The Supertex depletion-mode mosfets are interesting as current
limiting devices here, back-to-back LND150s maybe. But resistors would
be more reliable if, say, really big transients might be possible.
One nice thing about the push-pull opto thing is that it doesn't need
a lot of current into the optos, so you can use big current-limiting
resistors, or the LNDs, off the transformer, as long as you don't get
extreme and add too much time delay.
You could also do a cheap opamp-based diffamp to sense the transformer
voltage, if you don't need true isolation. That devolves to four
resistors (two dividers down to logic ground) feeding a comparator.
John
I do really need isolation. The reality is the output is a centre tapped
winding on a transformer, and I need to be able to tie either the centre
tap, or one of the outputs, to ground depending on application.
The LND150 looks interesting, they can used as constant current sources at
high voltage with a single resistor it seems. My concern with just resistors
is this has to work from 30VAC (possibly less) to 160VAC, and to get the
gain needed to switch quickly within a few volts would mean lowish value
resistors, and then at high voltages would dissipate power. This is why I
thought a constant current device would be better.
Having said that, replacing your resistors in your zero crossing circuit
with LND150s (using the intrinsic diodes to conduct when the polarity is
reversed) and using your push-pull approach with schmidtt buffer to reduce
noise might do the job! Thanks for suggesting that.
Mark