A small transformer operates based on the principle of electromagnetic induction. It only functions with alternating current and cannot convert direct‑current voltage directly. It is widely applied in power adapters, household appliances and industrial‑control equipment.
It mainly consists of an iron core, a primary winding and a secondary winding. When an AC voltage is applied to the primary winding, alternating current generates a constantly varying alternating magnetic field. The iron core is stacked with high‑permeability silicon‑steel sheets. It confines the magnetic field and transfers most magnetic flux to the secondary winding to reduce magnetic‑energy loss.
According to the law of electromagnetic induction, the changing magnetic field passing through the secondary winding induces an AC electromotive force. The output voltage depends on the turns ratio between the primary and secondary windings. More winding turns correspond to higher voltage. A step‑up transformer has more secondary turns, while a step‑down transformer has fewer secondary turns. Most consumer electronics adopt small step‑down transformers to reduce high‑voltage mains power to low voltage.
For an ideal transformer, input power is approximately equal to output power. Voltage rises as current falls, and voltage drops as current rises. Real‑world small transformers have inevitable losses: eddy‑current loss and hysteresis loss in the iron core, plus resistive heating loss in winding wires. Thus output power is slightly lower than input power, and mild heat is produced during operation.
Small transformers work exclusively for AC. Direct current produces a static magnetic field with no induced voltage on the secondary side, and may overheat and burn out the primary winding. Practical products are fitted with insulating layers and shielding housings to prevent electric leakage and electromagnetic interference for stable and safe operation.
