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From: Winfield Hill
Subject: Re: Watt Meter Project....Attn Win
Date: 21 Dec 2002 12:11:16 -0800
Organization: Rowland Institute
X-Newsreader: Direct Read News 4.20
Fred Bloggs wrote...
> R. Lewis wrote:
>> Winfield Hill
>>> 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
> a product.
Right. Let's look at a piece of the circuit again,
. | _ |
. +-|_|-+ meter
. | |
. V ~ Iac c c
. ,------ b b --,
. | e--+--e |
. ---+--/\/\----- | -------+----
. AC I ~ Vac AC
. LINE | LOAD
Note how the meter is located between the collectors and measures
the average unbalance of the LM934 pair. Imagine a current that's
90-degrees out of phase with the voltage. We'll examine half of an
ac line cycle. During half (of the half) the current and voltage
have the same polarity and the meter accumulates deflection energy
in one direction. Then during the 2nd half the current and voltage
have opposite polarities and the meter accumulates equal deflection
energy in the opposite direction. These two cancel out, you see no
net deflection, correctly indicating zero real power consumption.
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