Question

Full scale deflection current of a D’Arsonval movement is 10 mA and internal resistance is 100 Ω. If it is converted to a multirange voltmeter, the value of multiplier required for voltage of 0-50 V is:
1. 5.9 kΩ
2. 4.1 kΩ
3. 5.1 kΩ
4. 4.9 kΩ

Answer 1

Correct Answer - Option 4 : 4.9 kΩ

Concept:

To increase the range of a voltmeter, we need to the series resistance and it is given by

\({R_{se}} = {R_m}\left( {\frac{V}{{{V_m}}} - 1} \right)\)

Where V is the required voltmeter range

Vm is the voltmeter range

Rm­ is the meter internal resistance

Calculation:

Given that,

Meter full scale current reading (Im) = 10 mA

Internal resistance (Rm) = 100 Ω

Voltmeter range (Vm­) = I m­ Rm = 10 × 10-3 × 100 = 1.0 V

Required voltmeter range (V) = 50 V

\({R_{se}} = 100\left( {\frac{{50}}{1} - 1} \right) = 4.9k\;{\rm{Ω }}\)