Question

A 2 A full-scale PMMC type dc ammeter has a voltage drop of 100 mV at 2 A. The meter can be converted into a 10 A full-scale dc ammeter by connecting a

• A. $12.5m\Omega$ resistor in parallel with the meter
• B. $12.5m\Omega$ resistor in series with the meter
• C. $50.0m\Omega$ resistor in parallel with the meter
• D. $50.0m\Omega$ resistor in series with the meter

Answer 1

The correct option is A $12.5m\Omega$ resistor in parallel with the meter

PMMC resistance,
$R_m=\frac{100mV}{2A}=50m\Omega$

Now, Multiplying factor,
$m=\frac{10}{2}=5$

$\Rightarrow$ Shunt resistance required,

$R_{sh}=\frac{R_m}{m-1}=\frac{50}{5-1}=12.5m\Omega$