An autotransformer is an electrical transformer with only one winding. The winding has at least three electrical connection points called taps. The voltage source is applied to two taps and the load is connected to two taps one of which is usually a common connection that is also connected to the source. Each tap corresponds to a different source or load voltage. In an autotransformer a portion of the same winding effectively acts as part of both the primary and secondary winding. Operation The ratio of secondary to primary voltages is equal to the ratio of the number of turns of the winding they connect to. For example, connecting at the middle and bottom of the autotransformer will halve the voltage. Depending on the application, that portion of the winding used solely in the higher-voltage (lower current) portion may be wound with wire of a smaller gauge, though the entire winding is directly connected. Limitations

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