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Originally Posted by Al Skierkiewicz
Biff,
Op amps are pretty "vanilla" in that you can drop almost any device in place for a given circuit and it will work. In this application you do need a low input offset voltage specification. I remember at the time that a lot of discussion occured over different methods of measuring current. While the breakers do exhibit some resistance, this a variable especially if the breakers have ever tripped. We used the resistance across a 1 foot long #10 guage wire as it is close to .001 ohms/ ft. Remember to measure the current on the supply side of the speed controller as this doesn't change polarity.
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Thanks for the heads up about resistance variablity after tripping. The white paper I quoted was using "rail to rail" input op amps. These are findable but mostly in surface mount. If I get time I may take a run at it and go with a gain factor, for a given wire length. I am planning at this point to go with #6 right to the supply side of the Maxi 40 amps and drop to #10 or #8 to the Victors. If we can keep the wires from the Maxi's to the Motor victors the same length that would be the ideal sense point. Thanks Again.