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A 0–1 A moving iron ammeter has an internal resistance of 50 m\(\Omega\) and inductance of 0.1 mH. A shunt coil is connected to extend its range to 0–10 A for all operating frequencies. The time constant in milliseconds and resistance of the shunt coil, respectively, are

1. 2, 5.55
2. 2, 1
3. 2.18, 0.55
4. 11.1, 2

Correct answer: 2, 5.55

Solution

For all frequencies, the shunt branch must have the same time constant as the meter movement, so \(L_m/R_m = L_s/R_s\). Also, since the range is extended from 1 A to 10 A, the shunt carries 9 A when the meter carries 1 A, which fixes the shunt resistance. The resulting values are 2 ms and 5.55 m\(\Omega\).

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