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RE: [TCML] Twin coils primary lead design
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>
> > Peter,
> > I'm running my little twin UBTT:
> > http://home.datacomm.ch/m.schraner/TwinWithRotaryinAction.jpg
> > http://home.datacomm.ch/m.schraner/UBTT-Betrieb.pdf
> > 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
> > only(better).
> > 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.
> > Regards
> > Kurt
> > 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 winding.
> >> 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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