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As far I read, those sources give as output a stable 5V DC voltage. But I need to measure the exact variation of the 220 VAC by measuring a variation of that transformed 5V DC.
I'm not sure where an opto-coupler comes in to play but this is the method I would use:
1. Step it down with a conservatively rated transformer to say 6V AC. A small transformer is more likely to saturate and be non-linear.
2. rectify the AC, the variation will be the same regardless of whether it's AC or DC but DC is MUCH easier to work with.
3. use minimal filtering so that rapid variation in voltage is easier to see.
4. scale the voltage so the highest it will reach at maximum AC is just under 5V. Place a Zener diode across it just to be sure it can't go higher.
5. compare it with a 5V reference derived from an accurate source and amplify the difference so the +/-10% becomes 0 - 5V
6. measure the difference voltage !
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