To extend the useful range of an ammeter a shunt resistor R

To extend the useful range of an ammeter, a shunt resistor, R shunt, is placed in parallel with the ammeter as shown in the figure. If the internal resistance of the ammeter is Ri,A, determine the resistance that the shunt resistor has to have to extend the useful range of the ammeter. Then, calculate the resistance the shunt resistor has to have to allow an ammeter with an internal resistance of 2.00ohm and a maximum range of 2.00 A to measure currents up to 200. A.

Solution

If the internal resistance of the ammeter is Ri,A

the resistance that the shunt resistor has to have to extend the useful range of the ammeter by a factor N = Ri,A / (N-1)

Internal resistance 2 ohm, FSD current 2A. You want to read 200 A. as FSD.

The resistance that the shunt resistor has to have = 2/(200-2) = .0101 ohms