How to get a stable trigger out of constantly changing voltage

[PiotrMisiuna]

I want to set an opto-coupler to trigger only at the top of a fully rectified AC voltage, without smoothing. The output voltage can change anywhere from 5 to 60V DC dependently on load. I don't think a voltage divider could do, probably something with opamps, but I'm totally green there.

[innkeeper]

how are you going to use it?
just wondering if you could get away with leading or trailing edge trigger instead of the peak.

[sourcecharge]

I want to set an opto-coupler to trigger only at the top of a fully rectified AC voltage, without smoothing. The output voltage can change anywhere from 5 to 60V DC dependently on load. I don't think a voltage divider could do, probably something with opamps, but I'm totally green there.

First, I would feed the output voltage to a high impedance series resistor that feeds a 5V zener and a 10x parallel resistance than the series resistor. 

The output of this would be a stable 5V signal that can feed logic chips.

If that offsets the logic with a phase shift, use a low logic mosfet that triggers with only 1.5V on the gate, and change the zener to 1.5V.

Or you could use use an opamp to achieve your 5V logic which will practically eliminate all phase shift.

The "ucurrent" circuit actually is the same circuit you could use but with cheaper and less tolerant parts.

http://www.eevblog.com/files/uCurrentRev5schematic.pdf

Use the logic signal in a pll (cd4046) to multiply the frequency by 8 (cd4022) 

Take the 3rd and 7th output of the 1of8 outputs from the cd4022 for a positive slope edge triggered on the top of the fully rectified AC voltage. 
Take the 2nd and 6th output of the 1of8 outputs from the cd4022 for a negative slope edge triggered on the top of the fully rectified AC voltage.

If you do this with more n x frequency multiplication and 1of n outputs, you can fine tune where exactly you want to trigger, anywhere within a signal so even if you get slight phase shifting due to the voltage level input to the logic level,

hope that helps..

[JS]

What frequency?

To make a trigger just on top a differentiator could be used, when the output crosses zero is your trigger point, but you should ignore the other zero crossing at the bottom, if your frequency range is restricted it could be easier as they are much shorter so you could use that, if not use a zero detection at the signal and a zero detector at the derivative and some logic to get (zeroDerivative && ! zeroSignal)

Tuning the differentiator time constant might be tricky if the frequency is too broad as if it's too fast the noise on a slow signal might trigger it as well.

JS

[PiotrMisiuna]

What is it for - I have an old welder with a bridge rectifier connected to it. I also have a high voltage arc igniter for a gas furnace. I saw projects on Youtube where guys are using it to make an easy start to the welder. I want to make it into an arc stabilizer by injecting high voltage on top of each semi-sine wave. I could also use sine wave just after the transformer, 50Hz should not be a challenge. It's just for fun.

[mikerj]

I would arrange to sense the zero crossing on the AC side and then trigger the high voltage after a 90 degrees phase delay.  You could do this with a fixed delay if the welder only ever operates on a fixed frequency, or a small microcontroller could measure the period (including averaging and noise rejection etc.) and set the delay accordingly.

This should give reliable operation, unlike a simple differentiator which will very likely be prone to triggering on noise, especially on a welder.

[vk6zgo]

I would arrange to sense the zero crossing on the AC side and then trigger the high voltage after a 90 degrees phase delay.  You could do this with a fixed delay if the welder only ever operates on a fixed frequency, or a small microcontroller could measure the period (including averaging and noise rejection etc.) and set the delay accordingly.

This should give reliable operation, unlike a simple differentiator which will very likely be prone to triggering on noise, especially on a welder.

I agree, trying to trigger on a varying level voltage is just making it hard for yourself when there is a readily available signal you can "sync" to .
In fact, any handy source of AC voltage from the same supply will do .