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Safest way to measure AC mains 220-250 voltage?

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soldar:
Sorry, I expressed myself very poorly. I am sure there are specialized IC for this type of job because that is what Energy meters would use.

But when I wrote the second phrase what I meant is that maybe you could buy some cheap power or energy meter and hack it so you could get the info out. They can probably do most, if not all, of what you are trying to do.

Although, unless you are trying to do something very specific, energy counters, like the ones power companies use, can do pretty much everything, including interfacing with other devices.

The one my company has installed is Landis & Gyr but I'd have to check the model.

I also have about a dozen electro-mechanical meters which were to be dumped some years ago when they replaced them with the electronic remote control ones.

The old ones have a spinning disk and it would be simple to install a photodiode to check how fast it is spinning. Not to mention that it totals the energy right there. I have several installed in several devices just to check how much appliances use.

Again, it depends a lot on what you are trying to do and whether you are trying to be efficient or trying to have fun designing something.


--- Quote from: ogden on July 17, 2019, 06:11:01 pm ---http://www.ti.com/solution/smart-e-meter-amr-ami
https://www.st.com/en/applications/metering/single-phase-meter.html
--- End quote ---

james_s:
There was an IC made by Austria Microsystems for kWh meters but I don't know if it was widely used. It's not uncommon to use a standard microcontroller, a friend of mine did some experiments and using a 8 bit AVR he was able to capture fast enough to digitize a 50/60Hz sine wave and then calculate the RMS, power factor, crest factor, etc. For monitoring the voltage the power transformer powering the device was used.

paulca:
So the energy meter I have with it's wireless LCD display has no voltage tap, just the current clamp.  So it obviously assumes the voltage waveform.

I realise without a voltage tap I would be left assuming the voltage and current waveforms are exactly the same and thus I would miss account anything with a power factor not equal to 1.  That would include inductive loads like washing machines, dryers, fridges, freezers etc.  and capacities loads, which other than start up bursts of most things with capacitors would, I assume not be very common in a domestic setting.

I would also have to sample the current waveform, determine it's peaks, and crossing points to put my virtual voltage waveform in phase with it.

Or I could try and source a transformer that outputs 3V pp AC from 240V RMS mains.  There are ebay modules as I suggested in my original post.  Although any solution would need to be fused to prevent any significant current being able to pass through it, so that in the worst case, of say, a short in the device cannot possibly cause fire.

EDIT:  I do have a little dual tap 18V sealed 100mA transformer and I could always tap it and run it through a divider to get 3V pp.

soldar:
I can't see how an energy meter can work without measuring the voltage.

As discussed in another thread, the shape of the voltage is far from being a perfect sine.

And the shape of the current in devices with rectifier bridges is tiny bursts at the peak of the voltage.

To have any kind of accuracy you would need to sample V and I hundreds of times per cycle and then multiply.

And make sure you got them right because using a normal voltage transformer will introduce phase shift in the signal.

This issue is more complex than might seem at first sight.

paulca:
But you are assuming a very high level of accuracy.  A home energy monitor is probably not that accurate.  it might be out of 10% of so. 

I can't find a link to the one I have, but here is an example similar to it:
https://www.amazon.co.uk/Micro-CM180-Wireless-Energy-Monitor/dp/B007W0SQ3Y/ref=lp_1938287031_1_16?s=diy&ie=UTF8&qid=1563531804&sr=1-16

You will note it only has a current clamp.  So unless it can measure voltage via that and not just current, then it is assuming the voltage waveform.

If you google you will find dozens of similar devices with a current clamp and a transmitter.  Some integrate with smart home monitors, some with battery powered LCD displays.

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