One ratio governs voltage, turns, and current
An ideal transformer links voltage, turn count, and current through a single ratio: whatever factor the voltage steps down by, the turns step down by the same factor — but the current steps up by that same factor, because the power going into the primary has to equal the power coming out of the secondary.
- V1, V2 — primary, secondary voltage (V)
- N1, N2 — primary, secondary turn count
- I1, I2 — primary, secondary current (A)
More detail
Why current runs opposite to voltage
Voltage and turns move together (V1/V2 = N1/N2), but current moves the other way (I2/I1 = N1/N2 = V1/V2). Step a voltage down by 10× and the secondary draws 10× the current the primary supplies, for the same delivered power — that's why a step-down transformer's secondary winding is made of thicker wire than its primary, and why a step-up transformer (secondary voltage higher) always has a lower secondary current, never higher.
Sizing wire and fuses. Once you know the secondary current from the ratio, don't forget the primary side draws current too — a step-down transformer's primary current is at least I2/ratio (magnetizing current and losses add to it, and inrush is separate), and that's what determines the fuse or breaker rating on the input side, not the secondary current.
Frequently asked questions
A transformer steps 120V down to 12V — what's the turns ratio?
Ratio = V1/V2 = 120/12 = 10:1. If the primary has 1,000 turns, the secondary has N2 = 1000/10 = 100 turns.
Primary draws 2A at 120V, stepped down to 12V — what's the secondary current?
I2 = I1 × ratio = 2 × 10 = 20 A. Power in equals power out for an ideal transformer: 120V × 2A = 240W on the primary, 12V × 20A = 240W on the secondary.
I know the ratio is 10:1 and the primary has 1,000 turns — how many turns on the secondary?
Select "Primary turns & known ratio", enter ratio 10 and N1 1000. N2 = N1 / ratio = 1000 / 10 = 100 turns.
Does a step-up transformer ever have higher secondary current?
No — it's always the opposite. A step-up transformer raises voltage on the secondary (ratio below 1, since V1 < V2), which means the secondary current is always lower than the primary current, following the same I2 = I1 × ratio relationship.