A transformer with turns ratio Np:Ns = 240:120. If the primary is supplied with 240 V, what is the secondary voltage? If the secondary current rating is 2 A, what is the primary current (ideal transformer)?

• A. Vs = 240 V; Ip = 0.5 A
• B. Vs = 480 V; Ip = 0.5 A
• C. Vs = 120 V; Ip = 1 A
• D. Vs = 120 V; Ip = 2 A

Answer: (C)

The key idea is that a transformer’s voltage ratio matches its turns ratio, and power is conserved in an ideal transformer. With Np:Ns = 240:120, the secondary voltage is scaled by Ns/Np = 120/240 = 0.5, so Vs = 240 × 0.5 = 120 V. For an ideal transformer, Vp Ip = Vs Is, so Ip = (Vs/Vp) × Is = (120/240) × 2 A = 1 A. Therefore the correct result is Vs = 120 V and Ip = 1 A. The other options would violate either the voltage ratio or the power balance.