What is a Transformer in Electrical Device? A transformer is an electrical device that changes the voltage of alternating current (AC) to match the needs of different applications: Function Transformers increase (step up) or decrease (step down) voltage while maintaining the same power level. How it works Transformers use electromagnetic induction to create a changing magnetic field in one coil (the primary) that induces a current in a second coil (the secondary). The ratio of turns of wire in t