Electronics > Beginners
Safest way to measure AC mains 220-250 voltage?
paulca:
I intend to install a DIY mains electric monitor (or several) in my distribution board / meter box.
For current I hope to use one of these:
https://www.ebay.co.uk/itm/100A-SCT-013-000-Non-invasive-AC-current-sensor-Split-Core-Transformer-ODHN/264087793339?epid=7008163687&hash=item3d7cdbeebb:g:EMoAAOSw-CpYBowr
I gather this will output a 50mA (for 100A) AC current, so I still have open questions on how to correctly sample that to get a genuine current value.
For voltage, I believe I could use an unregulated transformer based AC/DC power supply and calibrate it's unloaded output with a multi meter. Obviously this will not allow me to measure things like power factor, apparent power etc. but I have no requirements to do so. A simple DC voltage, as close to linearly related to the AC input voltage as possible. I do not expect my voltage to fluctuate that much, the lowest I have ever measured locally was 239V and the highest 245V.
I also realise that if I just assumed 240V I would get a fairly accurate figure anyway, within a few percent.
paulca:
--- Quote from: paulca on July 17, 2019, 09:18:22 am ---I gather this will output a 50mA (for 100A) AC current, so I still have open questions on how to correctly sample that to get a genuine current value.
--- End quote ---
A quick google for example circuits and of course this should be as simple as a current sense resistor to convert the current to a voltage and a suitably sized capacitor to smooth the voltage out.
My AC is a little rusty (always has been), but if you pass 1A through a 1 Ohm resistor you get 1V, but if you then pass -1A through the same resistor you would get -1V across it, no? Surely this would just discharge the smoothing capacitor or reverse it's polarity. So I'm confused. Should there not be a diode in the circuit somewhere to filter out the negative voltage across the current sense resistor?
richard.cs:
You can put a bridge rectifier on the output of the current transformer then have your shunt resistor after that. The diodes introduce some extra voltage drop which is OK because the current transformer acts as a constant-current source. Your voltage across the resistor will be full-wave rectified so will be bumpy. Don't connect a capacitor directly across the shunt resistor, include it in a resistor-capacitor filter after the shunt resistor. That capacitor will then have a voltage across it that is proportional to the average current magnitude, as a reasonable approximation this is proportional to RMS current which is what you actually want.
Or, if you are feeding this signal into a microcontroller you don't need the diodes or the capacitors, instead you connect the shunt resistor directly across the current transformer, bias the signal (offset it positive so your signal goes from e.g. 1 to 3 V instead of -1 to 1 V) into the ADCs input range, and then sample multiple times per AC cycle to allow you to calculate the RMS current.
paulca:
--- Quote from: richard.cs on July 17, 2019, 09:56:38 am ---Or, if you are feeding this signal into a microcontroller you don't need the diodes or the capacitors, instead you connect the shunt resistor directly across the current transformer, bias the signal (offset it positive so your signal goes from e.g. 1 to 3 V instead of -1 to 1 V) into the ADCs input range, and then sample multiple times per AC cycle to allow you to calculate the RMS current.
--- End quote ---
Ah, yes. For some reason I was trying to shy away from dealing with the actual AC current waveform, but as you point out, if biased correctly and sampled fast enough, the rest just becomes mathematics.
I found this, which will help with the mathematics :)
https://learn.openenergymonitor.org/electricity-monitoring/ct-sensors/how-to-build-an-arduino-energy-monitor-measuring-current-only?redirected=true
soldar:
I have several of these https://www.ebay.com/itm/113704114092 and they work fine.
If you search ebay for -- panel meter ac 300V 100A -- you can find many similar items.
You can also find power meters and energy meters.
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