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I have a triac in a 60Hz 120V power line (as is often done for dimming) and want it to only switch on the power when the AC voltage is low (widely between -40V and +40V would even be fine).

This is your typical "zero-crossing" switching requirement. MOC3041M is well-known for exactly this purpose. It includes a small on-chip triac (designed to drive a second bigger external triac):

MOC3041 schematic

So, what is the schematic of the "ZERO CROSSING CIRCUIT" block?

If the true schematic is not known, I'm open to any working option. It's even fine if the triac never turned off again after turning on nicely (since a well-behaved repeatable turn on is useful even when I don't need dimming).

I can only think of complicated solutions which generate power by rectifying the line and then sensing the 60Hz swing, but I feel like something much simpler is well known. It's easy/natural to get a triac to turn off at a zero-crossing (as done in the standard dimmer), but turning it on at a zero-crossing seems much more difficult.

bobuhito
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  • https://electronics.stackexchange.com/questions/691617/whats-the-circuitry-inside-triac-driver-optocouplers#comment1842847_691617 – G36 Dec 11 '23 at 21:50
  • @G36 Ok, thanks, I was not aware of that question. I just commented on that question and hope that I can get some response there about its solution... – bobuhito Dec 11 '23 at 22:04

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enter image description here

Figure 1. Internals of a zero-cross detection circuit based on the G3MB-202P. It's not so simple really! Schematic based on that in EDN article and referenced in my answer to Using AC current to trigger Triac

If \$(V_{L1−L2}\$) is low (above but close to zero) and Q1 is turned on by photo-action from D1 then SCR1 will be triggered. This in turn will pass enough current through R6 to bring TRI1 gate voltage high enough to trigger.

When voltage exceeds a certain level Q2 will be biased on. The collector voltage will fall and there won’t be enough to turn on SCR1 even if Q1 subsequently turns on. The effect is that TRI1 can’t turn on unless it is triggered close to the zero cross.

The problem you might experience with operating on low voltage is that a certain minimum voltage is required on L1-L2 to make the circuit operate. At full mains voltage that's happens very close to the zero-cross. As the AC voltage decreases that voltage is reached later in the cycle and zero-cross action becomes less and less true!

enter image description here

Figure 2. Possible resulting waveform for zero-cross opto-isolator switching a low-voltage supply. Image source: mine.

For additional material see my article here.

Transistor
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  • How did you deduce that schematic since they don't publish it? By the way, it seems there must have been some problem with the G3MB part since it appears to be discontinued (and, another by the way, I don't understand why 45% humidity would pose a problem in their datasheet), the closest replacement is the much more expensive and perhaps very different G3CN. – bobuhito Dec 11 '23 at 22:21
  • Ahh, thanks. I'm always quite careful to correctly attribute images, etc., but I missed that one both in the linked question and my LEDnique article. I found it on another answer that I wrote on this site. I've updated my answer. See the caption under Figure 1. I don't have an answer to the humidity specification. It would be a good topic for a new question! – Transistor Dec 11 '23 at 23:26
  • Is this intended as a "this is an example of the kind of circuitry these might use" answer, or a "this is actually what's inside the MOC3041 (or G3MB-202P)" answer? I suspect the former, but it might be a good idea to make it a bit more obvious. – Hearth Dec 12 '23 at 14:57
  • @Hearth, I think that "Figure 1. Internals of a zero-cross detection circuit based on the G3MB-202P." is clear enough. I didn't reference the MOC3041 anywhere in my answer. Thanks (and keep up the good work). – Transistor Dec 12 '23 at 15:37