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I found this https://www.edn.com/isolated-circuit-monitors-ac-line/ circuit in my quest for a low cost isolated AC line voltage monitor and constructed it. On testing however, I am not getting the correct voltage as expected from the formula provided. Ve is supposed to be 48V but my calculation by rearranging the second formula to get Ve, is giving around 133V.

Is the formula wrong or have I made a mistake somewhere?

Additional information: I changed the values of R1 and R2 to 100k each as I found the 10k 0.5W resistors were getting quite hot and the 200k obtained should provide enough current to turn on the optocoupler. I used EL817 as I had a few and the diode is 1N4007. AC RMS voltage is 230V for which I got a low pulse width (Ttotal in the formula) of 0.0073s (it should be about 0.0091s if Ve is 48V). I used an STM8S microcontroller's input capture for measurement. Unfortunately I do not have a DSO to verify the pulse width. I did verify that the pulse width was changing using a variac and an old analog scope.

Also are there design errors in the circuit which should be corrected?

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  • \$\begingroup\$ I did use a 47V zener diode. The 1N4007 was used instead of 1N4004 in the diagram. \$\endgroup\$
    – Subin Roy
    Dec 17 '20 at 15:37
  • \$\begingroup\$ Why are you commenting on your own question? There's an edit link below the question if you need to add in additional information. Can you add the schematic into your question so we don't have to follow a link to understand it and so the question still makes sense if the link dies? Thanks. \$\endgroup\$
    – Transistor
    Apr 6 at 21:12
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There are known problems with the EDN circuit. See https://sound-au.com/appnotes/an005.htm , read near the end for Rod's experiences. There are plenty of working circuits available on the ESP page.

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