Working principle
A transformer transfers electrical energy between two circuits using a shared magnetic field, with no electrical connection between them. Alternating current in the primary winding sets up an alternating flux in the laminated iron core; that flux links the secondary winding and induces a voltage in it according to Faraday's law. Because the mechanism relies on a changing flux, a transformer works only on AC — connect one to DC and the primary is simply a low-resistance short circuit.
Turns ratio and impedance
Voltage is transformed in proportion to the turns ratio, and current in inverse proportion, so an ideal transformer neither creates nor destroys power. A 10:1 step-down transformer gives one tenth of the voltage and can supply ten times the current. Impedance is transformed by the square of the turns ratio, which is why transformers are also used for impedance matching in audio and radio-frequency circuits rather than only for changing voltage.
Losses and efficiency
Real transformers lose energy in two places. Copper losses are the resistive losses in the windings and rise with the square of the load current. Iron losses — hysteresis and eddy currents in the core — depend on flux and frequency and are essentially constant whenever the transformer is energised, which is why laminated, grain-oriented steel is used. Efficiency peaks where the two are equal and typically reaches 95–99 % in a well-designed unit.
Where it is used
Transformers make the AC grid possible: generation at medium voltage, transmission at hundreds of kilovolts to keep line current and losses low, then stepping back down for distribution and household use. Smaller single-phase transformers appear in power supplies, isolation and safety applications, welding sets, doorbells and audio equipment. Isolation transformers with a 1:1 ratio provide no voltage change at all — their whole purpose is safety and noise separation.