Transformer Voltage Ratio Calculator

Transformer voltage and turns ratio

A transformer links two windings through a shared magnetic core. In the ideal case, the turns ratio alone sets both the voltage and the current ratios: V1 / V2 = N1 / N2 = I2 / I1. Double the secondary turns and the secondary voltage doubles while the secondary current is cut in half, so the power stays put (V1·I1 = V2·I2, less the core and copper losses).

Take a primary of N1 = 100 turns fed by 127 V and a secondary of N2 = 200 turns. You get V2 = 127 · 200/100 = 254 V. That ideal-transformer picture leaves out leakage inductance, the resistance of the windings, and the losses from hysteresis and eddy currents. Real units land somewhere around 95–99% efficiency at rated load.

Applications

Nothing about the AC grid works without them. Distribution networks step 13.8 kV down to 220/127 V for homes. Isolation transformers keep sensitive gear away from earth references in hospitals and labs. Audio output transformers match the high impedance of vacuum-tube amplifiers to 4–8 Ω speakers, and current transformers feed the protection relays. The Brazilian standard NBR 5356 and the international IEC 60076 series lay out the performance, testing and tolerances expected of power transformers.

FAQ

Why does the current ratio invert? It comes down to conserving power. Multiply the secondary voltage by the turns ratio and the available secondary current has to be divided by that same ratio, otherwise V·I would not stay constant.

Will a transformer work on DC? No. The secondary voltage is induced by changing magnetic flux, and that needs alternating or at least pulsed current. Feed it steady DC and you just saturate the core and cook the primary winding.

What is impedance reflection? A load Z_L sitting on the secondary shows up on the primary as Z_L·(N1/N2)². That is exactly why output transformers handle impedance matching in audio and RF amplifiers.