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PowerCost Monitor

Ted Roche wrote:
> On 7/1/07, Matt Shields <mattboston-Re5JQEeQqe8AvxtiuMwx3w at> wrote:
>> I saw another place online selling these for $130.  It uses an optical
>> sensor that sits on your meter.
> It seems like quite a Rube-Goldberg way to do business. Is there a
> problem with wrapping a clamp-on ammeter around the incoming power
> cable to the breaker box? Use one with a digital interface and you
> should be able to pick off the values... sounds like a fun DIY
> project...
There are several AC ammeters that deliver a stable DC voltage 
proportional to current.  If you use a clamp on ammeter as is, you'll 
get an AC output proportional to the current flow which will make the 
process of reading this data potentially (pun) very complex.  However 
current measurement is done, it gives you just one piece of the puzzle. 
  The other is the RMS voltage at the sampling point.  Then you need to 
sample rapidly so that you get both values within a close time segment 
and take a number of these measurements to derive an average.

Then there's True RMS to contend with, depending on the load.

The WattNode is a good choice for dealing with these.  It provides n 
pulses per KWH and requires a simple current sampling loop only.  A 
simple (but well-isolated) counter chip would then do well as an 
interface.  Of course to get useful of activity at a certain point in 
time you'd need to sample the counter quite often.

We've got a working KWH system that uses the WattNode and a hardened 
1-wire counter and it's proving to be quite nice.


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