Measure High Voltage with arduino Nano (can i do that)

Hello,

i need to measure 230V AC, but transformers, analog optokopplers, ... are to big/expensive.

I dont want die if iam touching my Arduino!! So i measure the small resistor 33K between the two 1M Ohm resistors. If i touch the Board the current should be limited by the 1M Ohm Resistor to 0,11 mA ?!

Is that correct, possible and safe ? |500x225

I would not consider that to be safe and it is unlikely to meet the European electrical safety directive (LVD). Do you know the voltage rating of your resistors? Will they meet the required insulation test at 4000 V?

Furthermore you have to remember that the power supply to the Arduino may be grounded (if powered from usb). If not the Arduino will be floating at half 230 V!

Russell.

It’s NEVER considered “safe” to have a direct connection to line voltage, even through a resistor. You PROBABLY won’t die, but it won’t feel too good…

I just did a quick resistance measurement between my hands, and I got about 6 megohms.* Another megohm or two wouldn’t make much difference. Here in the US, our line voltage is “only” 120V… I’ve been shocked a few times and even though I didn’t die (or even get injured) it’s no fun.

If i touch the Board the current should be limited by the 1M Ohm Resistor to 0,11 mA ?!

If you are touching ground, there is only one resistor between the AC voltage and ground (plus your body resistance), and the peak of an AC sine wave is about 1.4 times the RMS (about 308V).

A small transformer should be less than $10 USD.

Or, you can put the whole thing in an isolated/insulated box with an isolated power supply (also safely inside the box) or power it with a battery (and never connect to USB when the AC is connected).

  • There are lots of variables when measuring body resistance but I knew that it was in the megohms and I know even with high body resistance you can still get dangerous currents

... If not the Arduino will be floating at half 230 V! --> Yes the board is grounded. So if the Current gets more than 10 mA the Residual current circuit breakers will switch off.

....A small transformer should be less than $10 USD. --> less $3 but to big

Yes i know its not the safest solution and in future the boards are behind the wallsockets and protected from touching.

Are the 0,11mA enouth current to measure the Voltage of the middle Resistor?

Yes i know its not the safest solution and in future the boards are behind the wallsockets and protected from touching.

That's safe. Just be careful during development and don't forget to disconnect the USB whenever it's connected to AC power.

If it was me, I'd use a transformer and do as much development, testing, and debugging as possible at low-voltage (you'd probably have to temporarily change your resistor values). I did something similar when I built a dimmer... The microcontroller was isolated but I tested the AC circuit at 12VAC before connecting the power line.

Are the 0,11mA enouth current to measure the Voltage of the middle Resistor?

The current isn't an issue, only the voltage. The Arudino inputs are around 100 meghoms, so almost no current flows into the input-pins (as long as you don't over-voltage it).

Well, let’s start at the beginning. What do you mean by measure? Are you really trying to read the actual voltage or are you just trying to tell if it is on? How would you use this measurement?

I assume you're in Europe. If so one side of the mains is grounded at the substation so will be close to ground potential. That will upset your potential divider if the Arduino is grounded. You need the Arduino to be completely isolated from ground and the supply for your circuit to work as expecte.

Russell.

outofoptions: Well, let's start at the beginning. What do you mean by measure? Are you really trying to read the actual voltage or are you just trying to tell if it is on? How would you use this measurement?

I think we really should not waste time posting further until the XY problem is resolved.