Question

A step-down transformer is connected to main supply 200V to operate a 6V, 30W bulb. The current in primary is

• A. 3 A
• B. 5 A
• C. 0.3 A
• D. 0.15 A

Answer 1

Answer: (D)

0.15 A

Answer 2

$V_{p} = 200\mspace{6mu} V,\mspace{6mu}\mspace{6mu} V_{s} = 6V$

$P_{out} = V_{s}i_{s} \Rightarrow 30 = 6 \times i_{s} \Rightarrow i_{s} = 5\mspace{6mu} A$

From $\frac{V_{s}}{V_{p}} = \frac{i_{p}}{i_{s}} \Rightarrow \frac{6}{200} = \frac{i_{p}}{5} \Rightarrow i_{p} = 0.15\mspace{6mu} A$