About opto-diac and AC loads.

Hello everyone.

I am working on a project which basically has a dimmer as output. I want to drive an AC load as filament lamps or LED lamps (if possible), just for lighting application.


I don't control the load brightness mannualy but with a voltage (already made the controller circuit), so I can't use that well known R-C potentiometer dimmer.

What I wonder is, can I do that with a MOC3011? It's an opto-diac (I don't know anything about zero or non-zero crossing ones) which is used to drive a triac. I saw dimmer circuits made with those but never tried them out. If I couldn't, which one would you recomend?

I read that "right edge aligned" dimmers aren't good for driving filament lamps since those would damage with a sudden peak voltage (instead being powered by a complete sine wave). But I don't know about any triac which can be switched off AFTER the zero crossing.

Is that information true? I don't think so but for some strange physical reason it could happen.

And another question I came across while writing this: Could the DIAC current peak duration turn the triac on after the zero crossing? I mean that if the pulse lasts 1ms and it happens 100us right before the zero-crossing, the 900us could turn the triac on for another cycle. Should I limit the pulse position to avoid that? Thay way I couldn't get the 100% of brightness; It doen't matter but I just realised.

Thank you in advance, ask if you don't understand something.

You can control some ac powered LEDs with triacs that are "dimmer compatible"

Some dimmers are zero crossing turn on and phase controlled with the trailing edge to avoid the surge current and possibly use IGBT or Gate Turn-off type switches with were once popular.

But you need to decide your load characteristics NOW before choosing a Diac Triac design.

If you want LEDs then it is best to go to ON Semi, Linear Tech, Micrel, or Cree and choose the best dimmer design there.

the zero crossing pulse can start just before or after the zero crossing as long as it continues after the zero crossing long enough for the 1.5V Gate voltage to be triggered.

Thanks for your response.

I still do not understand the difference between a zero-cross triac and a non-zero cross one.

Could a series connected LED array with its current limiting resistor (and the inverse protection or another array) be switched on and off at 50Hz? I think so because it's possible to do the same at higher frequencies. I think that the led array trigger voltage shouldn't be as close as the peak voltage because it would decrease the light intensity.

Could I do the same (left-alligned dimmer or zero-cross turn off) with a conventional filament lamp? Most cheap dimmers work like that and lamps don't seem to be damaged.

Opto-triacs with zero crossing logic (e.g. MOC3040) are switching complete half or full cycles and not suited for phase angle control (dimmers), only for on-off switching and multicycle control. The latter is the recommended method of proportional heater control but not useable for lighting.

As you already found out, the nature of triac operation allows a switch-off only at zero crossings. This kind of dimmers is working for filament lamps since half a century and should hopefully work for you, too.

A problem of extending a trigger pulse beyond the zero crossing can exist, but not for 100 % intensity, which is just permanently on. It occurs however near 0 %. It couldn't happen in classical analog dimmer circuits, but possibly with a digital trigger generator, e.g. a microprocessor.

Dimmers with turn-off capable switch elements are particularly needed for electronic lamps (LEDs with inverter circuit) but can't be implemented with triacs. In so far beyond the scope of this thread, I think.