[moved] Circuit to detect 240 or 380 volts AC

[Cabomba]

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.

 

[Romansh]

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

 

[BradtheRad]

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.

 

[Mozaaljh]

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

 

[schmitt trigger]

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.

 

[Cabomba]

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.

 

[BradtheRad]

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.

 

[schmitt trigger]

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.

 

[Cabomba]

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.