CT transformers - how is 180 deg out of phase achieved?

I've got a your typical small wall wart transformer that takes 110 VAC
and produces 13 VDC. The secondary winding is center tapped and uses
diodes/capacitor to provide DC - something like this
http://www.allaboutcircuits.com/vol_6/chpt_5/4.html.

How is it that the legs/coils off the center tap on the secondary
winding are 180 degrees out of phase? Is it the exact placement of
the center tap, turn ratio, or transformer construction? Does the
same apply for multitapped transformers?

Thanks in advance.

Bill

 

Each turn on the transformer produces essentially the same AC
voltage. The phase of that voltage depends on which direction that
turn uses to surround the common magnetic field. If the windings
continue to wrap around the core in the same direction after the
center tap connection point is reached, than the voltage continues to
develop in the same direction. Instantaneously, this is similar to
having a stack of cells (batteries) all turned the same way. If you
make a connection to the middle of the stack, then one end of the
stack is positive with respect to that point and the other end of the
stack is negative. That is just another way of saying that the two
ends of the stack are 180 degrees out of phase with each other.
Another analogy for a center tapped winding is a see saw. With
respect to the center point, one end has to go up (positive height)
when the other end goes down (negative height) if the see saw is rigid
(constant slope versus length = constant volts per turn).

 

i.e The winding doesnt change direction at the centre point, from end to end it
may be 100vac. So the center tap will be at the 50v point. measuring center to
one end will thus give +50vac, and to the other end, -50Vac. Try it with four
batteries, negative probe of your meter between batteries two and three, and
youll see it measure +3v at the top end, and -3V at the bottom end. Your
negative then is at the 0v point!! ...a 6 volt center tapped supply, without
you reversing the direction of any batteries, windings are the same in a
transformer.
....John.

Drop the QRM from my addy to mail me...

 

Thanks for the replies. I did come across the following link that
further clarifies this thread http://www.tpub.com/neets/book2/5e.htm.

Bill

 

They're not. They're simply the inverse of each other. There is no
phase shift, just a polarity inversion.

Cheers!
Rich