Question

A moving coil ammeter requires a potential difference of $0.4V$ across it for full scale deflection. It has fixed shunt resistance of 0.01 ohm with a coil circuit resistance of $R=1kohm$. The value of shunt required to give full scale deflection when the total current is $10A$ is equal to

• A. 0.02 ohm
• B. 0.04 ohm
• C. 0.05 ohm
• D. 0.06 ohm

Answer 1

The correct option is B 0.04 ohm

Shunt Resistance($R_{sh}$)

$I$=curent to be measured

$I_g$=Full scale deflection current

$R_{sh}$=$\frac{I_{g}R}{I-I_g}$ Using KVL,

$I_g=\frac{V}{R}=\frac{0.4}{1\times {10}^3}=0.4\times {10}^{-3}$

Second case, $I=10A,I_g=0.4\times {10}^{-3}$

$R_{sh}$ =$\frac{0.4\times {10}^{-3}\times 1{\times 10}^3}{10-4\times {10}^{-3}}\approx \frac{0.4}{10}$

$R_{sh}=0.04ohm$