A transformer has 400 turns in the primary and 200 turns in the secondary. If 240 volts applied to the primary winding, the voltage measure of the secondary winding will be?

• A. 120 volts
• B. 240 volts
• C. 480 volts
• D. 60 volts

Answer: (A)

Voltage on the secondary is proportional to the turns ratio: Vs/Vp = Ns/Np. With Ns = 200 and Np = 400, Ns/Np = 200/400 = 0.5. Applying 240 V to the primary gives Vs = 0.5 × 240 V = 120 V. This configuration is a step-down transformer (half the number of turns on the secondary), so the secondary voltage is half the primary. In an ideal transformer, power is approximately conserved, so the secondary current would be about twice the primary current.