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From: "John Jardine"
Subject: Re: Watt Meter Project....Attn Win
Date: Sat, 21 Dec 2002 20:59:57 -0000
NNTP-Posting-Date: 21 Dec 2002 20:46:57 GMT
X-Newsreader: Microsoft Outlook Express 5.00.2615.200
R. Lewis wrote in message
> "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.
Yes, it does just calc the VxA but *only* when it can 'see' a bit of both of
them at the same time. (that's the 'trick')
If you've got some say inductance kicking about, then the current wave won't
be rising at the same time as the voltage wave. Eg The voltage could be
couple be a couple of volts above zeroV while the current's still rising
towards zeroV. The circuit at that moment, therefore says '0.0Watts' of
power as there is a measured voltage present but the 'current is not yet
registering cos it hasn't moved into the half cycle where it can be
measured. Hence the time shifted 'reactive power' bits of the waves are
(This only works when looking at one half of a waveform . It falls apart
with a full cycle measurement.)
Also ... Yes, power is being used continuously, even when the electronics
can't detect it. But, the averaging filter will average over many half
cycles only those odd shaped snippets of power that were both positive at
the same time, to give a DC running average of the 'true volts x amps' power
used. (these pos snippets should of course balance identical neg snippets).
All the charging and discharging, reactive stuff that didn't appear above
the line in the same half cycle simply couldn't be measured in the first
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