Autotransformer:

Sometimes, it is not necessary to provide direct current isolation between the primary and secondary windings of the transformer. Then an autotransformer can be used. This comprises of a single, tapped winding. The symbol of it is shown in the Figure given below for an air core, and for a ferromagnetic core.

An autotransformer can be a step down or a step up device. In the given figure the autotransformer present at A is step down, and the one present  at B is step up. An autotransformer can have an air core, or it can be wound on any of the above mentioned types of the ferromagnetic cores. You will sometimes see this type of transformer in the radio frequency receiver or transmitter. It works well in impedance matching applications, and in solenoidal loopsticks as well.

Autotransformers are occasionally, used at the audio frequencies and in 60 Hz wiring. In the utility circuits, autotransformers can step down by the large factor, but they are not taken in use to step up by more than some percent.

Figure--   Schematic symbols for the autotransformers. At point A, air core and at point B, ferromagnetic core. The unit at A steps down and one at B steps up.