If you are measuring current, and only want current (not watts or VA's),
it doesn't seem that you'd ever need to measure the AC voltage. An RMS
calculation of instantaneous current samples should give you the same
result as a thermocouple ammeter. If you do indeed want watts, take
instantaneous voltage times instantaneous current. Sum up the products
and divide by the number of samples. There's watts. This automatically
takes power factor into account.
        By the way, didn't Maxim make a thermal RMS to DC converter? I looked
around their site earlier today and could not find it. As I recall, it
was a bit like a hall effect current sampler in that it used a balancing
mechanism. There were actually two heaters and two temperature sensors.
The voltage to be measured was put on one heater. An opamp would drive
the other heater until the two temperatures matched. The op-amp output
was then the RMS of the voltage driving the other heater. This was pretty
neat in that it handled pretty high frequencies and large crest factors.
I only wish the "unknown" heater was electrically isolated from the rest
of the circuit for a few thousand volts.

Harold


On Tue, 13 Nov 2001 22:00:16 +0200 "Peter L. Peres" <plp@ACTCOM.CO.IL>
writes:
> > Since you have a processor, you can compute the true RMS.
>
> The problem with the RMS current is not the arbitrary waveform, it
> is the
> cos(phi) value in the AC circuit. The only 'safe' way to get RMS
> current
> in an AC load is to compute RMS power (using both voltage and
> current) and
> then divide by the AC voltage value (Vef). Measuring the heat
> produced in
> a shunt is one way. An electrodynamic instrument may be another but
> people
> will frown here ;-) (this is exactly the same mechanism used in
> electricity power meters but the disc turns against a spring and
> moves
> a needle - it's a true RMS power meter that can be calibrated in
> Amps if
> the AC voltage is constant). The automatic overcurrent breakers used
> in
> normal panels (slow type - for lighting) and wire fuses both measure
> 'RMS'
> current.
>
> Peter
>
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