MikeX:
Well, not very high AC voltage, around 50 P-to-P from a transformer. The voltage is expected to fluctuate between 49v and 51v, and I'd like to see the changes with high resolution. That is voltage dividing 50v to 5v range would not work as the steps would be 5/1024*10 = 0.05v resolution. If I could bring the edge of the AC wave into the 0-5v region, that would offer 0.002v resolution.What would be the easiest way to do this?
Some alternatives to high res ADC and the op-amp peak detect circuit:
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Variations in transformer output voltage comes from variations on the supply or load side. If you consider the supply side constant, you could measure variations in load current (such as voltage drop across a shunt resistor). A change in load current will cause an inverse proportional change in transformer output voltage.
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You can use a string of zener diodes (plus a resistor) to drop the output voltage into the AtMega ADC range. Zener diodes around 5V are good picks as they’re temperature neutral. You measure the peak voltage across the resistor which also defines the required zener current.
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Use an opto-coupler to trigger at a voltage level close to peak and measure the time between the rising and falling edge of this pulse. Pulse width will be proportional to changes in transformer output voltage.