- Jan 1, 1970
markp said:John Devereux said:markp said:message
I have an sinusoidal AC signal between 30V and 160V AC and between
600Hz in frequency, and I need an opto-isolated zero crossing
AC is actually from a transformer output but I don't want to add
windings to it. I do however have a centre tap on the AC output.
have any suggestions?
This should work.
You don't need galvanic isolation, you just need a little common-mode
Thanks for this. I think this circuit might work, but I worry about
opamp/comparitor (which I think might have to be an instrumentation
being very high impedance and the possibility of noise picked up in
area from elsewhere. I'd rather have true isolation to be honest.
The impedance only 2kohms.
Yes, but the variation in input impedance of the opamp inputs (and hence
input biasing and offset currents) will negate the balancing of the
resistor dividers. To get accuracy from this circuit the impedance of
opamp has to be orders of magnitude greater than 1M.
Don't think so - it just needs to be orders of magnitude greater than
2k, which is a lot easier As a worst case imagine there is a 1M
resistor across one of the 2k. It only makes a ~0.2% difference to the
OK. Are you sure that input offset currents are not going to cause any
problem? If the common mode causes the inputs of the opamp to both be
above 2.5V then to get positive input bias and offset currents this has to
come from the 1M resistors. Is that not going to perturb the accuracy?
Actually forget that. Since I'm strapping one of the outputs or the centre
tap to ground, the point of accuracy will be when both inputs are at ground.
As long as I don't strap these to any other potential and my logic ground is
close to the strap potential it looks like it would be OK.