Explain the difference between a transformer and auto-transformer

[PLW]

Hi there, I understand that a transformer is used to either step up or step down the supply voltage. I know that the basic principle is that if there are more windings in the primary coil than the secondary coil then, the voltage is stepped down and if there a more windings in the secondary coil then the voltage is increased. Please could someone explain the differences between a transformer and an auto-transformer. Also please could you also give examples of their applications and why you would use one over the other. Thanks

 

[Wilko]

Hi - An auto transformer has one winding with taps. Pic is worth a thousand words

 

[Lucien Nunes]

Because an autotransformer only has one winding, it can be made smaller and cheaper than a double-wound transformer of similar rating, or conversely one of the same size can be wound with heavier wire and have lower losses. But it does not provide isolation between input and output. Examples: For reducing 230V to 208V for American 3-phase machine, an autotransformer is much more economical. OTOH a 230V - 110V tool transformer cannot be an auto, as isolation is required for safety and so that the centre-tap can be earthed.