Question

1. A 150 v moving iron voltmeter is intended for 50HZ, has a resistance of 3kQ. Find the series resistance required to extent the range of instrument to 300v. If the 300V instrument is used to measure a d.c. voltage of 200V. Find the voltage across the meter?​

Answer 1

Voltmeter reactance = 2π × 50 × 0.7 = 220ΩImpedance of voltmeter = 3000 + j220 = 3008Ω When the voltmeter range is doubled its impedance has also to be doubled in order to have the same current for full-scale deflection. If R is the required series resistance then 3000 + R2 + 2202 = 2 × 30082 ∴ R = 3012Ω When used on d.c. supply if the voltmeter reads 200 V the actual applied d.c. voltage would be = 200 × Total A.C. Impedance/total d.c. resistance = 200 × 2 × 3008/3000 + 3012 = 200 × 6016 × 6012 = 200.134V.