Question

A basic 1’ Arsonval movement with internal resistance Rm = 100 Ω and full scale deflection current If = 1 mA is to be converted into a multi range d.c. voltmeter with voltage ranges of 0−10 V, 0−50 V, 0−250 V and 0−500 V. Find the values of suitable multipliers.

Answer 1

Full-scale voltage drop = (1 mA) (100 Ω) = 100 mV. Hence, sensitivity of this movement is 100/100 × 10−3 = 1 kΩ/V.

(i) 0−10 V range

Total resistance required = (1 kΩ/V) (10 V) = 10 kΩ. Since meter resistance is 1 kΩ additional series resistance required for this range R1 = 10 − 1 = 9 kΩ

(ii) 0-50 V range

RT = (1 kΩ/V) (50 V) = 50 kΩ; R2 = 50 − 9 − 1 = 40 kΩ

(iii) 0−250 V range

RT = (1 kΩ/V) (250 V) = 250 kΩ; R3 = 250 − 50 = 200 kΩ

(iv) 0−500 V range

RT = (1 kΩ/V) (500 V) = 500 kΩ ; R4 = 500 − 250 = 250 kΩ