>I am planning to op-amp this to get about 0 to 3V out for the
>PIC AD input. This is easily achieved using a cheap op-amp
>such as an LM358, esp. if a negative rail is supplied to get
>the output down to zero.  If I forego the negative rail, I
>cannot quite get down to zero (not that that would be any
>great hassle anyway) - I figured I would need a more expensive
>rail-to-rail op-amp for that.

Check out the Maxim Semiconductor web site for chips they make for doing
"hot side" current measurement. You are still going to need some form of
high wattage low resistance shunt, but as I understand it, these chips will
convert the voltage across the shunt to a ground referenced voltage for you.
See http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3597 for an example. There
may be others suitable as well, I noticed that there is one which has what
looks like a PWM output instead. Also check out the application note link at
the bottom of the page.

The only thing I would suggest about the resistors you are proposing is to
check out the temperature co-efficient of the resistor. Without knowing what
accuracy you want, the resistors you propose may be sufficient, but if you
want better accuracy, then you may have to use more resistors to keep the
temperature change down to a level where the resistance change becomes your
biggest source of inaccuracy. OTOH you may be able to have a table in the
PIC to compensate for such a change if this is needed.

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