Transformer Turns Ratio Calculator

How it is calculated

For an ideal transformer the voltages follow the turns: Vs = Vp × Ns / Np, where Vp is the primary voltage in volts, Np the primary turns and Ns the secondary turns. The turns ratio is shown as Np / Ns : 1. If you enter the primary current Ip in amperes, the secondary current is Is = Ip × Np / Ns, because the power in equals the power out.

Turns must be positive and the voltage cannot be negative. The current line is left empty when no primary current is given. An ideal transformer has no losses, so real secondary voltage under load is slightly lower and real currents slightly differ. Alternating current is required, since a transformer does not work on steady DC.

Common mistakes

• Swapping primary and secondary turns, which turns a step-down into a step-up.
• Entering the secondary current in the primary-current field; the field expects the primary current, and the secondary one is calculated.
• Expecting the ratio to give a higher current and a higher voltage together; power is conserved, so one goes up as the other goes down.
• Ignoring losses and the maximum VA rating of a real transformer, and trying to draw the calculated current from it.