time the sine duty cycle on my MCU?

  3. Looking at the schematic I posted, Since I am handling a Triac from an opto-triac with zero crossing detector, I can assume that the opto-triac’s zero crossing detector does not do anything since it’s on the DC side?

  4. From the schematic I posted, is there any other suggestion or change in terms of how I hooked things up and why?

So if I understand correctly: on board zero crossing devices do not allow dimming since it will always start the firing at zero cross of the sine wave? If I want to dim a heater or attenuate the power I would need to detect zero crossing with a Device and read it from the MCU and work the duty cycle from there.

Again, thanks a lot for your help. It really helps me out! And I appreciate it! Sorry for my knowledge thirst! :blush:

  1. Why an opto triac behind a triac?
    The opto part takes care of the galvanic separation of you, your circuit and the dangerous mains voltage. It is mandatory! otherwise you will get electrocuted.

  2. Zero crossing.
    Yes it is automatic, just write a ‘HIHG’ to the io pin the opto is connected to and the triac wil fire at the next zero crossing.

  3. Correct.

  4. NOP, it is a standard schematic and works fine.

  5. Zero crossing switches efficiently on or off. No dimming!


Hi @frlobo

I see @marcus answered while I was typing this, but I wanted to add a few things:

The optoisolator protects you and your Spark core from the high-voltage. Your Spark has to get 3.3V or 5V from somewhere and isolating that from the high-voltage AC is a good idea. If you do as you suggest, the entire core and everything connected to it has to potential to be at high-voltage and very dangerous. Please don’t do this. You should take extreme care to make sure all of the high-voltage wiring is physically separate from the low-voltage Spark side with the optoisolators being the only things that cross the domains. Even then if you are working with 220V, you may need to have an air gap milled in your circuit board to improve isolation.

Unless you are dimming a light bulb, you want to switch AC at the zero crossing to avoid problems with the load seeing partial cycles and high inrush currents and to avoid generating RF interference. Triacs are funny devices that require special handling on their gate input, a function normally provided by diac either inside the triac or a separate device. The zero-crossing detector in optoisolator removes the need for a separate diac. Here is a good description I found quickly by Googling:

I don’t understand your third point: the optoisolator zero crossing detector is on the high-voltage side and is a good thing to have if you don’t want to dim a lamp.

The optoisolator is a small low-power device that can control milliamps of current, not the many amps required for a typically load.

Finally, maybe you should just buy a solid-state relay. They are essentially all the components you have in your diagram in a nice package for not too much money. You do have to heat sink them (like you would your triac) for big loads. A 240V 10A rated SSR is under US $3 on ebay and a 40A SSR is less than $10 at

Why not just buy it? That will be much easier in the long run.

Hi @bko

Im just asking because I need to learn. :smile: I would use SSR in my design for sure.

I didn’t ment not to use opto isolation from the mains… I ment to say that why use an opto-triac specifically… Can I use an opto coupler (transistor based) for isolation and drive the triac that will handle the large load from there?

You said that unless I was dimming a lightbulb, I should switch AC at zero crossing. What about a heater? Can a heat element be “dimmed” ?

Disregard the question on point 3… I took another look at my schematic and you are correct.

Thanks for the answer!


Quick Question:

Let’s assume the heater produces 2000 watt of heat. This means it consumes 2000 / 230 = 9 ampere. If you live in a 110 country the calculation is: 2000 / 110 = 18 ampere.

Your result came to 18 ampere, but you are suggesting we get a 4 times the current TRIAC. Well the TRIAC you suggested has 25A rating… That does not satisfy your 4 times the amp requirement or did I miss something?

Maybe I am looking at the wrong spec.

My calculation came to this:
1250WATTS @ 127V = 9.9A x 4 = 39.6A

So I picked this TRIAC (40A) for the project… Is that ok, or did I miss something?

Thanks for the great answer BY THE WAY.

ok ok ok,
everything in electronics is a bit fuzzy. When you look at the specs of triacs you will find a much higher rush in current tolerance, usually 10 times then the rated current.
So 25 amps continuous current is a good choice, read the datasheet :relaxed:

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Hi @frlobo

The short answer as to why you cannot use a transistor to drive the main triac is that you want to drive the main triac on both the positive and negative halves of the AC waveform.

As to “dimming” a heater: you will be much better off having a temperature control that turns on the heater when a lower limit is reached and turns it off when an upper limit is reached. Don’t make the two limits the same or too close together or else the heater will cycle too quickly and won’t last.

Your other question about factors for safety can be hard to answer. Oversizing the current capacity by four is good for a heater, but would not be enough for a motor, for instance.

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Got it! Otherwise it would not be possible to turn the TRIAC off…

@marcus As for the Datasheet… What is the value we are looking for as to amps:
It - (On state RMS Current ) ?
Itsm - (Non repetitive surge peak on-state current) ?

So 25 Amps in It is enough because the Itsm is much larger?

This triac will do, the RMS current is 40 Amps. that’s a whole of a lot more than your mains will supply.

So the spec we are looking for in a TRIAC that should have the 4x factor of the actual application requirement is the Itsm (Non repetitive surge peak on-state current) ?

So in that case (for example) This TRIAC:

25A RMS continuous
250A (Non repetitive surge peak on-state current)

Will suffice for 1200WATT application @ 127VAC?

Just checking to understand :smile:

Hi @frlobo
That part will do
get it and test it with a lightbulb !