transformer schematic

[v0ltair3]

Thank you all once again 

What do you mean wiring it up as an autotransfomer ? I was planing to add a variac in "front" of it, but I didn't got any good deal on one so far...

[m k]

Transformer is finally just nested loops.
If you put all wires in series you can still put voltage in and take it out freely, or sort of freely, insulation is still what it is and wire thickness also.
It happens so that you connect one end of primary and secondary together, winding direction is meaningful here, one end increase and other end decrease the overall voltage.

In early days galvanic connection was not an issue, so cheap transformers had only one winding.
There middle taps were used for different voltages, the picture has a regular way.
But it doesn't need to be from 0 to something.

[MrAl]

Thank you all once again 

What do you mean wiring it up as an autotransfomer ? I was planing to add a variac in "front" of it, but I didn't got any good deal on one so far...

Hello again,

An autotransformer has just one winding with taps for different voltages.  Because it's just one winding you can not get isolation.

For example, say your primary is 200vac and you have a secondary winding of 20vac.  You can connect one lead of the 20vac winding to one lead of the 200vac so that they are in series (series aiding) and then you get 220vac out of the combined windings.  This provides a lot of other possibilities too.  If you take that same connection and apply 220vac to the open ends, then where the 200vac lead was you get 200vac out.  There are many other possibilities with so many windings to work with.

There are some rules though.  All of the windings used have to be able to handle the current you intend to draw off of it, and NO winding can be used for a higher voltage (across itself that is) than what it was originally capable of.  For example, if a winding was for 200vac then you cannot apply 300vac maybe not even 220vac as that would overdrive the core and cause a high excitation current.  If you do have a 220vac winding though of course that's ok to apply 220vac to.
The other rule is the windings have to be in phase if you want the voltages to add (series aiding) or if you want the voltages to subtract then reverse one phase (series opposing).  For example, with that same 200vac winding if you connect a 20vac winding in series opposing the total voltage would now be 180vac.  Series opposing just means the second winding has its phase reversed with respect to the other winding.

When you consider all this and all the taps on that transformer, you might be up all night trying to figure out all the combinations (ha ha) because you can use three or more windings to get even more output voltage combinations.

The main thing is though that you lose the isolation as mentioned.  The other thing is that you have to consider the loading factor.  As you add windings, the loading factor becomes more significant.  It may not be too bad though but you do have to be aware of that and test it for whatever you intend to use it for.  Losing isolation though may mean the circuit you use with it becomes more dangerous to a user.  That's something to be very aware of.

The kind of transformer I think you are looking at is a variac which is also usually an autotransformer but you can get isolated versions too at higher cost.  Check out Circuit Specialists (online store) if you haven't already they have (or had) good prices on variacs.  Variacs are very handy for testing things like this transformer.