A transformer has 400 turns of wire in the primary coil and 40 turns in the secondary col. If the input…

Step 1: Analyze the question We have a transformer with:

• Primary turns, Np = 400
• Secondary turns, Ns = 40
• Input (primary) voltage, Vp = 150 V
• Output (secondary) voltage, Vs = ?

Step 2: The concept For an ideal transformer, the voltage ratio equals the turns ratio:

Vs / Vp = Ns / Np

Since Ns < Np, this is a step-down transformer, so the output voltage should be smaller than 150 V.

Step 3: Solution

Vs = Vp × (Ns / Np)

Vs = 150 × (40 / 400)

Vs = 150 × 0.1

Vs = 15 V

Step 4: Answer The output voltage is 15 V, which is option D.

Tip: Look at the turns ratio first. Here 400 : 40 is 10 : 1, so the voltage is divided by 10 (150 ÷ 10 = 15). If the secondary has fewer turns, it is step-down, so the output voltage is smaller. If the secondary has more turns, it is step-up, so the output voltage is larger. This lets you eliminate wrong options quickly in the exam.