Question

In a step-down transformer having primary to secondary turns ratio of $20:1$, the input voltage applied is $250\text{V}$ and output current is $8\text{A}$. Assuming $100\%$ efficiency, calculate current in the primary coil and the output power.

• A. $0.4\text{A}$, $100\text{W}$
• B. $0.8\text{A}$, $200\text{W}$
• C. $0.2\text{A}$, $50\text{W}$
• D. $0.1\text{A}$, $50\text{W}$

Answer 1

The correct option is A $0.4\text{A}$, $100\text{W}$

Using ideal conditions and transformer equation, we have

$\frac{I_p}{I_s}=\frac{N_s}{N_p}$

$\Rightarrow I_p=\frac{N_s}{N_p}\times I_s$

Substituting the values,

$\Rightarrow I_p=\frac{1}{20}\times 8=0.4\text{A}$

Since. the transformer is ideal,

$\therefore \text{Output power = Input power}$

$P_s=P_p=V_p{I}_p$

$\Rightarrow P_s=250\times 0.4=100\text{W}$

Hence, option $\left(A\right)$ is correct.