RE: intro + coil questions

["Tesla list" <tesla@xxxxxxxxxx>]

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

A correction to my post below:

If the NST is hooked across the capacitor, the series inductance of the
tank capacitor doesn't matter so far as blocking the zero-crossing
transients.  The NST won't see them because the gap is (really) off and
the NST is not connected to the primary.  To correct another typo in
that paragraph:  So, the possibility exists that having the NST across
the >capacitor< may actually be safer, _IF_ you don't use an R-C filter.

Regards, Gary Lau
MA, USA

> 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
> >
> >
> >