True or False: Power is conserved across an ideal transformer, so if the voltage decreases, the current increases proportionally.

• A. True
• B. False
• C. It depends on the transformer
• D. Not enough information

Answer: (A)

Power conservation in an ideal transformer means the power on the primary side equals the power on the secondary side: P_in = P_out. The voltages and currents are linked by the turns ratio, with Vp/Vs = Np/Ns and Ip/Is = Ns/Np. When you combine these relationships, you get Vs Is = Vp Ip. So if the secondary voltage drops, the secondary current must rise to keep the same power transfer, and the current change is the inverse of the voltage change dictated by the turns ratio (Is = (Np/Ns) Ip). For example, stepping down from 120 V to 24 V (a 5:1 voltage reduction) with a given primary current results in a secondary current five times larger to maintain equal power. In real transformers there are losses, but in the ideal case the statement is true.