Homebrew PCBs

Not correct.  An autotransformer absolutely does have a core.  The 
advantage of an autotransformer is in the size of the windings and core 
for a given power level. For example:
1) Assume a standard 110V to 110V  isolation transformer with a 1 amp 
rating.   So 110V/1A  to 110V/1A rating.
2) A simple wiring connection of the two windings in series gives a 
220V  ability.  At the junction of the windings is a 110V 2A connection!
3) To achieve this with a 220V/110V isolation transformer the windings 
would be a 220V/1A winding plus a 110V/2A winding, requiring about twice 
as much copper and a larger core to allow the larger size of the 
windings.  This is the economic reason for the auto-transformer 
construction .

Regards,
Charles R. Patton

On 2/25/2016 7:39 AM, rolohar@... [Homebrew_PCBs] wrote:
>
> Variacs and autotransformers only have one winding. There is no 
> primary/secondary winding
> configuration. There is no iron core to facilitate induction between  
> the primary and the secondary
> windings.