Autotransformers

An autotransformer is a "transformer with at least two windings with a common part". In concrete terms, it is a transformer with only one winding, the secondary being part of the primary winding.

The current supplying the transformer flows through the primary in its entirety and a bypass at a given point in the transformer determines the output of the secondary. It is smaller, cheaper and has a higher efficiency than two-winding transformers.