Voltage transformers, commonly called transformers, are devices capable of increasing or reducing voltage values.

A transformer consists of a core made of a highly magnetizable material and two coils with different number of turns isolated from each other, called primary (coil that receives the mains voltage) and secondary (coil where the transformed voltage comes out).

Its operation is based on the creation of an induced secondary current from the flow variation generated by the primary.

Input and output voltage is proportional to the number of turns in each coil. Being:

Where:

- is the tension in the primary;
- is the secondary voltage;
- is the number of turns of the primary;
- is the number of turns of the secondary.

From this proportionality we conclude that a transformer reduces the voltage if the number of turns of the secondary is less than the number of turns of the primary and vice versa.

If we consider that all energy is conserved, the power at the primary should be exactly the same as the power at the secondary, like this: