About transformer calculator
A transformer steps voltage up or down using coils with different numbers of turns. The ratio of voltages equals the ratio of turns.
Formulas
- Vp ÷ Vs = Np ÷ Ns.
- For an ideal transformer, Vp × Ip = Vs × Is, so the side with higher voltage carries less current.
Example
Stepping 230 V down to 12 V needs a ratio of about 19.2 to 1. A 12 V, 5 A secondary draws roughly 0.26 A from the 230 V side.
Real transformers
Real transformers lose some energy as heat, so allow for efficiency below 100 percent.
Another worked example
To step 230 V down to 12 V, the turns ratio is 230 ÷ 12 ≈ 19.2. If the secondary has 100 turns, the primary needs about 1,917 turns. A 12 V, 5 A secondary (60 VA) draws about 0.26 A from the 230 V side, ignoring losses.
Rating and heat
Transformers are rated in VA or kVA. Running one beyond its rating makes it overheat, so choose a rating with some margin above the load.
