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    Transformer Turns Ratio Calculator

    GCSE PhysicsAQAEdexcelOCRHigher tier, GCSE Physics only

    Find the primary or secondary potential difference, or the number of turns on either coil, using Vp ÷ Vs = np ÷ ns.

    Quick answer

    Vp ÷ Vs = np ÷ ns. To find the output, Vs = Vp × ns ÷ np. For example, 230 V across 1000 primary turns with 50 secondary turns gives 230 × 50 ÷ 1000 = 11.5 V (a step-down transformer).

    By Engr. Mehmood Kakar · Updated

    Vp ÷ Vs = np ÷ ns
    Find:

    Simulation

    Step it up or step it down

    Set the primary voltage and the number of turns on each coil. More turns on the secondary steps the voltage up; fewer steps it down.

    230 V
    500 turns
    50 turns

    Vp/Vs = np/ns. A transformer only works with alternating current, because it needs a changing magnetic field in the core.

    Put these numbers in the calculator above

    How to use the transformer equation

    A transformer has two coils of wire wound on an iron core. The ratio of the potential differences across the coils equals the ratio of their numbers of turns:

    Vp ÷ Vs = np ÷ ns

    • Vp and Vs are the potential differences across the primary and secondary coils in volts (V)
    • np and ns are the numbers of turns on each coil

    A transformer with 1000 turns on the primary and 50 on the secondary, connected to 230 V mains, gives 230 × 50 ÷ 1000 = 11.5 V. Fewer turns on the secondary means a smaller output p.d.: a step-down transformer.

    A step-up transformer has more turns on the secondary, so it increases the p.d. The National Grid uses step-up transformers so electricity is sent at a high p.d. and low current, which reduces energy lost heating the cables. Transformers only work with alternating current.

    Common mistakes that cost marks

    1. Flipping one side of the ratio. Keep primary over secondary on both sides.
    2. Not converting kV. 25 kV is 25,000 V.
    3. Mixing up step-up and step-down. Step-up has more turns on the secondary and a bigger output p.d.

    Practice questions

    Try each one on paper first, then reveal the worked answer.

    Q1A transformer has 1000 turns on its primary coil and 50 on its secondary. The primary is connected to 230 V. Calculate the secondary p.d.[3 marks]
    1. Vs = Vp × ns ÷ np
    2. Vs = 230 × 50 ÷ 1000
    3. Vs = 11.5 V
    Q2A step-up transformer has 200 primary turns and 4000 secondary turns. The input is 400 V. Calculate the output p.d.[3 marks]
    1. Vs = 400 × 4000 ÷ 200
    2. Vs = 8000 V
    Q3A transformer changes 230 V to 12 V. The primary coil has 920 turns. How many turns are on the secondary?[3 marks]
    1. ns = np × Vs ÷ Vp
    2. ns = 920 × 12 ÷ 230
    3. ns = 48 turns

    Endless practice

    A new question every time, marked instantly. Work it out on paper, then type your answer.

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    Frequently asked questions

    What is the transformer equation?

    Vp ÷ Vs = np ÷ ns: the ratio of the potential differences equals the ratio of the numbers of turns on the primary and secondary coils.

    What is the difference between a step-up and a step-down transformer?

    A step-up transformer has more turns on the secondary coil, so Vs > Vp. A step-down transformer has fewer turns on the secondary, so Vs < Vp.

    Why do transformers need alternating current?

    A changing current in the primary coil makes a changing magnetic field in the core, which induces a p.d. in the secondary coil. Direct current gives a steady field, so nothing is induced.

    Are transformers Higher tier?

    Yes. In AQA, transformer calculations are in GCSE Physics (separate science) only, Higher tier.