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Originally Posted by seanc
eprigge:
Your values are off.
The formula for parallel resitance is: (R1 X R2)/(R1 + R2)
In order to read 10x the original amperage, you need 1/10 the original shunt resistance. Your numbers produce just under 1/5 the resistance, and the meter would be way off.
The correct value for the second resistor would be @ 0.11 x the first value
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No, my numbers are right, 0.11 is (close to) the 1/9th as I said... the formula for the second shunt resistor is Rs = Rm/(n-1) where n is the factor of division you want to get on your meter readings and Rm is the meter shunt resistance.
I would hazard a guess that a DMMs 10A range shunt resistance is around 0.01 ohms. Any more than that and you'd have too much power dissipating inside the meter. You might be able to find out from the meter docs or the manufacturer instead of measuring it yourself. If it was 0.01 ohms then the second shunt would need to be 0.00111 ohms and would need to handle 9 watts to carry 90A.
I would use a constant, known current source like a current controlled power supply to make the new shunt. It's such a low resistance you will probably just make it out of a length of copper wire. If you know you've got 10A flowing through the load, diddle with the shunt resistance until your meter reads 1.00. Voila. In the example above, about a foot of 10-12 gauge wire would be in the 0.001 ohm ballpark.