However....
I just went through troubleshooting a power controller whose gate firing
opto's were being killed by DV/DT. Some are less immune to high DV/DT than
the devices they are used to control or output to....But then again, that
was on the output, or detector side, not the emitter as is being discussed.
I have never seen a properly applied opto used to detect ZC fail probably
because the transient events are so fast that the LED's don't have time to
dim out.

Even slow opto's are fast enough for most AC control apps. There are a
number of parts available with bi-directional LED's also, called AC
optocouplers, I think. Each one protects the other and they provide a nice
quick pulse a +/- the diode voltage - really nice for triacs, a bit of extra
effort to use them with SCR's, though...

Chris

>
> > The safest way of doing it is  via an opto-coupler, which
> needs a reverse
> > diode accross the internal LED, and suitable current limit
> resistor so as
> not
> > to get too much delay on the sync relative to the 0 point
> on the wave.
>
> Yes. Or someother "proper" isolation system. Note that even
> optos have their
> limitations in very high rise time situations, but that's not
> liable to be
> much of a problem here.(very bad input transients may need to be
> considered).
>

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