[time-nuts] Power monitoring

Dennis Tillman dennis at ridesoft.com
Thu Apr 17 20:26:18 EDT 2008

You're in luck! Much of what you want to do can be done for almost nothing.

Several years ago when I was interested in knowing more about the power
coming to my PCs I had a Uninterruptable Power Supply made by APC (American
Power Conversion). Most of their models, except for the very lowest price
ones, came with the ability to tell you just about everything you could be
interested in about the power line voltage through a DB-9 connector on the
rear. These models went by the name 'Smart UPS'. The software, provided by
APC, was called 'Power Chute'. Any company that made UPSes for servers had
features like this and software to go with it.  

The status of the power line is important for a computer server and an IS
manager to know. So, the UPS would report the line voltage at intervals you
selected via its built in A/D, the magnitude and duration of all over
voltage transients or drop outs, the line frequency, the status of the
batteries, etc. In the event of an imminent loss of power the UPS could tell
the server how long the batteries would last. Many of these messages had
settable limits so you could specify a 'normal' range for each of these
events. In addition, for each condition you could specify an action to be
taken (email someone, dial a pager number, shut the server down, etc). Lots
of interesting things can be found in the logs kept by the UPS software
about the status of the power coming into a server.

In addition the server could tell the UPS to test itself periodically to
verify and recalibrate the charge on the batteries, etc.

The batteries in a UPS wear out in 5 years (just like a car battery). Most
people don't know that the bateries are easy to replace. You can buy
replacements on-line or through a local alarm company. But because the
batteries wear out people think the UPS went bad and usually they throw the
UPS out or take the UPS to a local recycling station. You ought to be able
to find one with the smart features in it for next to nothing. You don't
need to have good batteries in it to be able to use the smart interface to
start monitoring your power line voltages. 

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Hal Murray
Sent: Thursday, April 17, 2008 11:10 AM
To: Discussion of precise time and frequency measurement
Subject: [time-nuts] Power monitoring

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?

If I was doing it myself, I'd start with a low power (quiet) PC and a UPS.  
Then I'd have a platform that could monitor other things too, like 

  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.   
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 

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