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.
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 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. 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.
