autotransformer current transformer custom transformers AUTOTRANSFORMER An autotransformer is a type of transformer that has the primary winding employed as a secondary winding, or vice versa; This process is also named a compensator or balancing coil. An autotransformer is a simple and inexpensive way to correct problems that arise when a secondary voltage is difficult to obtain or a transformer you have is either too high or too low. A transformer that has a part of its winding included in both the input and output circuit is known as an autotransformer. In a simple autotransformer connection there is a common portion of the winding that has no isolation between the input and output circuits, and there is only one winding.