In the transformer example, what is the secondary current if the primary current is 1 A and the input is 28 V with 400 primary turns and 100 secondary turns?

• A. 1 A
• B. 0.25 A
• C. 2 A
• D. 4 A

Answer: (D)

In an ideal transformer, power in equals power out, and the voltages relate to turns by Vs = Vp × (Ns/Np). Here, Ns/Np = 100/400 = 1/4, so Vs = 28 V × 1/4 = 7 V. The input power is 28 W (28 V × 1 A). To keep the power balance, the secondary must supply Is = Power / Vs = 28 W / 7 V = 4 A. Equivalently, Is = Ip × (Np/Ns) = 1 A × (400/100) = 4 A. So the secondary current is 4 A.