Hey guys, my website has moved, I will no longer update this one.  you want to go here for my new site.

     Sometimes a Tesla coil operator wants to vary the voltage going into his step up transformer.  This is done with a "variac".  The term Variac is actually a name brand for a device called a variable auto transformer.  An auto transformer is a transformer with only one winding.  A conventional transformer works by current creating magnetic fields in one winding which induces a current in another winding (this may or may not involve a steel or ferrite core).  The voltage put out is relative to the number of turns in either windings (the turn ratio), and the voltage put in.  In an auto transformer, there is only one winding with taps at various points along it.  A magnetic field is induced by current through the winding (carried by a core, usually toroidal in shape (dough nut)) and voltage can be pulled off of it by tapping at different points.  A variac simply has a movable tap (the hot out).  This is the schematic for a typical variac.
Well I'll put one here as soon as I draw it, so chill out Ok.
The line neutral and the load neutral are shared, but the hot in and out are not.  If one taps the winding beyond the neutral out, one can get higher voltages than put in (due to induced current and "turns ratio" like a conventional transformer).  Here are some happy pictures of my variacs.

I built a variac once, and it did work but the turns would overlap and heat up and melt, which is not good, but I have included a picture of it anyway.  If you want construction details of it, e-mail me.

Tesla Coil, Hobbies, Home.

Scott Bogard. 2007