1) Within the range of 200-300 V there are plethora of cheap and accurate analog measure-heads, magnetodynamic or other.
They never needs any power supply
2) If the digital is mandatory, why not to use any battery as isolated power source. At reasonably low amperage battery may serve up to 3 years without of exchange.

 

HI,

I am trying to design a V-meter for a comercial product, that can accuratly measure voltages between 180-250V.
Since this is high voltage we are working with, the standards require that it has to be galvanicly isolated from the low-voltage section of the circuit with at least 4kV isolation strenght. The target precision doesnt need to be amazing. Errors of +-2V would still be acceptable

The current design i am using looks something like this:

View attachment 289956

It works reasonably well and can measure with +-2V precision, but it eventualy likes to drift over a long period and screws up the measurment.

This is the working principle:

the ZMPT101B transformer reduces the +-250V AC signal to +-15V AC and isolates the mains voltage from the circuit

this then goes on to an opamp, that removes the negative voltage values and outputs the difference between 12V dc and +-15V AC signals.

This outputs is 0-3V output voltage, which then goes to an ESP32 for adc conversion and measurment.

0V here represents 200V while 3V represents 250V. This way the sensitivity translates to +1V at mains = +60mV at output

I assume that the issue is with the transformer, because it saturates when its output voltage exceeds 2V already. This causes it to output spikes instead of a smooth sinus signal, which is a lot harder to measure the peaks of.
View attachment 289958


I have not been able to find a reasonable replacement part that would not saturate at 15V output voltage. If anyone has any recomendations i would highly appreciate.

I would also be open to any other solutions for isolated mains voltage measurment that you might have. I would also like to avoid using any niche IC chips, since they usualy have problem with stock availability, and this is a comercial product.

I have done it reasonably well, here are a few points to consider:
1.The more volts you want from secondary, the more its possible you screw up with the waveform and will become triangular in shape.
2. Keep the output in limits of around 1 volt, choose burnout resistor accordingly (in range of few ohms to max 1 Kohm), you will get near to perfect sinewave. the more the burnout resistor, the more wave shifts towards like a spike.
3. The wave will be little edgy, put a capacitor in parellel, I got very smooth edge by adding 1uf cap.
4. Now we can detect the voltage on ADC of esp32 with 1.1Volt internal reference (attuenation of 0 db) which is very linear compared to the 11db. The actual or original adc reference of esp32 is 1.1V, all the others are manipulated and mapped that is why they screw up the readings on low and high end, also they are not correct, for near to perfect readings use 0db attuenation which means 0 - 1.1V max.

I have been running this setup for past 2 months as a trial and i am satisfied with it. I have also tried for 440VAC measurement with 780Kohm input resistor and results are very stable.
I have also tried the only half wave measurement, which also is very clean signal.
Actually I am using it for 3 phase voltage measurement, phase detection and phase reversal detection and ZMPT107 is a savior.
If you have any questions, ping me at < Mod: email deleted > Happy to help.