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From: Winfield Hill
Subject: Re: Watt Meter Project....Attn Win
Date: 24 Dec 2002 05:03:01 -0800
Organization: Rowland Institute
References: <3E04C4D7.email@example.com> <firstname.lastname@example.org>
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> I wonder about the harmonic content too, when one component
> is negative when the other is positive. It seems to me that
> the negative component(s) result in a zero when multiplied.
> Without a four-quadrant measurement I'd doubt the accuracy.
No problem. Carl Nelson's circuit properly handles both
current polarities during one ac-line-voltage half-cycle.
The other ignored voltage half cycle has an identical power
waveform, so nothing would be gained from performing the
calculation during this half. So for symmetrical loads the
circuit really is equivalent to a four-quadrant measurement.
. | _ |
. +-|_|-+ meter
. | |
. V ~ Iac c c
. ,------ b b --,
. | e--+--e |
. ---+--/\/\----- | -------+----
. AC I ~ Vac AC
. LINE | LOAD
As for high harmonic content, there's the issue of how high
a crest factor the circuit handles. The circuit has a 15mV
full-scale current-shunt drop, yet the multiplier's clipping
occurs above 120mV. Although the circuit becomes non-linear
(reduced gain) it performs the power calculation with crest
factors above the 5x crest factor usually assumed necessary.
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