[moved] Circuit to detect 240 or 380 volts AC

Hi, everyone!
I need to make a circuit that can sense if the mains voltage is 240 or 380 volts. In case it detects about 380 volts, a relay should turn on.
I guess I need a way to measure the mains voltage. It doesn´t have to be precise. Can I rectify and filter the mains, and use a voltage divider to scale down the high DC voltage,
to be read by a comparator?

Thank you.

Sure, you can do it that way, just make sure you are using components with the right voltage rating.

You might install back-to-back 340V zener diodes, to set a threshold for current to start flowing. Then that current may be sufficient to energize the relay, if you rate components properly.

You could step down the voltage and use a Comparator LM339 to trigger a relay ..

For the energy level you are talking about, I would recommend to play it safe, and use a transformer to step down the voltage to a safe level.

Thank you for the help, guys!

schmitt trigger, I agree with you. I´d rather not to work with such a high voltage, and a transformer would make my work much easier.
The problem is that I cannot find a transformer with a primary rated for 380VAC. There are only models for 220V input.

My plan is to use a capacitive power supply to run a microcontroller with an ADC. The voltage to be measured will be rectified by a 1N4007 and filtered by a 200 nF capacitor. The DC output will be reduced by a voltage divider, so that the ADC can see around 1.5V DC when the input voltage is 240 volts, and 3.5 volts when the input is at 380V (just examples).

I know the precautions we must take when dealing with such circuit, but if my idea is totally trash, please let me know.

There are relays whose coils are made to activate at 120V, or 230V, so possibly 380V too. Certainly DC, maybe they'll work with AC as well. A relay typically pulls in at 70-80 percent of its rated voltage. So if you test a 380V relay, you may find that it rejects 240V. All of this depends on what you do about peak voltages in your setup. Peak is 1.4x the nominal AC voltage.

Well.......there are control transformers for industrial apps that will cover 380v and even higher voltages. Like this one:

https://www.newark.com/hammond/phc2...0000005178&searchView=table&iscrfnonsku=false

If you apply 380V, its secondary output will then be 24*380/600 = 15.2 v.

This was the first hit in a Google search. I'm pretty sure that if you refine the search, you can actually find less expensive devices.

That transformer would be just perfect for what I need.
Unfortunately is almost impossible to find such parts in my country, so I´ll try the capacitive PSU.
Thank you, schmitt.