Auto transformer is a one type of transformer which has one winding.

In an auto transformer, the portions of the same winding act as both the primary and secondary sides of the transformer. In contrast, an ordinary transformer has separate primary and secondary windings which are not electrically connected

Transformer that act like an Isolation transformer by changing-

  • voltage level
  • current level
  • Impedance values

But does not isolate between the Primary and Secondary.

 The Auto-transformer can be used to either step-up or step-down the applied voltage depending upon the winding configuration.

step-down:  In the step down device, the whole of the winding serves as the primary winding whilst the lower half of the winding serves as the secondary winding. In this case, there are fewer turns in the secondary than the primary. So, the voltage is stepped down, but the current is stepped up. This configuration is typically used in power aircraft instruments where the voltage is stepped down from 115v 400Hz to 26v.

The disadvantage of this format is that the full voltage is placed across the load if the coil goes open circuit so it cause may very dengerous. Since there is no voltage isolation between the two winding.


Step-up:  This will be opposite to the other one So, in the step down devices, the lower half of the winding serves as a primary winding side and the whole of the winding serves as a secondary winding at the secondary side. In this case, there are fewer turns in the primary than secondary so, the voltage is step up but current is stepped down.


In the Auto-transformer one winding is made to benifit for reducing copper requirment or copper cost as while as copper loss. This type of transformer is used where the required voltage varry for the load.

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