A 120V; 60W lamp is to be operated on 220V A.C supply mains. Calculate the value of non-inductive resistance that would be required to ensure that the lamp is run on correct value?

A. 100Ω
B. 200Ω
C. 300Ω
D. 500Ω

Correct Answer:

Option B = 200Ω

Explanation

Power of the lamp = 60W;

I=P/V

I=60/120
= 0.5A

R=V/I

Rmains = 220/0.5 = 440Ω

Rlamp = 120/0.5 = 240Ω

∴ The non-inductive resistance to keep the lamp = (440−240)Ω

= 200Ω