Date: 05-07-94 22:57
From: Richard Quick
To: Dave Mcknight
Subj: 10KVA Tesla Coil
DM> Well, I'm nowhere near where I need to be on 1/4 wave coils,
DM> but I'd appreciate the step by step on building a demo 3
DM> coil system. If nothing else I'll stash the info until that
DM> point in the future where I CAN use it.
Well the first thing you need to do to build an efficient three
coil Magnifier is to wind the secondary or "driver" coil. I have
found that the driver coil works best with a very low aspect
ratio (height to width ratio) usually around 1.5 to 1 on a fat
coil form. I like the following coil design for general
Cut a two foot (24") high section of 14" diameter PVC pipe. It
make no difference here if this is thin wall, thick wall, or
whatever wall. I have seen no performance variations dependant on
material type or wall thickness with this coil design. No sealing
or prep work of the coil form is required.
Wind on about 200 turns of heavy stranded insulated wire. I don't
have a coil or wire chart handy, but I believe #12 or #14 PVC
insulated wire will give the right number of turns per inch (8
TPI) to get you in the ballpark. I prefer #12 PVC jacketed, so
you may have to juggle the coil form diameter (or height) to get
the proper length of wire on the form. You are shooting for a
coil with a frequency right around 450 KHZ, which will require a
little over 730 feet of wire.
Slap on a few coats of polyurethane sealer to glue the wire on to
the coil form. Once the sealer is dried, cap the coil top and
bottom with a couple of plexiglass disks glued down with clear
epoxy, and wind on a roll of clear plastic wrap (Saran wrap) to
protect the coil: flashover from the closely coupled primary coil
will cause headaches. If necessary I will also wind a layer or
two of 30 mil polyethylene sheet around the driver coil to
protect it from primary coil flashovers.
The primary coil for this system should be a classic vertical
helix, just like the driver. Use a 75-100 foot length of 3/8"
copper refrigerator tubing. Build a plastic coil form about four
inches larger in diameter than the OD of the completed driver
coil. Space your primary turns so that the primary coil is 2/3rds
the height of the driver coil (another way to look at this is the
primary should be a little higher than the driver is wide). You
should have a driver that nests neatly inside the primary, with
about two inches of spacing between the two coils. This arrange-
ment provides the close coupling required.
A little trick that you would find useful: grease the top five or
so turns of the primary with mineral oil or vaselene, and slide a
25 foot length of heavy wall vinyl tubing over the copper pipe.
This will really help prevent flashovers; tap your primary from
the bottom. Make your permanent connection at the top of the
primary, remove any sharp edges or points, and insulate it well.
On top of the driver coil I build a little steeple out of
plastic, and run the output wire here for connection to the
Your extra coil needs to be close wound, and fairly high
inductance. If the extra coil is lower in inductance than the
driver coil, the system will not function. I like an 8" diameter
extra coil, about 14" high, wound with about 400 turns of #20
double Formvar coated magnet wire. This coil should resonate
below 400 KHZ, and have the higher inductance necessary to
function well with described driver coil. BTW, this coil form
should be low loss, or well dried and sealed thin wall PVC drain
pipe, and should have plenty of top coats of sealant.
The transmission line connecting the top of the driver coil to
the bottom of the extra coil must be low loss, I like 1/2" hard
copper water pipe for a transmission line.
To lift the extra coil spark discharge up away from the
transmission line, set the extra coil up on a tall insulated
pedestal, and top the coil with a small toroid (say 12" diamtr).
Set a 2' length of stove pipe on the center of the small toroid,
and cap the whole balancing act with a 30-35" toroid. You may
need some juggling with the top capacitance, but this should
bring the coils into a rough frequency match.
The extra coil requires a huge top capacitance (toroid
discharger) to function well. This is where an O'scope, signal
generator, and frequency counter come in very helpful. The
addition of top capacitance is required to bring the system into
frequency match with the driver. As top capacitance is added to a
coil, the resonate frequency of the coil drops. I grid dip the
driver coil alone, then grid dip the extra coil alone. Add top
capacitance to the extra coil until the frequency is brought down
to about 1/2 that of the driver coil. In this instance, 225 KHZ
is about right. This frequency matching will facilitate 1/8th
wave current production from the driver coil, where the
transmission line feeds the current into the base of the tuned
1/4 wave resonator.
The tank circuit is tuned to operate at the resonate frequency of
the extra coil, not the driver coil. I use the signaling
equipment to grid peak the tank circuit, then final tune by seat
of the pants. For high powered Magnifier operation I like a
variable speed rotary gap capable of at least 1000 breaks per
second on the balanced Tesla Tank circuit. Large capacitors are
not required for really huge sparks: higher input voltage from
the step up xfrmr, combined with higher break rates, keeps the
processing power up (more, bigger sparks) while the capacitors
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
> Dr B.
> Brian H Le Page, Ph.D, Fleet, England
> 'Scientific progress goes boink', Bill Watterson, 1991