# Winding Toroids Phasing

Discussion in 'Electronic Basics' started by Hammy, Dec 22, 2009.

1. ### HammyGuest

I'm constructing a multiple output flyback transformer using a Toroid.
My input voltage is 12V to 20V output is 5, 12 and -12, Po = 7W.

My question is about the phasing. See picture

http://i49.tinypic.com/a5f614.gif

Referring to step one of the picture for the primary side, if I start
where "start" is and pull the wire through from start (left) to right
does that make the "start" side the DOT. I'm putting 12 turns on the
primary.

All I have is standard electrical tape should I bother using it or
will it reduce coupling? I'm using insulated magnet wire.

My secondary turns are 4,5 and 9. The 5V and 12V are DC stacked and
will be bifilar tap.

Should the secondary windings start on the same side or does it
matter?

2. ### ehsjrGuest

The dot placement does not matter with only one coil wound
on the core - you can place it at either end. However,
when you put the secondary winding on, the placement of the
dot for the secondary winding depends on where the dot is
on your first winding. If you put the dot on the start
end of the primary, then put the secondary dot on the
start end of the secondary, assuming you keep winding in
the same direction.
No need for electrical tape, unless you are looking for mechanical
strength for the connecting leads between the xformer & circuit.
Sometimes you wind with fine wire, and make the connecting leads
out of something heavier. It that case the leads need to be
mechanically well attached to the toroid.
I'm not sure what you have in mind in that question, so
I'll try to answer it as a general statement. Imagine
that the toroid starts out as a rod, that will magically
be bent into a toroid when all windings are completed.
You want to end up with the primary winding spaced so
that it occupies about 90% of the length of the rod.
Then, you want to wind your secondary, and space the
windings so that it, too, occupies the same ~ 90% of
the length of the rod. In both cases, the winding starts
near the left end of the rod, and ends near the right
end of the rod. The windings have to be in the same
circular direction around the rod.

Ed

3. ### HammyGuest

Thanks for the detailed response.

I built it up on a breadboard and it works after some playing around
with the toroid. I just started all windings on the same side primary
and secondary.

I wanted to know before hand about the phasing so I could plan out the
easiest way for laying it out on the breadboard, 8 windings on a
toroid then cramming it on a breadboard gets pretty cramped.

It's noisy as hell but that is always the case when doing SMPS's on a

I know now it works so I can do a layout for a PCB.

I'm just goint to use the tape on the primary to hold the windings so
they dont shift all over when winding the secondaries.

4. ### ehsjrGuest

With small wire, I've "glued" it to the toroid with clear
nail polish. Dries pretty quick, and works fine.

Ed