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.
The line neutral and the load neutral are shared, but the hot in and out are not. If one taps the winding beyond the hot in, one can get higher voltages than put in (due to induced current and "turns ratio" like a conventional transformer). If you are confused, this is impossible in the above image, the hot in is at the end of the transformer (some variacs are set up this way), but if we move the hot in down a little (about where the hot out is now), we can slide the hot out further left and get a higher voltage than what we put in! 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.
The line neutral and the load neutral are shared, but the hot in and out are not. If one taps the winding beyond the hot in, one can get higher voltages than put in (due to induced current and "turns ratio" like a conventional transformer). If you are confused, this is impossible in the above image, the hot in is at the end of the transformer (some variacs are set up this way), but if we move the hot in down a little (about where the hot out is now), we can slide the hot out further left and get a higher voltage than what we put in! Here are some happy pictures of my variacs.