Electronics > Beginners
Safest way to measure AC mains 220-250 voltage?
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paulca:

--- Quote from: soldar on July 18, 2019, 06:41:58 am ---I may be mistaken but I believe the OP is looking for transducers which will convert AC voltage and AC current into DC signals proportional to the RMS values of the originals and which can then be fed and measured.

--- End quote ---

True, but as the device (MCU) will be doing nothing much else it can probably sample a 50Hz waveform fast enough to calculate the RMS values.

I bought SCT current clamp, took note of the warning about clamping it to anything while un-burdened!  I'll start with just a periodic measurement of my mains voltage and calibrate it using something like a 100W incandescent lightbulb on the bench before putting it in the meter box.

The OpenEnergyMonitor project has lots of gudies and code libraries that will help.

Last night I dusted off my soldering iron which has been boxed up for months due to house move.  Took an hour to make a temp sensor for my living room using an ESP32.  Next job is the energy monitor.
soldar:
There are many ways to go about this depending mostly on what you need. If you need to know polarity that is one thing. If you just need to know absolute magnitude that is different.

If just magnitude is enough then optoisolators can come in handy.

If you need polarity and do not want to handle negative voltages then you can have an input of -2.5 to +2.5V and offset it by +2.5V which gives you 0 - 5 V.

paulca:
All I'm after is a fairly accurate electrical usage figure, so I can display it, log it and graph it with my home data logger system.

As power is current * voltage I figured I'd need a safe way to measure the mains voltage with a 5V MCU, well actually a 3.3V MCU.

However as pointed out by others if the mains voltage is stable, and it pretty much is here, there is no real need to measure the voltage, just add it as a calibration configuration value.
soldar:
That won't work because you need instant value. Phase counts.

You need to measure voltage and current at least several hundred times per cycle and multiply those values.

Or how would you measure the power used by a bridge rectifier followed by a capacitor where you get very short bursts of current?

You would probably be better off getting a specialized IC. You are aware that these things are sold off the shelf commercially.


paulca:

--- Quote from: soldar on July 18, 2019, 06:42:15 pm ---You would probably be better off getting a specialized IC. You are aware that these things are sold off the shelf commercially.

--- End quote ---

Do you have an example?
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