A transformer has 300 turns of wire in the primary coil and 30 turns in the secondary coil. If the input voltage is 100 volts, the output voltage is

Analyze the question We have an ideal transformer with 300 primary turns, 30 secondary turns and an input voltage of 100 V. We need the output voltage. The turns ratio is 300:30, so this is a step-down transformer.

The concept For a transformer, the voltage across each coil is proportional to its number of turns:

Vs / Vp = Ns / Np

so

Vs = Vp × (Ns / Np)

• Ns > Np: step-up transformer (voltage increases)
• Ns < Np: step-down transformer (voltage decreases)

Solution Given: Np = 300, Ns = 30, Vp = 100 V

Vs = 100 × (30 / 300) Vs = 100 × 0.1 Vs = 10 V

This agrees with the voltage falling, since the secondary has 10 times fewer turns than the primary, so the voltage is 10 times smaller.

Answer: B. 10 volts

Exam tip Voltage follows the turns, so just divide the primary voltage by the turns ratio. Ask yourself: "How many times fewer (or more) turns?" and scale the voltage the same way. Here the turns drop by a factor of 10, so 100 V becomes 10 V. Also remember that for an ideal transformer, VpIp = VsIs (power in = power out), so a step-down transformer gives a larger current in the secondary.