An autotransformer is an electrical transformer with only one winding. In an autotransformer a portion of the same winding acts as part of both the primary and secondary winding.The winding has at least three taps where electrical connections are made. The voltage source and the electrical load are each both connected to two taps with one tap at the end of the winding being a common connection to both circuits. Each tap along the winding corresponds to a different source or load voltage.