ASVAB (Armed Services Vocational Aptitude Battery)Electronics Information (EI)Medium
A transformer has 500 turns in its primary coil and 100 turns in its secondary coil. If the primary voltage is 120 V AC, what is the voltage across the secondary coil?
- A24 V
- B240 V
- C60 V
- D600 V
Show answer & explanationAnswer & explanation
Correct answer: A. 24 V
The voltage ratio in a transformer is equal to the turns ratio: Vp/Vs = Np/Ns. Rearranging for Vs: Vs = Vp * (Ns/Np). Given Vp = 120V, Np = 500, Ns = 100. Vs = 120V * (100/500) = 120V * (1/5) = 24V.
Why the other options are wrong
- B. This would be if the transformer was a step-up (2x), not a step-down (1/5x).
- C. This would be 120V * (250/500), which implies a turns ratio of 1:2 not 5:1.
- D. This would be 120V * 5, incorrectly applying the ratio as a step-up by Np/Ns.
Transformer
An electrical device that transfers electrical energy between two or more circuits through electromagnetic induction, typically to change voltage levels.
- Consists of a primary coil and a secondary coil wound around a common core.
- Operates on AC (alternating current) only.
- Turns ratio (Np/Ns) determines voltage ratio (Vp/Vs) and current ratio (Is/Ip).
Memory trick: Turns-Voltage-Current: Np/Ns = Vp/Vs = Is/Ip (inverse current!).