Transformer with bifilar secoundary?

[DongBeetle]

I've just finished rewinding an old toy transformer, I think it had 3-5-9-15V/1,2A output, but what I wanted, is a center tapped +/- 12V output to use with a pair of uA7812-7912 regulators, in a power supply like this:
Build your Gadget: Dual Voltage Power Supply by 7812 and 7912 regulator

I used 2 identical secondary coils(0,55mm wire) coming form 2 old 12V/1A wall transformers, and I wound them on bifilar (both wires at the same time) on the empty bobbin.
I placed a 500mA fuse on the primary side and tested it out.
From the center to each end I get 12,4-12,6Vac, but from end to end I don't get any voltage..
is this because it's bifilar and I made both start windings into the center tab?
If that's the case, is the solution then to take 1 start and 1 end from each wire and make that the center tab, or do I have to redo it, one wire at a time?
hope my Q's makes sense, and remember.. there are no stupid questions
Regards.

 

[JimB]

OK, no problem.

What you need to do is connect the start of one winding to the end of the other winding.
Then you will have 12v from centre to each end, and, 24v from end to end.

What you have at the moment is both windings in-phase with each other, reconnect them as I have suggested and they will be out of phase.

JimB

 

[ericgibbs]

hi,
Transformer secondary windings have a Start and Finish, this dictates the 'phase' of the voltage for the winding.

By winding bifilar both secondary windings Starts are in phase, so if you connect the two Finish windings to make a centre tap, the windings will cancel each other out.

Connect the Start of one winding to the Finish of the other winding and make that your centre tap..

EDIT:
Jim beat me to the post,,, no pun intended

 

[DongBeetle]

Yes just as I thought!
Thanks for your replies.
Regards.