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Re: Magnifiers

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  Date: 28 Dec 93  12:51:00
  From: Richard Quick
    To: All
  Subj: 10KVA Tesla Coil
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Well I hope everyone had a happy holiday. I am now recovered
enough to post some more specific info on Magnifier construction
and operation. I have covered some general material on this
revolutionary Tesla coil system, but will now endeavor to throw
out some specific pointers for those of you who have an interest
in this much more efficient RF power processor.

As I have mentioned the Magnifier is a three coil system. The
primary is excited from a Tesla tank circuit, the secondary coil
is inductively coupled to the primary, and the extra coil is top
loaded on the secondary in such a fashion as to be uncoupled
completely from the primary/secondary "driver" coils. The extra
coil is base fed by transmission line, which can be anything from
Litz wire, copper pipe, or heavy DC transmission wire.

The keys to getting an efficient Magnifier setup in operation can
be listed fairly easily: Close coupling between primary and
secondary, good gap quenching, and proper impedances in the
secondary driver and the extra coil.

Coupling in Tesla magnifiers must be tight. Classic style
vertical helix primaries are very practical for use in Magnifier
systems, but heavy insulation between the coils must be used to
prevent flash-over. The close coupling reduces somewhat the VSWR
in the driver secondary, and the 1/8th wave output contains
substantially less voltage than a normal 1/4 wave Tesla system,
but the close physical proximity between coils means flash-over
will be a problem. Use several layers of polyethylene plastic
around the secondary, and insulate the top primary turns with
heavy vinyl hose. It has come to my attention that tapping the
primary coil from the bottom turns, and heavily insulating the
top turns, reduces flash-over. (now gee, why didn't I think of
that?)

Good gap quenching is a must. Because the coupling is so much
tighter in Magnifier systems, more strain is put on the main
system spark gap. If ever there was a need for exotic spark gap
systems this is it. Quench times must be low, low, low, in order
to trap the maximum energy into the secondary driver. An un-
quenched (closed) gap will allow energy in the secondary back
into the primary/tank circuit. This must be prevented. In
Colorado Springs, Tesla used a pair of air blast gaps in series
with a massive rotary. He kept pushing his line frequency higher
(he had a massive, variable speed alternator in the lab),
increased his break rate (up to 50,000 bps) and kept the energy
delivered per pulse down to a minimum. The reason he strove to
keep the energy per break down was to prevent overloading his
gaps. As energy per break (voltage * current) increases, quench
times decrease and efficiency in the Magnifier drop off. The
increase in the number of breaks per second allowed the total
energy processed to climb to unheard of levels, with unheard of
performance and efficiency.

The next key point to building and operating a small to medium
Tesla Magnifier is proper coil impedances in the secondary driver
and extra coil. The extra coil must have a higher impedance than
the secondary driver. The extra coil must be lower in resonate
frequency.

I have mentioned here that the 1/8th harmonic output of the
secondary driver coil matching the natural 1/4 wave resonate
frequency of the extra coil is the most efficient mode of
Magnifier operation. This is true, but don't get bogged down with
it. This system will work and fire (within reasonable limits)
with any extra coil that has a higher impedance and inductance
that the driver secondary. Let me give some tips.

I have found that most high performance 1/4 wave secondary coils
make lousy driver secondary coils in the Tesla Magnifier. The
reason is the impedance and inductance are too high, and the 1/4
wave coil is not designed or constructed to handle high current
outputs. 1/4 wave coils built per my instructions (posted here
several times) are designed to produce excellent 1/4 wave voltage
peaks, not 1/8th wave current. Your best bet is to wind a special
coil for use as a driver. With this in mind what type of coil
design would work best? Well my first decent driver coil was
built for use as a 1/4 wave resonator, but it dropped into the
role of a Magnifier driver without complaint; it had a nice low
aspect ratio, and it was wound with heavy stranded wire...

A good general purpose Magnifier secondary has a very low aspect
ratio for good tight coupling over the entire winding length (say
about 1.5:1). It is wound with heavy stranded insulated wire (say
#18 or larger) to help carry heavy RF currents with lower losses.
The frequency of the coil should not be too low, say in the area
of 400 kHz for most medium systems.

For the extra coil you want to pack a lot of inductance into a
small unit volume, but not too much. You will end up with an
amazingly short resonating coil if this is done properly. Use a
6" or 8" coil form with an aspect ratio a little over 2:1. For
winding the coil use say #20 -#24 double Formvar magnet wire. The
extra coil must be heavily top loaded with toroid discharger in
order to produce really killer performance, as is the case when
pumping a lot of energy through any magnet wire coil. The
impedance in the bare extra coil must be higher than the driver
coil by some margin, and the frequency of the extra coil should
be brought down around 200 kHz by top loading the coil with
discharger. The impedance of the driver must be low (heavy
stranded wire spaced by insulation) while the impedance of the
extra coil must be high (thin magnet wire, close wound turns)

The tank circuit of the Magnifier must be tuned to the same
frequency as the extra coil with discharger. Set the driver
secondary inside the primary and run a transmission line some 6-
8' to the extra coil. The extra coil must be away from any
field damping effects such as ground, large metal objects, other
coils etc. It must be allowed to resonate as freely as possible.



Tesla List wrote:

> Original Poster: "Brian H Le Page" <brian-at-oneau-dot-com>
>
> I am looking for information to start me on the way to designing a
> magnifier.  Can anyone point me at relevant web sites etc that would get me
> started.  When I know a bit more I may then be able to ask some intelligent
> questions.
>
> Thanks.
>
> Dr B.
> Brian H Le Page, Ph.D, Fleet, England
> 'Scientific progress goes boink', Bill Watterson, 1991