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Subject: Re: Watt Meter Project....Attn Win
X-Newsreader: Microsoft Outlook Express 5.50.4133.2400
Date: Tue, 24 Dec 2002 13:50:33 GMT
NNTP-Posting-Date: Tue, 24 Dec 2002 13:50:33 GMT
Organization: AT&T Worldnet
I copied the following from the National AN 222 pdf.
"The power meter in Figure 12 is a good example of
minimum-parts-count design. It uses only one transistor pair
to provide the complete (X) • (Y) function. The circuit is
intended for 117 VAC ± 50 VAC operation, but can be easily
modified for higher or lower voltages. It measures true
(non-reactive) power being delivered to the load and requires
no external power supply. Idling power drain is only
0.5W. Load current sensing voltage is only 10 mV, keeping
load voltage loss to 0.01%. Rejection of reactive load currents
is better than 100:1 for linear loads. Nonlinearity is
about 1% full scale when using a 50 µA meter movement.
Temperature correction for gain is accomplished by using a
copper shunt (+0.32%/°C) for load-current sensing. This circuit
measures power on negative cycles only, and so cannot
be used on rectifying loads."
The last sentence says;
"This circuit measures power on negative cycles only, and so cannot be used
on rectifying loads."
How about a fullwave bridge rectifier with capacitor filter.
Wouldn't both halves of the cycle be identical ?
Also fullwave bridges have large short current pulses, would
this be accurate for this type of signal?
> > 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.
> - Win
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