I've had good luck with current sense transformers from
http://www.toroid.com/standard_transformers/current_sensing_transformers/current_sensing.htm
. A lot of current sense transformers you see out there do not work well
at 50/60 Hz.

On circuitry, I'd take the secondary of the transformer, run it through a
full-wave bridge, then into the terminating resistor. Doing full wave
rectification would help eliminate core saturation. Doing the termination
after the rectification improves linearity, as diode drops "disappear" as
the secondary forces current through the terminating resistor. You can
then send the voltage across the terminating resistor into the PIC A/D and
measure current in a variety of ways. If the current is sinusiodal, you
can accumulate an average of instantaneous voltages and scale that to RMS.
Or you can capture the peak voltage and scale that to RMS. Or, you can
accumulate a sum of squares of samples, divide by the number of samples,
then take the square root to get the RMS. I've done this in a couple PIC
products, and it's worked out well.

Good luck!

Harold



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