A transformer has a turns ratio of 20:1 and a secondary voltage of 120 V. What is the primary voltage?

A transformer has a turns ratio of 20:1 and a secondary voltage of 120 V. What is the primary voltage?

• A. 240
• B. 2400
• C. 4160
• D. 7620

Answer: (B)

Voltage on primary and secondary scales with the turns on each winding. In an ideal transformer, Vp/Vs = Np/Ns. With a turns ratio of 20:1, the primary has 20 times as many turns as the secondary, so the primary voltage is 120 V × 20 = 2400 V. So the primary voltage is 2400 V. The other numbers would require a different turns ratio or mixing up which side has more turns.

Voltage on primary and secondary scales with the turns on each winding. In an ideal transformer, Vp/Vs = Np/Ns. With a turns ratio of 20:1, the primary has 20 times as many turns as the secondary, so the primary voltage is 120 V × 20 = 2400 V. So the primary voltage is 2400 V. The other numbers would require a different turns ratio or mixing up which side has more turns.