Common mode Choke for AC voltage measurement

Question

I'm trying to measure Mains AC voltage of 230V RMS/50Hz with the ADC of a MCU. Therefore I need some isolation and voltage division (+/-325Vp down to +/- 1V).

The voltage division is easy to solve, but for the isolation I was thinking about using a Common mode Choke.

The idea is to use the Common mode choke as a small transformer, by connecting it to the Mains power (230V RMS), in series with a resistor.

On the secondary side I would place a resistor, and hopefully the voltage across this resistor would be dependent on the 230V RMS.

But can I use a Common mode choke in this way (at 50Hz)? These chokes are normally for RF.

Answer 1

Not a good idea. It is not designed to carry much magnetic flux in the core at 50 Hz nor have the isolation between windings that you need.

Think about how the common mode choke is used. With live and neutral running through it in opposite phase, there would normally be very little resultant magnetic flux at 50Hz and all other frequencies. The core is only designed for the small induced RF currents which come in common mode.

The easiest solution is a mains to low voltage transformer, with safety agency approval (kilovolts of dielectric strength between windings). Other solutions are also possible eg voltage divider into an ADC on the live side, and pass the digital signal through an opto coupler to the rest of the circuit (low side micro). But then you have to make a small power supply for the ADC.