- A transformer generally utilized to couple a power supply to components, circuits, devices, equipment, or systems. The primary winding of such a transformer is usually connected to a power line, and each of the secondary windings provides a different current and/or voltage output. The rating of a power transformer is stated in terms of the maximum voltage and current-delivering capacity of its secondary windings. Alternatively, there may be a single secondary winding with multiple taps. Also called power-supply transformer.