A transformer trades voltage for current at constant power. The exchange rate is the ratio of the turns on each winding, because both windings share the same changing magnetic flux and each turn develops the same induced voltage.
Step the voltage down by ten and the available current goes up by ten. Power in equals power out, minus losses. Nothing is amplified.
The impedance ratio is the square
This is the part that surprises people and the reason transformers appear in audio and RF work at all. A 10:1 transformer transforms impedance by 100:1. That is how a valve amplifier with an 8 kΩ output impedance drives an 8 Ω speaker, and how an antenna is matched to a feedline.
Transformers only work on AC
The induced voltage depends on the rate of change of flux. Apply DC and the flux stops changing, the secondary voltage collapses to zero, and the primary becomes a low-resistance winding across your supply — which is how transformers get destroyed.
Where the ideal model breaks
- Core saturation. Past a certain flux density the core stops responding and primary current rises sharply. This sets the minimum operating frequency.
- Copper and core losses. Winding resistance and hysteresis turn some power into heat. Small mains transformers are often only 70–90% efficient.
- Leakage inductance. Flux that misses the other winding shows up as series inductance and causes the output to sag under load.
- Regulation. A small transformer’s no-load secondary voltage can be 10–20% above its rated full-load figure.