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 ___________.

Solution

Physics sir · 3 steps

The transformer only changes the voltage. Set turns ratio equal to voltage ratio and solve for the primary.

1. Which voltages. The primary takes the 220 V mains, and the secondary must supply 12 V to the rectifier. The rectifier converts AC to DC. It does not change the size of the voltage, so take Vₛ = 12 V.

   Nₚ/Nₛ = Vₚ/Vₛ

2. Put in numbers. Nₚ = Nₛ × Vₚ/Vₛ = 24 × 220/12 = 2 × 220 = 440.
3. So. This is a step-down transformer, so the primary needs more turns than the secondary. The primary has 440 turns.

The idea to keep. In an ideal transformer the voltage per turn is the same on both coils, so Vₚ/Vₛ = Nₚ/Nₛ. A step-down transformer always has more turns on the primary.

Answer from the NTA final answer key. Solution by PrepWiser.