Electronics > Beginners
Safest way to measure AC mains 220-250 voltage?
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soldar:
I do not understand how those things work. It would seem to me that it would not distinguish between reactive and resistive loads.

I would check to see how precise they are because bad information is worse than no information.

Whatever trick they use I see they have a USB model which you can connect to a computer if that's what you want to do.

Personally I have a similar device but it is placed at the mains panel and is connected to the wires so it does have voltage information as well as using that to supply power so it does not need batteries. I find that is good enough for me as the panel is right at the front door and I feel no need to carry the receiver around with me around the house.

Most of the time I am only concerned with the current being used because in Spain we have current limiters and pay by the max current you can pull so I have a lower current limit but have to watch how much current I am pulling at any one time and disconnect something if I am over the limit.
paulca:
Peak demand based billing is only applied to large commercial and industrial supplies here.  Thankfully.

I think we have different aims.  If the energy monitor I have tells me I'm drawing 450W and that figure it out by 10% because one of the loads is inductive like the washing machine, I'm fine with that. 

I do not need this to verify or cross check the electric utility meter (magnetic rotating disc and clockwork digits type).  I really just want indicative, comparable measurements.   

If logged and graphed this can show me an estimate of the base load and see where and how hard the spikes hit when the "big hitters" are used, shower, washing machine, cooker, space heaters etc.

As to hacking into an off the shelf module, it's took fiddly.  As to the USB style monitors; they often save a CSV file to a memory card and so not realtime data, that or a prepriatory file which you have to pen with their "app".

I might just follow the open energy monitor spec and see how far I get.
richnormand:
Interesting timing for your post. 

A few days ago Ontario Hydro had a statement about a test where they would lower the mains voltage by 5% to test their various systems across the province.
So, using stuff in the misc and spares drawer, I put a test system to monitor the line voltage behaviour.
I was not comfortable going straight to the mains so I located a wall-wart that had a non-regulated AC output (ie: just a transformer).
I then put together a full bridge rectifier with very low filtering  and a resistive load on it so it would react fast to changes in voltage (just the opposite of what you do for a good supply). I then put a resistor/LED to signal the system is on.

For monitoring I used my Lascar voltage monitor. Could have used the computer monitoring on the DMM but the Lascar is simpler.

Photos:
Setup.
Results.
Used a Variac to change the line voltage and get a calibration reading rms voltage from DC generated.

Edit: just realised that the dip was in sample units. The 5% voltage drop was from 2:00 to 3:00 pm.
james_s:
Is there a reason you put a rectifier and capacitor on it instead of just taking the raw AC out of the transformer? I suppose it shouldn't make much difference either way but I figure it's less stuff to potentially influence the reading.
richnormand:
No not really, but in my case the Lascar only take 0 to 30VDC as an input.
All depends on what you have to monitor mains voltage.

Edit: just added in my previous post that the voltage dip was one hour long. Most people probably never noticed.

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