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.

Alternative methods include measuring the voltage drop across a series
resistance (including using the cable resistance) and using magnetic
sensors. The series resistance approach is the simplist but involves
direct connection to the measured circuit with obvious safety
concerns.

RP

On 05/03/2008, Michael Cunningham <michael.cunningham.usa@gmail.com> wrote:
> I am rather new to PIC development but I am learning fast. I would like to
> build a circuit to measure the amount of AC current in AMPs flowing through
> a wire. Any suggestions on how to measure the current without directly
> connecting to the wire which will be carrying up to 125vac @ 30 amps.
>
> Can the PIC ADC be used for this purpose? Eventually I would like to drive an
> LCD with the value in amps.
>
> Any suggestions on where to begin?
>
> Thanks,
> Mike
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