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What is an Autotransformer?

Alexis W.
Alexis W.

An autotransformer is an electrical transformer containing only a singular winding, rather than separated primary and secondary windings. The single winding still allows for the performance of the same function as an ordinary transformer. In autotransformers, each opposing side of the coil acts as the primary side or the secondary side, depending on which side of the coil is connected to the load versus the power supply. There are at least three connections made to the coil between the two opposing sides that are draws on the voltage created by the autotransformer in order for the transformer to function as it was intended.

The primary function of a transformer is to transform the amount of voltage directed to a circuit by attaching the connection from the power supply to the load at a certain point within the transformer’s winding. Due to the fact that the amount of winding or turns within a transformer’s coil determines the amount of voltage released at any point throughout the winding, the connections can be monitored for output voltage before the connection is made. For instance, if the voltage entering an autotransformer is 100V, the halfway point, or the center of the transformer’s coil, will allow an output voltage of 50V. The output voltage of an autotransformer depends directly on the point at which the connection to the circuit is made in the transformer’s coil.


That doesn’t mean that autotransformers all have each of the same capabilities as an ordinary transformer, however. There are limitations that accompany autotransformers that don’t limit ordinary transformers. For instance, the isolation that an ordinary transformer provides between its individual windings is not available with the use of autotransformers. As a result, failure of the winding’s insulation is possible, which creates a united voltage across the entire winding, making the input voltage the same as the output voltage across the entirety of the winding.

If the winding inside an autotransformer is at a ratio that allows the output voltage at the end of the winding, the total output voltage at the winding’s end can result in an output voltage greater than the input voltage supplied to the autotransformers. This is an important variable to be aware of when creating a power source for multiple circuits through the use of autotransformers. Connecting a circuit to the end of the winding that is unable to tolerate the amount of voltage at the winding’s end could result in damage to the circuit.

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