Question

A transformer is used to light a 120 W, 24 V lamp from 240 V a.c. mains. The current in the main cable is 0.6 A. The efficiency of the transformer is :

• A. 48%
• B. 63.8%
• C. 83.3%
• D. 90%

Answer 1

Answer: (C)

Answer is C.

The efficiency of a transformer can be calculated using Equation,
$Efficiency=\frac{Outputpowerofthetransformer}{Inputpowerofthetransformer}=\frac{P_s}{P_p}\times 100$
In this case, the output power is 120W.
The input power is calculated as follows.
The input voltage is 220 V and current is 5 A
$Power=VI=240\times 0.6=144W$.
So, $\eta =\frac{120}{144}\times 100=83.3$.
Hence, the efficiency of the transformer that lights the bulb is 83.3%.