An 8 Ω resistive load is connected to the secondary of an ideal transformer with turns ratio N1:N2 = 10:1. What impedance is seen from the primary side?

Answer: A. 800 Ω

In an ideal transformer the primary voltage is n = N1/N2 times the secondary voltage and the primary current is 1/n times the secondary current, so the impedance seen at the primary, V1/I1, is n² times the secondary's V2/I2: 10² × 8 Ω = 800 Ω. Because of this impedance transformation, transformers are also used for impedance matching, for example between an amplifier and a loudspeaker.

• A. Correct. It scales by the square of the turns ratio: 10² × 8 Ω = 800 Ω.
• B. 80 Ω multiplies by the turns ratio only once, ignoring that both voltage and current change.
• C. 0.8 Ω divides by the turns ratio—the transformation in the wrong direction.