Turns set the voltage, the core sets the power
Turns ratio, winding currents, secondary turns and an indicative core cross-section.
A transformer changes voltage in the same ratio as the turns of its windings. Stepping 220 volts down to 12 means a secondary with 18.3 times fewer turns. Current moves the opposite way, rising by the same factor.
Power is not created by a transformer; it is limited by the core. The larger its cross-section, the more flux it can carry without saturating — which is where the rule of thumb comes from: area in square centimetres is roughly the square root of the power, scaled a little.
What this calculator shows
- The turns ratio and what kind of transformer it makes
- Primary and secondary winding currents
- Secondary turns when the primary is known, plus core cross-section
What to keep in mind
- Losses are ignored: a real transformer runs at 85 to 95 percent efficiency, so primary current is slightly higher than calculated.
- The core area formula is indicative and suits mains transformers at 50 Hz.
FAQs
How do I find turns per volt?
Divide 50 by the core cross-section in square centimetres — the classic approximation for 50 Hz transformers. A 10 cm² core gives five turns per volt.
Why is the secondary current higher?
Because power is conserved. If voltage fell by a factor of 18, current must rise by the same factor or the power balance breaks.
What is an isolation transformer?
One with a ratio of one, leaving the voltage unchanged. It exists purely for galvanic isolation, so the load has no electrical connection to the mains — which matters for safety.
What wire should the windings use?
Sized by current density: 2 to 2.5 amps per square millimetre for small transformers. Eight amps on the secondary needs about 3.5 mm², a little over two millimetres in diameter.
Worked example
A 12 volt step-down
Input: 220 V to 12 V, 100 W
Output: Ratio 18.33, secondary current 8.33 A, core 11.5 cm²
Note: Eight amps out against 0.45 in — exactly 18.33 times more. Which is why the secondary is wound in visibly thicker wire.