Question

A common transistor radio set requires 12 V (D.C.). The D.C. source is constructed using a transformer and a rectifier circuit operated at 220 V (A.C.). The number of turns of secondary coil are 24, then the number of turns of primary are _________. (Assume ideal transformer)

Hint:

In a step-down transformer, the number of turns in the primary is greater than the number of turns in the secondary ($N_p>N_s$).

Answer 1

Correct Answer: 440

Step 1: Transformer turns ratio formula: $\frac{V_p}{V_s} = \frac{N_p}{N_s}$.
Step 2: $V_p = 220$ V, $V_s = 12$ V (peak D.C. output roughly corresponds to secondary RMS).
Step 3: $\frac{220}{12} = \frac{N_p}{24}$.
Step 4: $N_p = \frac{220 \times 24}{12} = 220 \times 2 = 440$.