FAA Aviation Mechanic General (AMG)Fundamentals of Electricity and ElectronicsHard
A step-down transformer has a primary winding with 100 turns and a secondary winding with 20 turns. If 120 volts AC is applied to the primary, what voltage is induced in the secondary winding?
- A24 V
- B600 V
- C2,400 V
- D20 V
Show answer & explanationAnswer & explanation
Correct answer: A. 24 V
Transformer secondary voltage is found using the turns ratio: Vs = Vp × (Ns/Np) = 120V × (20/100) = 24 volts.
Why the other options are wrong
- B. This results from dividing incorrectly (120×5) rather than using the ratio.
- C. This results from an inverted ratio calculation (120×20).
- D. This is only the number of secondary turns, not the voltage.
Transformer Turns Ratio
A transformer changes voltage between primary and secondary windings based on the ratio of turns, following Vs/Vp = Ns/Np.
- Step-up transformers have more secondary turns than primary
- Step-down transformers have fewer secondary turns than primary
- Vs = Vp × (Ns/Np)
- Power is conserved: Vp×Ip = Vs×Is (ideal transformer)
Memory trick: More turns, more volts — fewer turns, fewer volts.