GATE 2015 EE – Question 16
A (0-50 A) moving coil ammeter has a voltage drop of 0.1 V across its terminals at full scale deflection. The external shunt resistance (in milliohms) needed to extend its range to (0 - 500 A) is ________.
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Correct answer: 0.22 to 0.23
Explanation
The meter drops 0.1 V at 50 A, so its resistance is $\frac{0.1}{50} = 2\ \text{m}\Omega$. For a 500 A range the shunt must carry $500 - 50 = 450$ A at the same 0.1 V, so $R_{sh} = \frac{0.1}{450} = 0.000222\ \Omega = 0.222\ \text{m}\Omega$.