Re: braided secondaries???

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Dr. Duncan Cadd by way of Terry Fritz <twftesla-at-uswest-dot-net>" <dunckx-at-freeuk-dot-com>

Hi S, Ray!


>Original poster: "Ray von Postel by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <vonpostel-at-prodigy-dot-net>
>
>S:
>
>What you have is "Litzendraht wire"  also called "Litz wire".  It is
>used in coils operating at low  radio frequencies to
>minimize the skin effect and thus the  r. f.. resistance.

With fine enough (50swg / 0,01mm) wire, it is/was used to as high as
3Mc/s.  At these higher frequencies, the success depends to a large
extent on the even-ness of the braiding and how well the individual
strands are insulated from each other, a bit of space between each
wire becomes important.  Often the strands were enamelled and fabric
covered before being braided together in order to get that bit of
extra space.

>I have seen it as much as 2 inches in diameter in the coils of
>transmitters and in small sizes as the wire
>used to wind low frequency i.f.. (intermediate frequency) coils in
radio
>receivers.

There's a photo from the Rugby "wireless" station in the early 1920s
of a big, low loss transmitting coil using what I take to be very
thick litz cable as per Ray's description on my page at:

http://home.freeuk-dot-net/dunckx/wireless/maxpower2/maxpower2.html

The station still exists but I doubt they have anything like this
there any more.  I could be wrong though, they have a 60kc/s
transmitter plus massive masts/aerial system for the standard time
signal from the National Physical Laboratory and it's just possible
they might still have a big LF litz coil like this one for that very
purpose.

>I have no idea where you could get a current price for Litz wire,
>but 30 years ago it was as much as 10 times the cost of
>enameled wire.

A lot more now, as the cost of fabric (real or artificial silk)
covering has increased dramatically and few firms have the machinery
to do it; typically one ounce costs around 10 UK pounds, so if you
have five lbs of it, you have a real find :-)  A few hundred $$$ worth
at a guess.  You might make a bit by selling it at a hamfest!  I'm
assuming that S's litz is fabric as well as "enamel" insulated.

>You will find it hard to solder because each wire end has to be
cleaned.  There
>is a procedure for that in some old editions of "Chemistry and
>Physic's HandBook"  published by the Chemical Rubber Publishing Co.
Your
>local library should have a copy.


For the skin effect to be reduced, each individual wire of the litz is
insulated from its neighbours and you have to get that insulation off
before you can solder it, a real pain when it's real enamel.  In my
Father's day, you could either heat it to red heat in a flame
(difficult without melting the wires clean through if they were thin)
and plunge into alcohol (difficult without starting a fire because
thin wires don't hold heat very long and so the flame has to be
perilously close to the alcohol ;-) or you could take a razor blade
and carefully scrape the enamel off (difficult, and a real pain if the
litz contains dozens of individual strands, but the way Dad did it!)
Difficult all round basically.

These days you probably don't need to bother with all this palaver.
The litz I bought a few years back was insulated with modern
solderable "enamel" i.e. plastic and I expect if your wire is of
recent manufacture, all you will need is a good, hot soldering iron
and some flux-cored solder.  Have good ventilation when doing this as
the fumes from decomposing polyurethane (isocyanates) are nasty.

<snip>

I would guess that a spark-driven Tesla coil will see little
improvement from litz (my baby coil of ~15W didn't seem to notice the
difference though YMMV) since the secondary Q is of relatively little
importance.  A CW coil is more likely to benefit as Q really does
matter here, the higher the better.  It would be an interesting (and
normally very expensive!) experiment to do.

Dunckx