Question

A projector lamp can be used at a maximum voltage of 60 V, its resistance is 20 $\Omega$ , the series resistance (in ohms) required to operate the lamp from a 75 V supply is

• A. 2
• B. 3
• C. 4
• D. 5

Answer 1

Answer: (D)

Current flowing through the lamp, $I=\frac{V}{R}=\frac{60}{20}=3A$ The current in the lamp must remain 3A, when it is operated at 75 V supply. Let R be the resistance connected in series, then $\frac{75}{20+R}=3$ $\Rightarrow$ $75=60+3R$ or $3R=75-60$ or $3R=15$ or $R=5\Omega$