Question

The efficiency of a transformer is 90%. The input voltage is 20 V and produced 100 W of the output power. What is the currents in the primary coil?

Answer 1

From the formula for the efficiency of the transformer($\eta=\frac{P_2}{P_1}$ ) we determine the input power

$P_1=\frac{P_2}{\eta}=\frac{100}{0.9}=111\space{ }W$

From the formula for the input power,($P_1=U_1 \cdot I_1$ ) we determine the current in the primary coil

$I_1=\frac{P_1}{U_1}=\frac{111}{20}=5.55 \space{}A$