[time-nuts] Power monitoring

Jim Lux james.p.lux at jpl.nasa.gov
Thu Apr 17 14:34:54 EDT 2008

Quoting Hal Murray <hmurray at megapathdsl.net>, on Thu 17 Apr 2008  
11:10:11 AM PDT:

> My power went out the other day.  That reminded me that I've always been
> slightly curious about that area.
> Are there any not-expensive boxes made for this?  Or something that shows up
> on eBay occasionally?

Lots of dataloggers out there in the <$200 range.
Check out things like the HOBO from Onset.

you can also get embedded style computer from someone like Tern or  
Z-world or Vesta that has all the hardware needed, and comes with some  
basic software that you could use.

> If I was doing it myself, I'd start with a low power (quiet) PC and a UPS.

Battery and single board computer or data logger is much better...  
such an application doesn't need a video monitor, keyboard, etc.

> Then I'd have a platform that could monitor other things too, like
> temperature.
>   Step 0 is just to measure when power is/isn't there.
>     I assume the UPS has a signal for that.
>   Step 1 is to measure the voltage.
>     This takes an A/D.  The standard PC audio input might be appropriate.

no DC coupling on the audio card.  Much better to either get something  
with the appropriate A/D OR a "one-wire" style interface.

You could transformer down the 60 Hz, digitize as an audio signal, and  
process it appropriately.

> I'd probably use an AC wall-wart transformer for isolation and a couple of
> resistors to get down to a reasonable voltage.
>   Step 2 is to catch dips and spikes.
>     That's just software behind the A/D.  (assuming the A/D is fast enough)
> As long as I'm dreaming...  Suppose I wanted to measure the power my whole
> house is drawing.  What's available along the lines of a current transformer
> on the main lines?  My first thought is that nobody does that (for homes) so
> it's probably horribly expensive.  On the other hand there is a lot of
> interest in energy conservation these days so it might only be somewhat
> expensive.

Lots of these available, in the few hundred dollar range..


you can also do something like watch the wheel goaround on the meter  
with a photocell
put a suitable current transformer ($5 surplus, $50 new) on the mains  
coming in.

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