RE: intro + coil questions

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Lau, Gary" <Gary.Lau@xxxxxx>

Hi Ben:

As far as spark-performance is concerned, it makes no difference whether
the NST is across the capacitor or gap.  Where it differs is what sort
of abuse the NST is subjected to.

With the NST across the gap, the gap is "for the most part" a short
circuit for the time that the primary is oscillating at the tank
frequency, thereby protecting the NST from this tank frequency RF.  But
less obvious are the VHF transients that occur at the zero-current
crossings of the gap.  At these crossings when the gap briefly shuts
off, the primary rings briefly with its C-self in the high megahertz
range.  The initial amplitude is the same as the voltage on the tank
capacitor Vc, so with the ringing primary in series with the charged
tank cap, the NST sees a transient starting at 2Vc.  But the good news
is that these transients are easily cut down to nothing by the R-C low
pass filter if you're using a Terry filter.

If you choose to hook the NST across the capacitor, it's possible that
the capacitor will filter out the VHF transients.  But it's also
possible that the series inductance of the tank capacitor may limit the
filtering ability (no one has ever looked into this).  So, the
possibility exists that having the NST across the NST may actually be
safer, _IF_ you don't use an R-C filter.  It is unknown whether the tank
frequency RF or the shorter duration but twice-the-voltage VHF
transients present the greater NST hazard.

But of course, the safest thing is to rig the NST across the gap, AND to
use an R-C filter between them.

Regarding splicing secondary wires - bummer!  But if you must, try to
make the splices as low-profile as possible.  Lay the two wires
side-by-side for the shortest length possible (you don't need a
super-robust connection) and use only enough solder to join them.  Any
bumps on the secondary may tend to promote corona and be a starting
point for racing sparks.  I'd skip the sleeves and just use epoxy or
polyurethane to insulate.

Regards, Gary Lau
MA, USA

> Original poster: Ben Buxton <bb@xxxxxxxxxx>
>
>
> I've just joined this list and thought I'd introduce myself before
> asking a question or 2 on coil design.
>
> I hail from Sydney, Australia and have been interested in electronics
> for a long time now. I've played a lot with high voltage stuff as
> well, having done things involving he-ne lasers, ignition coils,
> LOPTs and jacobs ladders from NSTs. Getting good HV gear is
> reasonably easy here so there's plenty of room to play.
>
> My most ambitious project is, naturally, a Tesla coil. I'm currently
> building a 6" coil, powered from a 15kv, 60mA NST. I've sourced all
> the parts (including TDK pulse capacitors) and am slowly getting it
> all assembled.
>
> One aspect I've not yet found an answer to is whether to place the
> NST across the gap or the caps. Some designs have it one way, and
> others the other. Is there any guideline as to what's best? I can see
> things involving less kickback to the NST with it across the gap, but
> there's probably more to it than that.
>
> I've also got a bit of a concern with my secondary. As I'd not been
> able to (read - didnt research hard enough!) find long enough single
> spools of wire to wind with, I've used 3 x 250metre spools, and made
> splices along the length. These consist of the ends soldered together
> (with minimal 'sharpness') and sleeving places over the join, then
> epoxy sealed using araldite. Will this give me any problems? After
> winding, I only now know where i could find 700 metre spools, but too
late!
>
> Thanks for any help!
>
> Ben
>
>
>