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From: "R. Lewis"
Subject: Re: Watt Meter Project....Attn Win
Date: Sun, 22 Dec 2002 12:48:31 -0000
Organization: Nextra UK
NNTP-Posting-Date: Sun, 22 Dec 2002 12:48:18 +0000 (UTC)
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
"Fred Bloggs" wrote in message
> R. Lewis wrote:
> > "Winfield Hill" wrote in message
> > news:firstname.lastname@example.org...
> >>R. Lewis wrote...
> >>>Winfield Hill wrote
> >>>>Gary Lecomte wrote...
> >>>>>I have just put a good Watt-meter design on my site. It is also
> >>>>>capable of measuring wattage into complex loads! And its quite
> >>>>>simple to make.
> >>>> As an alternate solution, NSC offers the LM13700, which features
> >>>> a pair of linearizing diodes that allows the use of higher signals,
> >>>> such as from a conventional 50 or 100mV current shunt.
> >>>> http://www.national.com/pf/LM/LM13700.html
> >>>How does this meter work? At first sight it just seems to do VA
> >>>rather than watts.
> >> You're referring to the LM394 circuit? First off, it only operates
> >> for half the ac cycle, as it assumes both are the same. That said,
> >> at every instant it multiplies V (proportional to the LM394 current)
> >> and I (proportional to the unbalance of the long-tail pair) together
> >> and then the meter takes averages this. So it's a legitimate vector
> >> power calculation.
> > I appreciate that it just works on the half cycle but doesn't it just
> > compute volts x amps - which is not watts for a reactive load. The
> > notes say it works for a complex load but I don't see how.
> > Am I missing something here.
> You are missing the fact that the average of the instantaneous i x v
> product is real power- the quadrature component in the product averages
> to zero. Volt-Amps is a product of averages (RMS) and not an average of
> a product.
You are correct in that I wrote VA as a lazy mans way of writing sigma di*dv
but what I still cannot see is how the power in a reactive load can be
deduced from looking only at two consecutive quadrants.
No problem as long as the load is not reactive - make the assumption that
the each half wave is the same as the next - but is it not the point with
complex loads that this is not so - and cannot be so else they would not be
At a glance, since the claim for the circuit is that it is valid for complex
(reactive) loads, I wondered if there was something clever going on that
somehow predicted the non-monitored quadrants behaviour.
On further inspection I see that it doesn't and the claim is invalid.
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