Transformer Turns Ratio Calculator
Find output voltage or required turns for a transformer.
Calculate
Formulas
V1/V2 = N1/N2 = I2/I1Voltage ratio equals turns ratio. Current is inversely proportional.
Transformer Basics
Step-down: more primary turns. Step-up: more secondary turns. Ideal transformer: P_in = P_out. Real efficiency: 95-99% for power transformers. Used in power supplies, chargers, and grid distribution.
How Transformer Ratios Work
A transformer changes AC voltage up or down by the ratio of turns in its primary and secondary windings. Voltage scales with the turns ratio while current scales inversely:
V1/V2 = N1/N2 = I2/I1V is voltage, N is the number of turns, I is current; subscript 1 is the primary, 2 the secondary.
If the secondary has fewer turns, voltage steps down and current steps up proportionally (ignoring losses), because power in equals power out for an ideal transformer.
Worked Examples
Primary 230 V with 1000 turns, secondary 100 turns:
V2 = V1 × N2/N1 = 230 × 100/1000 = 23 V
If the secondary delivers 5 A at 23 V, the primary current is:
I1 = I2 × N2/N1 = 5 × 100/1000 = 0.5 A
Power checks out: 230×0.5 = 115 W in, 23×5 = 115 W out.
Ideal vs Real Transformers
The turns-ratio relationship assumes an ideal, lossless transformer. Real transformers lose a few percent of power to winding resistance (copper losses) and core magnetisation (iron losses), so the actual secondary voltage under load is slightly lower than the ideal calculation. Efficiency for well-designed mains transformers is typically 95–99%.
Frequently Asked Questions
Do transformers work with DC?
No. Transformers rely on a changing magnetic field to induce voltage in the secondary. Steady DC produces no change, so no transfer occurs (and the primary may overheat).
Why does current go up when voltage goes down?
Power is conserved. If voltage halves, current must double to deliver the same power (minus small losses).
What is a 1:1 transformer for?
Isolation. It transfers power without changing voltage, electrically separating two circuits for safety or noise reduction.