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Re: [TCML] Twin coils primary lead design


I part your concerns about using stranded cable in the tank circuit, and it would be interesting to replace the audio cable by i.e. a parallel arrangement of 6mm copper tubing, in order to see, if the result is better. Perhaps I'll do that, if the "tinkering virus" attacks me...

However, the heavy gauge stranded audio cable is really easy for practical use of the coil (postitioning the terminals, transport etc.). I will post some pic's and data about the cable, which I bought in an ordinary consumer electronics shop "media market", which is widespread here in Switzerland.

Best regards,

Lau, Gary wrote:
I would advise caution before choosing the finely stranded audio
cable in the tank circuit.  I have found that stranded wire has a
much higher AC resistance compared to a solid-wire equivalent
conductor.  The measurements I took were with wire that had parallel
strands.  I suspect that the ultra-flexible cable would be even more
lossy, as the strands twist and alternately become inner and outer
strands in the bundle (reference skin-effect in the recent discussion
of silver-plated stranded wire).  A short length of it to attach a
tap lead is no big deal, but a lengthy 2-conductor span between twins
would be significant.

I'm not sure that minimizing inductance is critical, unless the
pri/sec geometry does not permit the coupling to be increased to
compensate for the additional off-axis inductance.  Off-axis
inductance is not a source of loss, as is the skin-effect loss that a
stranded conductor would incur.

Regards, Gary Lau

-----Original Message-----
From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
Behalf Of Peter Terren
Sent: Monday, August 03, 2009 7:56 PM
To: Tesla Coil Mailing List
Subject: Re: [TCML] Twin coils primary lead design

Excellent craftsmanship and write up. Far better than anything I
have ever done.  I had wondered about that HiFi cable although the
ones I had seen were a lot of "fluff" with big insulation and
connectors but only a small amount of copper.
It is good to see real data on the inductance of the lead in of twin
cable. I found the parallel connection to be better than the serial
one as well. My other source of inductance is that 4 boards of the
SISG are about 2 feet long. Perhaps I should put these inside a
coax. Hopefully, zig-zagging the current path will minimise this
without compromising cooling.

Peter www.tesladownunder.com

----- Original Message -----
From: "Kurt Schraner" <k.schraner@xxxxxxxxxx>

I'm running my little twin UBTT:


with heavy gauge (10mm^2 cross section) ultraflexible
car-loudspeaker cable (the one HiFi-Freaks are using) as connecting
line between the 2 primaries. This parallel-line cable can easily
take the primary voltage stress, and keeps the cross-sectional area
of the  of the current loop in the primary low, in order to have
low stray inductance. In my example:

The seriesed primaries have a total inductance of 23.2uH, of which
the cable was responsible for about Lcable=2.6uH(Ccable=88pF). This
was when I used a "T" for the feeder (unfavorable), which was later
corrected to a shorter, direct "---" connecting parallel-line
Data with the "T" are in the above report PDF on page 22/23.

I can make Pic's of the primary lead system, if desired.


Peter Terren wrote:
I am working on a small twin coil system with a cylinder primary
and small spherical topload. Using a single coil only, run by a
single MOT SISG, it puts out a 60 cm spark for a 50 cm secondary
I am finding results a bit limited when run as a twin system due to
the length of the primary leads.  With a series primary
arrangement I should tune to 3 turns on the primary but with loose
leads this drops to about 1.5 turns which is not efficient and
spark length is less than 30cm despite double the power.
With parallel primary windings, I should tune to 6 turns but it is
reduced to 3 turns with the lead in wire inductance. It seems like
I am wasting half of my inductance.
Best performance of 90cm sparks with 2 MOT SISG is with a reduced
tank capacitance to allow greater inductance in the primary using
8 turns in parallel connected primaries with lead in wires taped
together sort of transmission line like.
My question: How can I minimise the effect of primary lead
inductance. Should I use two close parallel conductors (sort of
transmission line like) or would a coax arrangement be better?  I
can make up a coax system with copper pipe and plastic tubing
covering 1/4 inch tubing. I don't really understand the concept of
impedance matching as applied to Tesla coil primaries though or
whether it is even likely to be relevant.
Any thoughts?

Peter  www.tesladownunder.com

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