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From: "R. Lewis"
Subject: Re: Watt Meter Project....Attn Win
Date: Sat, 21 Dec 2002 19:30:47 -0000
Organization: Nextra UK
NNTP-Posting-Date: Sat, 21 Dec 2002 19:30:36 +0000 (UTC)
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
"Winfield Hill" wrote in message
> 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.
> >> ........................(snip)
> >> 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 circuit
notes say it works for a complex load but I don't see how.
Am I missing something here.
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