Hi Michael,

I know, capacitive droppers are widely used, especially in household applications. But the disadvantage is, that the dividing cap looks like a short-circuit for fast transients and overvoltages like surge and burst. So, I would try to avoid capacitive droppers.

I would suggest you the following scheme instead:



The idea is, to charge a sample cap with a little current (<1mA) from mains and to carry out a strobed measurement of the cap's voltage by the help of a second opto. This is based on an idea from Erik given in another thread some time ago.

A simulation with TINA shows, that the cap charges up to about 10V at 85VAC input voltage and to about 17V at 260VAC. Charge up time constant is less than 130msec and discharging time constant, in the absence of mains voltage, is about 220msec. Heat dissipation of 470kOhm resistor is less than 100mW.

If you want to carry out a measurement, then fire the lower opto and check the output of upper opto within 500µsec.

If you want to make the circuit sensitive to mains voltages exceeding a minimum threshold, then add a zener in series to the 1k6 resistor.

Of course, the resistors directly connected to mains (470k, 220k) must be special HV-types, which can easily withstand overvoltages like surge and burst. As you said, that you have lots of switchings on mains, good filtering by the help of x-caps and varistors might be wise.

Kai Klaas