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Detecting AC VOLTAGE [INPUT (0-200) up to 1000 V AC] [OUTPUT: Any low DC is ok]

andychriss123

New Member
Hello!
I need to build a circuit with a DC output that is ON whenever the AC input is active (a.i. voltage is present at the input) ideally from as low as possible but it could be as high as 200, 250 VAC and still be acceptable but needs to survive up to 1000VAC spike and some 600VAC constant. How do you voodoo?
The only idea I got involves many transformers and many zenners which makes me feel like there should be a much much easier way to do this hence why I am here. I just want to detect the voltage, thinking about using then an optocoupler with the output in order to get the signal out or whatever isolation. I looked for Voltage Detection Relays but nothing is within my range and an AC sensor 20-1000VAC is way too overengineered/expensive for this application, I think at least.
What is your idea? Thank you for your time!

Summary:

(0 (max 250) - 1000 [VAC] INPUT RANGE)----| INSERT CIRCUIT HERE |----(SOME CONSTANT LOW VOLTAGE DC OUTPUT)

EDIT: Or whatever idea you might have, i just need my PLC to know when there is tension present on those wires (many cases there will be tension but no current unless I connect something so wireless/magnetic detectors are no good unless some small load is used)
 

Diver300

Well-Known Member
Most Helpful Member
I guess you don't need much power. You haven't told us the ratings of the PLC input.

One way you could do this is to put the inputs of four 230 V transformers in series, with the outputs in parallel.

https://cpc.farnell.com/multicomp/mcfm16-12/mini-toroidal-1-6va-2x-12v/dp/FF01518 would work.

You should not connect the centre taps to anything.

That will give 24 V ac output with a 1000 V input, so about 5 V ac with a 200 V input.

That can be rectified, smoothed and regulated to 5 V. If you needed to take 100 mA from the output, you would get around 3 W of heat generated in the regulator.

If you could manage with much less power, you could just use a potential divider from the input, but that would get hot with no load and would not provide isolation.

I would probably go for a neon light with suitable series resistors, and a photo-transistor connected to the input of the PLC. The photo transistor would need some low voltage supply, but that would be isolated from the input voltage, and you could check that the photo transistor worked by testing it with an external light source.
 

andychriss123

New Member
Thank you for taking the time to reply!
It seems like a very good solution, I think I will use this though I'm also thinking about just using a drop down capacitor instead of the transformers.
The plc works on 24VDC but I was thinking of using an AC optocoupler anyway, controlled by the output of this circuit to supply 24VDC to the Digital Input.
I just need to be careful so that the current is suitable for the whole range of aprox. 5~24 that you have stated as not to burn the LED and it should be fine I think.. Do you think there's anything else I should be worried about?
I'm a little bit reluctant to use the transformers as the frequency is variable. The impedance of the cap is also variable with frequency but I think it will offer a little bit more play
 

danadak

Active Member




Don't forget to do calculations on dropping R's and make sure they can handle high V w/o arcover.

A fuse is always a good idea.


Regards, Dana.
 
Last edited:

Diver300

Well-Known Member
Most Helpful Member
I just need to be careful so that the current is suitable for the whole range of aprox. 5~24 that you have stated as not to burn the LED and it should be fine I think.. Do you think there's anything else I should be worried about?
I'm a little bit reluctant to use the transformers as the frequency is variable. The impedance of the cap is also variable with frequency but I think it will offer a little bit more play
I was suggesting a voltage regulator to reduce the voltage to 5 V. Then the LED brightness will be constant. As long as there is a large enough heat sink on the regulator so that it doesn't burn out at the largest sustained voltage, it will be fine.

Mains transformers are fine at up to about 500 Hz. If the frequency is the less than the rated frequency, the voltage rating drops as well. For instance, a 230 V, 50 Hz transformer should not be used over 46 V on 10 Hz.
 

ronsimpson

Well-Known Member
Most Helpful Member
AC sensor 20-1000VAC
Really you want to see 20V to 250VAC with spikes to 1kv. The wider the operating range the harder the circuit.
AC optocoupler
I like the "AC Optocoupler" not a DC one. It gets you off the power line and isolated.

Here is an idea. Constant current source driving a LED opto isolator.
Q5 is a high voltage low current transistor used in TV sets to set focus.
R9, just enough current to turn on Q5. R7 sets the current.
Q5 could be a small HV MOSFET.
1651105424785.png

This will pulse at the power line frequency. About 40% duty cycle. That can be fixed.

Don't like this I have 5 more ways.
RonSimpson
 

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