A common transistor radio set requires 12 V D.C. for its operation. The D.C. source is constructed by using a transformer and a rectifier circuit, which are operated at 220 V A.C. on standard domestic A.C. supply. The number of turns of secondary coil are 24, then the number of turns of primary are ______

We have a transformer that converts 220 V A.C. to 12 V D.C. The secondary coil has 24 turns.

For an ideal transformer, the voltage ratio equals the turns ratio: $$\frac{V_1}{V_2} = \frac{N_1}{N_2}$$.

Here, $$V_1 = 220$$ V, $$V_2 = 12$$ V, and $$N_2 = 24$$.

Substituting, $$\frac{220}{12} = \frac{N_1}{24}$$.

$$N_1 = \frac{220 \times 24}{12} = \frac{5280}{12} = 440$$

Hence, the number of turns in the primary coil is 440.

So, the answer is $$440$$.