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I have in my box of parts a phototriac coupler, namely a Sharp PC3SD21NTZ (datasheet), which is a zero cross type. I don't know how to use it, but I've been meaning to learn for a while.

This part is specifically intended for triggering a main triac in load control applications and "are ideal isolated drivers for medium to high current triacs". Typical application from the datasheet:

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When triggering the main triac directly, one would typically have a zero-cross detection circuit which a microcontroller can use as an input to decide when to trigger the triac. However, this part does not appear to offer any way for the microcontroller to detect zero crossing, which leaves me wondering what its purpose is in the first place.

What kind of input would one typically drive onto the diode side when using this part? Is it intended for on/off control only?

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up vote 3 down vote accepted

It's for on/off control only. After the opto-coupler sees the LED going on it waits for the next zero-crossing of the mains voltage, at which point it switches on. After switching off, like any triac it remains on until the current goes below a hold-value. The circuit doesn't need to tell te microcontroller; it takes care of it all by itself.
Opto-couplers with zero-crossing switch are useful for switching resistive loads, like incandescent bulbs. They don't offer advantages for switching reactive loads.

Note that with typical PCB pads there may be not sufficient creepage distance between the pins on the control side (1, 2) and the load side (4, 5, 6). In Europe this should be 6 mm. The NVZ version may offer a solution.

The PC3SD21NTZ sounds like a somewhat exotic part. If you need more of them, you may consider the compatible MOC3031/3041 (3031 for 115 V, 3041 for 230 V).

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