> Probably a current transformer would be your best bet. This is
> basically a step-up transformer with a single turn primary ( the wire
> being sensed is just fed through the core) and a multi turn (can be
> 100s of turns) secondary. The secondary current is then 1/n times the
> primary current.
>
> The critical requirement is that there is a "burden" or load resistor
> fitted to transform the secondary current into a secondary voltage of
> sensible value.
>
> For measuring AC current you will need to include some sort of
> rectifier and don't forget that the peak current is well in excess of
> the rms value. One trick is to place the burden on the DC side of a
> bridge rectifier so that the diode non-linearities are minimised. It
> is still reccomended that a (higher value) resistor be placed on the
> AC side as well though in case the bridge becomes open circuit or very
> high voltages can be generated.
>
> This method is commonly used as it not only provides a suitable
> voltage range, but also a high degree of isolation can be obtained.
> With appropriate design, the PIC ADC can be used without having to use
> additional amplifiers etc.
>

I'd use this approach with one of these transformers:
http://www.toroid.com/standard_transformers/current_sensing_transformers/current_sensing.htm
. I've done this, then do RMS calculations in software.

Harold



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