Autotransformers are special types of transformers in which the primary and secondary circuits are electrically connected.
Autotransformers are commonly used in test circuits, when it is desired to vary the voltage applied to a circuit continuously, or in steps. In the design stage, connecting the circuit to the full voltage at once is not recommended, as some unpredictable accidents might happen.
Figs. 9-31 and 9-32 show the schematic diagrams of two autotransformers used to step up and step down the voltage, respectively.
In the step-down autotransformer (Figure 9-31):
In the step-up autotransformer (Figure 9-32):
The relations for voltage and current transformations will be the same as those for ordinary transformers, i.e.,
where is the turns-ratio.