A transformer is a static device which transfers AC electrical power from one circuit to another at the same frequency but the voltage level is changed usually. When the voltage is raised up to on the output side (V₂> V₁), the transformer is called step-up transformer, whereas the transformer in which the voltage is lowered on the output side (V₂< V₁) is called a step-down transformer. Transformers are used in electronic equipments not only to step-up or step-down the voltage but also to electrically isolate, the electronic circuit from the mains.