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lakee911:
A typical transformer consists of a primary and one or more secondaries on a common magnetic core. Inherently the primary and secondary are isolated. Thus, no DC path (low resistance) between them. There could be a potential difference between the primary of any value that does not breakdown the insulation between the primary and secondary. You can deliberately provide a low resistance path, bond, one end of the primary to one end of the secondary if desired. Now no isolation. This is done internally on a single post utility transformer.
An autotransformer is a transformer with one coil and a tap somewhere on the coil. The wire size may be different on each side of the tap. If you classify the primary as one outer end and the tap, and the secondary as said outer end the other outer end, then you have a step-up transformer. Interchange primary and secondary and it is a step down. No DC isolation possible.
A commercial product called a Variac is a variable autotransformer.
If instead of the autotransformer we have a standard step-down transformer, for example 110 V to 10 V, a 110 V AC source, might be the secondary of a transformer, but just call it a voltage source, then if the voltage source is connected to the primary of the step-down transformer there will be 10 V output from the step-down transformer. If one side of the secondary is connected to one side of the source voltage, then the sum of the source and the secondary is 100 V or 120 V depending upon the phasing of the connection. Functionally looks very much like an autotransformer. When used this the transformer is called a buck-boost transformer.
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