bAt 03:46 PM 7/15/2007, David Mathog wrote:
Greg Lindahl <[EMAIL PROTECTED]> wrote
> So, the thing that logs power usage over time seems to be the "Watts
> Up Pro", which says that it plugs into USB and has a Windoze program that
> graphs power usage. Does anyone have one, and can you access the data
> sans Windoze program? Are there any other cheap logging power meters?
The Watts Up Pro is only $130 at Amazon right now - if your
time is worth anything just buy one.
If this is a one time deal and you have a USB Camera you
set it to snap pictures of the display at fixed intervals, and
then scan through those later.
Or, if you already own a USB data logger and a Kill-a-watt, try
this at your own risk. Most likely you'll end up filing the effort
under "I wish I'd just spent the money on the data logging version"!
On opening up a Kill-a-watt you'll find that one side has all the power
goodies and a tiny amount of logic, and the other side contains the
display, the buttons, and what looks like the logic to control the
display. The two sides are connected by a 6 wire cable, where
all of the wires are the same diameter, with no shielding and no
twisted pairs. It seems likely that if one attached a voltmeter
to those 6 wires one would find a ground, 5V (or 12V) dc, and
4 measurement lines, probably representing Line Voltage, Current, Line
Frequency, and Power. If you're lucky the measurement values are
encoded as a DC voltage. That would be easy to test, and if true,
you could solder leads onto those 6 lines and bring them outside
the case to attach to the input leads of your existing USB data
logging device.
Whoa there cowboy!! You're risking death, dismemberment, and
egregious excitement from the destruction of your datalogger. The
entire insides of the Kill-A-Watt are floated at line
potential. Providing isolation to a connection to the outside world
(i.e. to provide a serial or USB port) costs money, not only for the
parts, but because the design process is more rigorous and you need
to have a lot more regulatory oversight (testing labs, etc.)
There's a good description of the insides of this beast posted by
Terry on the Tesla Coil Mailing List at http://www.pupman.com/ where
he figured out how to recalibrate it, the differences between the
110V and 220V versions, etc. There's been a post on this list with
the exact URL, too.
It's not just a analog current and voltage measurement that you can
log, because you need to measure power factor, and that requires
digitizing the waveform (or at least fancy electronics to measure
instantaneous I*V)..
If one wanted an interesting project.. Get two isolation transformers
and send V and I to Left and Right inputs on the sound card, digitize
it, and process it in software. SOX could do a nice job digitizing,
Matlab, C, Python, LISP, what haveyou to do the processing.
For isolation transformers: Current transformers are available new
from Newark, Mouser, etc.. For a voltage transformer, get a low
power transformer used in power supplies. A 5-10V output (from 120V)
is typical, then use a resistive divider to get it down to something
reasonable for the sound card (about 1 V p-p)... Check the waveform
with a resistive load, because some of those small cheap transformers
saturate. If you get a little PC mount transformer with 120V/240V
dual primary, you can run it as a 240V transformer across the 110V
line, which will almost guarantee no saturation (doubling the turns per volt)
James Lux, P.E.
Spacecraft Radio Frequency Subsystems Group
Flight Communications Systems Section
Jet Propulsion Laboratory, Mail Stop 161-213
4800 Oak Grove Drive
Pasadena CA 91109
tel: (818)354-2075
fax: (818)393-6875
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