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From: Fred Bloggs
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Subject: Re: Watt Meter Project....Attn Win
Date: Sat, 21 Dec 2002 19:44:38 GMT
NNTP-Posting-Date: Sat, 21 Dec 2002 11:44:38 PST
Organization: EarthLink Inc. -- http://www.EarthLink.net
R. Lewis wrote:
> "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.
>>>> 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.
>>>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.
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
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