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From: "John Jardine"
Subject: Re: Watt Meter Project....Attn Win
Date: Tue, 24 Dec 2002 15:18:40 -0000
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amdx wrote in message
> 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
> 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 would have thought they 'meant' any waveform having asymmetry between the
top and bottom half cycles. This implies a DC component, level shift due to
even order harmonic distortions. A full bridge is symmetrical so no even
order components and the circuit would still work OK. The sharp cap charging
current pulses (load related) could cause a measuring prob' due to crest
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