How to use it
- Choose what to find: the secondary voltage (you know both turn counts) or the secondary turns (you know the voltage you want).
- Enter the primary voltage and the number of primary turns.
- Enter the secondary turns or the target secondary voltage, and the load in VA.
- Read the result, the turns ratio, both currents and the turns per volt.
The ideal transformer equations
- Voltage: Vs = Vp × Ns / Np.
- Turns: Ns = Np × Vs / Vp.
- Turns ratio: a = Np / Ns = Vp / Vs.
- Currents: Ip = S / Vp and Is = S / Vs, so Ip / Is = Ns / Np.
- Turns per volt: Np / Vp.
The ideal model has no winding resistance, no leakage and no magnetising current, and input power equals output power.
Worked example
A 230 V primary with 1,000 turns and a 50-turn secondary: a = 1,000 ÷ 50 = 20, Vs = 230 ÷ 20 = 11.5 V. With a 100 VA load, Ip = 100 ÷ 230 = 0.4348 A and Is = 100 ÷ 11.5 = 8.6957 A. The core takes 1,000 ÷ 230 = 4.3478 turns per volt.
To get 12 V from a 230 V primary of 1,150 turns: Ns = 1,150 × 12 ÷ 230 = 60 turns, ratio 19.1667.
Tips and common mistakes
- Use the same kind of voltage on both sides: RMS with RMS. A 12 V RMS winding rectified and smoothed charges a capacitor to about its peak, 12 × √2 ≈ 17 V, minus the diode drops.
- VA is not watts. Transformers are rated in VA because heating depends on current, whatever the power factor of the load.
- Allow for losses. Real voltage under load is a few percent lower than the ideal figure, more in small transformers.