In Auto Transformer, one single winding is used as primary winding as well as secondary winding. A diagram of auto transformer is shown below, the winding AB of total turns N1 is considered as primary winding. This winding is tapped from point ′C′ and the portion BC is considered as secondary. Let's assume the number of turns in between pointís ′B′ and ′C′ is N2. If V1 voltage is applied across the winding i.e. in between ′A′ and ′C′.