With a transformer having primary turns 200 and secondary turns 50, if the secondary current is 2 A, what is the primary current (ideal transformer)?

With a transformer having primary turns 200 and secondary turns 50, if the secondary current is 2 A, what is the primary current (ideal transformer)?

• A. 0.5 A
• B. 2 A
• C. 0.25 A
• D. 1 A

Answer: (A)

In an ideal transformer, the voltage and turns relate by Vp/Vs = Np/Ns, and the currents relate inversely by Ip/Is = Ns/Np (so power remains the same: Vp Ip ≈ Vs Is). Here the turns ratio is Np:Ns = 200:50 = 4:1, meaning Vs is one quarter of Vp and the primary current is one quarter of the secondary current. With Is = 2 A, the primary current is Ip = Is × (Ns/Np) = 2 A × (50/200) = 0.5 A.

In an ideal transformer, the voltage and turns relate by Vp/Vs = Np/Ns, and the currents relate inversely by Ip/Is = Ns/Np (so power remains the same: Vp Ip ≈ Vs Is). Here the turns ratio is Np:Ns = 200:50 = 4:1, meaning Vs is one quarter of Vp and the primary current is one quarter of the secondary current. With Is = 2 A, the primary current is Ip = Is × (Ns/Np) = 2 A × (50/200) = 0.5 A.