The autotransformer

Sometimes, it’s not necessary to provide dc isolation between the primary and secondary windings of a transformer. Then an autotransformer can be used. This has a single, tapped winding. Its schematic symbol is shown in Fig. 1A for an air core, and Fig. 1B for a ferromagnetic core.
An autotransformer can be either a step-down or a step-up device. In Fig. 1, the autotransformer at A is step-down, and the one at B is step-up.
An autotransformer can have an air core, or it can be wound on any of the aforementioned types of ferromagnetic cores. You’ll sometimes see this type of transformer in a radio-frequency receiver or transmitter. It works quite well in impedance-matching applications, and also in solenoidal loopsticks.
Autotransformers are occasionally, but not often, used at audio frequencies and in 60-Hz utility wiring. In utility circuits, autotransformers can step down by a large factor, but they aren’t used to step up by more than a few percent.

Fig. 1: Schematic symbols for autotransformers. At A, air core and at B, ferromagnetic core. The unit at A steps down and the one at B steps up.