transformer split secondary winding, how to wind?

Hello,
I have a transformer with a lot turns primary and a low turns secondary. I need to find a way to cancel out transformer action from the secondary to the primary, i.e. if AC is fed to the secondary it should not affect the primary.
The book I am reading states a split winding secondary (center tapped?), with the two windings wound in the opposite direction to cancel out their fields in the core.
I would like to ask how should I make these windings?

1. A center tapped secondary? Imagine it like a single winding, with a tap in the center.
OR
2. Wind the first winding right hand side. When its end comes, start wind the second winding in opposite direction

Which of the two are correct?

Neither, a transformer is essentially bidirectional so anything on the secondary that changes the magnetic flux around the core will be picked up in the primary. I suspect you are reading something different though, that balancing the current through two secondaries can be used to cancel each others magnetic field and hence make no difference in the primary. This is often used in small RF transformers in mixer circuits.

The easiest way to make identical secondaries, providing the voltage is relatively low (in case of insulation breakdown) is to wind the secondaries at the same time. Start with two wires and keep them side by side as you wind the turns around the core. When you have the desired number of turns you should have two identical windings, not only in terms of their inductance and magnetic properties but also capacitive to nearby objects as well. If you want a center tap, simply join one of the secondary wires to to a wire on the other secondary, you can swap one winding connection if necessary to reverse it's phase.

Brian.

betwixt said:

I suspect you are reading something different though, that balancing the current through two secondaries can be used to cancel each others magnetic field and hence make no difference in the primary.

If you want a center tap, simply join one of the secondary wires to to a wire on the other secondary, you can swap one winding connection if necessary to reverse it's phase.

Brian.

Click to expand...

Yes, I mean exactly that. I did not know how to better describe it.
It is a saturable reactor. The book states that by having a split winding in the secondary, cancels these fields and this has the advantage that the primary is not sensing any (ideally) signal. Also, this has the advantage that the core is not accidentally saturated by heavy load currents in the load winding (secondary), since they will always cancel out. so the only thing that saturates the core is the control primary. I think this is what happens there.

So I wind up a bifilar secondary, that is clear now. The center "tap" is either the connection of the two coils in the right (or left) side, or the connection of the end of the first coil to the beginning of the second coil (which of the two, is to be determined experimentally). Am I getting it right?

Do I need to twist wires? This is not to be used on more than 100KHz and thick wires are more hard to twist and maintain their position onto the core when twisted.

Brian a mains center tapped transformer I have is not wound bifilar, I do not know why they made it the other way, but the way you describe seems better to me as it is more symmetrical.