Topband: Teflon tubing in binocular ferrites

[David Sinclair]

kurt-erich.finger at arcor.de wrote:
> I read the intereting discussion about using binocular type ferrites for
>transformers in this reflector (Feb 2006). I understand that the wire
>should be in an additinal tube to avoid scratching the enamel from the
>wire. But why is teflon recommended  in receiving  transformers? Shouldn't
>PVC isolated wires do the job?
 >
 >
I think that PVC-insulated wires could be OK sometimes. It is true that
some of the high perm cores are conductive. In addition, they do indeed
have sharp edges, unlike most toroids. What I use for this purpose is
wire wrap wire. It has a tougher jacket than plain PVC wire. Two trade
names I have used personally are Tefzel and Kynar. They are pretty
rugged, and they should stand up to those sharp edges so long as you
don't over tighten the turns. Loosely tight is good enough. I would
not choose enameled magnet wire for this application. It is OK for
toroids, however- they rarely have sharp edges.

No doubt the Teflon tubing could help, but only if carefully cut
(square ends, and no shorter than the core) and placed in the core
openings, with a good, snug fit. If it is too short, it does you
no good at all, as the magnet wire could still be cut into by those
sharp ferrite edges if you draw the wires too tight. You might want
to try to glue the tubes in place so they don't shift. I think that
Teflon tubing really only belongs in high power transformers where
high voltage insulation is truly needed, or if it is needed as a
dielectric to obtain a particular line impedance (see Jerry
Sevick, W2FMI's excellent book on Transmission Line Transformers).

Tubing reduces the space left over for the actual windings. This
can only reduce your design options with a given core. I like the
wire wrap wire a lot better. Its insulation is surprisingly thin,
yet tough.

73, David K3KY