An auto transformer is a type of transformer that shares a portion of its winding between the primary and secondary sides. It differs from a normal (or conventional) transformer primarily in the way the windings are configured. Here's how they compare:

1. Construction:

   • Normal Transformer: In a normal transformer, the primary and secondary windings are completely separate and insulated from each other. They do not share any part of the winding.
   • Auto Transformer: In an auto transformer, a portion of the winding is common to both the primary and secondary sides. This shared winding serves as both primary and secondary, with the remaining portion acting as either the primary or secondary winding, depending on the application.

2. Voltage Transformation:

   • Normal Transformer: A normal transformer provides electrical isolation between the primary and secondary circuits and can step up or step down the voltage without a direct electrical connection between them.
   • Auto Transformer: An auto transformer can also step up or step down the voltage, but it does not provide electrical isolation between the primary and secondary circuits since a portion of the winding is shared. This can lead to potential drawbacks in terms of galvanic isolation and insulation requirements.

3. Efficiency and Size:

   • Auto transformers tend to be more efficient and compact compared to normal transformers, primarily because they use fewer winding turns and require less copper for construction. However, this efficiency advantage comes at the cost of reduced isolation and insulation capabilities.

4. Applications:

   • Auto transformers are commonly used in applications where voltage adjustment is required without the need for complete electrical isolation, such as in voltage regulators, motor starters, and power transmission systems.
   • Normal transformers are preferred in applications where electrical isolation between the primary and secondary circuits is critical, such as in power distribution, safety isolation, and impedance matching.

In summary, while both auto transformers and normal transformers are used for voltage transformation, they differ in construction, voltage isolation, efficiency, and application suitability. The choice between them depends on the specific requirements of the application, considering factors such as voltage regulation, isolation needs, efficiency, and size constraints.