RE: Proposal: Wattmeter project

There's an application note at www.national-dot-com which describes a circuit
for an electronic watt-hour meter.  The actual address for the file is 


-----Original Message-----
From:	Tesla List [mailto:tesla-at-pupman-dot-com]
Sent:	Friday, 15 January 1999 4:15 PM
To:	tesla-at-pupman-dot-com
Subject:	Re: Proposal: Wattmeter project

Original Poster: "Steve Young" <youngs-at-konnections-dot-com> 

John & all watt-burners,

> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Proposal: Wattmeter project
> Date: Wednesday, January 13, 1999 11:55 AM
> Original Poster: FutureT-at-aol-dot-com 
> All,
> I think every coiler could benefit from having an electronic wattmeter
> that would accurately measure their TC input power.


> The perfect design for the wattmeter would be the one presented
> by and built by Dave Sharpe of the TCBOR some time ago.  It uses
> an optocoupler IC that costs about 15 cents or so, and a few other
> components.  It's a simple circuit overall.  (Dave, any objections?)
> Just a thought,
> John Freau
Great idea!  In the April 14, 1997 issue of Electronic Design, in the
"Ideas for Design" section, is just such a circuit.  It uses a quad-channel
optoisolator in a peculiar variant of the "Gilbert cell" analog multiplier
to compute 4 quadrant V x I.  Simple circuit--also uses a couple of voltage
regulators and op amps and a handfull of discretes.  The output is 0 - 1200
Hz corresponding to 0 - 1200 watts.  But by changing the current shunt
resistor (#16 copper wire), it could read higher power.  There is even an
optional interface to a PC comm port and simple BASIC program to average
the pulse rate and display accurate power.  The article also shows how to
add a few more components to measure 3 phase power.

Sorry, I don't have a way to scan the circuit, but the magazine should be
available in libraries.  And I don't have time to build a bunch of them,
but I would be interested in buying one.