Question

A transformer is used to light 140 watt, 24 volt lamp from 240 volts ac mains. If the current in the mains is 0.7 A,

then the efficiency of transformer is

• A. 63.8%
• B. 84%
• C. 83.3%
• D. 48%

Answer 1

Answer: (C)

83.3%

Answer 2

$P_{out} = V_{s}i_{s} = 140W,V_{s} = 24V,V_{p} = 240V,i_{p} = 0.7A\eta = \frac{P_{out}}{P_{in}} \times 100 = \frac{P_{out}}{V_{p}i_{p}} \times 100 = \frac{140}{240 \times 0.7} \times 100 = 83.3\%.$