> And measuring a complete bulb+ballast fixture with a
> Kill-a-Watt style meter may not be very accurate because of
> the power factor of a magnetic ballast, and non linearity of
> an electronic one.
While I have not tried one myself, it's my understanding that the
Kill-a-Watt meter measures both apparent and true RMS power. It has
displays for watts, volt-amps, power factor, etc.
Fairly easy to do, all it takes is a small microprocessor and a couple of
A/D converters (one measuring current, the other voltage). We're only
dealing with a 60 Hz waveform, so a cheap 50kHz A/D will give us almost 1000
samples per cycle. That's plenty to give us a very good approximation of
true power. I'm sure there are better techniques available, but it wouldn't
take much processing power at all by today's standards to capture a few
cycles worth of data points; multiply each voltage,current pair together;
and do a brute-force RMS calculation on them. (Multiply each value by
itself, add up the squares, divide by the number of samples, and take the
square root of that.)
Compared to what my digital camera does every time I push the button, that's
nothing.
Randall
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