The formula
V₁ / V₂ = N₁ / N₂
| Symbol | Quantity | Unit |
|---|---|---|
| V₁ | Primary voltage | V |
| V₂ | Secondary voltage | V |
| N₁ | Primary turns | |
| N₂ | Secondary turns |
What it means
The ratio of voltages equals the ratio of turns. Power is conserved, so raising the voltage lowers the current by the same factor — a transformer trades one for the other and creates neither.
How to work it out by hand
- Divide the secondary turns by the primary turns
- Multiply the primary voltage by that ratio
- The current changes by the inverse ratio
What is worth knowing
Transformers are why the grid runs on alternating current: they only work with a changing magnetic field, so they cannot step DC up or down directly, and cheap voltage conversion is what makes long-distance transmission viable. The efficiency of large transformers exceeds 99 %, but 'power is conserved' is the point rather than the caveat — a step-up transformer gives you volts at the cost of amps, and no arrangement of coils produces energy. Anyone offering a transformer that does should be treated the way you would treat a perpetual motion machine.