Transformer Turns Ratio Calculator
Calculate transformer turns ratio, secondary voltage or current from primary values, and verify impedance transformation for audio and power applications.
How a Transformer Works A transformer operates on the principle of electromagnetic induction, discovered by Michael Faraday in 1831 in England. Alternating current flowing through the primary coil creates a constantly changing magnetic field in the iron core. This changing field induces a voltage in the secondary coil. The key: only AC works in a transformer — DC current creates a static magnetic field and induces nothing.
The Turns Ratio Law The ratio of primary to secondary voltage exactly equals the ratio of turns: V1/V2 = N1/N2. This is precise for an ideal transformer. Real transformers are 95–99% efficient, with losses from resistance in the copper windings (copper loss) and from eddy currents and hysteresis in the iron core (iron loss). High-frequency switching power supplies use ferrite cores instead of iron to reduce these losses.
Current Goes the Other Way While voltage steps up with more secondary turns, current steps down by the same ratio. This is required by conservation of energy: power in = power out. If you double the voltage, you halve the current. A step-up transformer with a 10:1 turns ratio (secondary has 10× more turns) increases voltage 10× and decreases current 10×. This is why power is transmitted at high voltage — to reduce current (and therefore resistive losses in the transmission lines).
The Power Grid’s Remarkable Efficiency In the US, power plants generate electricity at 15,000–25,000V. Transmission substations step it up to 115,000–765,000V for long-distance lines. Distribution substations step it down to 4,000–35,000V for neighborhoods. Pole or pad transformers step it down to 120/240V for your home. This multi-stage system loses only about 5–8% of power in transmission — an engineering feat that took decades to perfect.
Impedance Transformation An often-overlooked property: transformers transform impedance by the square of the turns ratio. A turns ratio of 10:1 transforms impedance by 100:1. This is critical in audio engineering — a vacuum tube amplifier output might have an impedance of 5,000 ohms, while a speaker is 8 ohms. An audio output transformer with the right turns ratio matches these impedances for maximum power transfer. This is why vintage hi-fi equipment has those large output transformers.