The images referenced in this
article were never scanned and placed into the file. Refer to pictures of magnifier system at Bert
Pool’s web site - http://hot-streamer.com/pool/may97_3.htm
Copyright © 1997, by Bert Pool
Warning and disclaimer:
Tesla coils are high voltage
devices that incorporate voltages and currents that can be lethal. This file is intended for persons over the
age of 21, who are familiar with the construction and safe operation of high
voltage apparatus and who build and operate Tesla coils. Extreme caution is
advised, especially when building and operating those coils using potential or
distribution transformers as a power source. Every attempt has been made to
insure that the information contained in this file is accurate and safe. The
author assumes no liability whatsoever in the event of injury or death or
damage to property resulting from the construction or operation of apparatus
built using any of the information contained in this file. The builder should
be aware of the many risks involved and should take reasonable personal
precautions to insure his or her own safety, as well as the safety of
bystanders and personal property. If you a re careless, or if you are stupid,
or if you have no experience working with high voltage apparatus, then you
should not build Tesla coils or attempt to work with high voltage.
Theory in a nutshell
The Tesla Magnifier is an air core
transformer, which drives a resonant, freestanding "Extra" coil. The
main power source is a step-up transformer, which supplies power to a capacitor
and primary coil, which together form a resonant tank circuit. A fast quenching
spark gap is used to commutate the high voltage in the primary circuit. As the
tank circuit oscillates, currents in the primary inductively drive the tightly
coupled secondary coil. The secondary coil is a low impedance, high voltag e, high current source, which is used to base-drive
the third, or "Extra" coil. The primary/secondary is designed to form
a 1/8 wavelength helical driver pushing a ¼ wavelength resonator. The Extra
coil has been carefully built so that its inductance, self-capacitance, and top
load capacitance form a resonant circuit whose frequency is double that of the
primary tank circuit. The "Extra" coil, which is not magnetically
coupled to any other coil, forms a high-Q resonant circuit, whose peak voltages
can reach hundreds of thousands or millions of volts potential. The electrical
currents produced are high frequency, high voltage, and are relatively low
current.
The components in a Tesla Magnifier
A Tesla Magnifier is NOT just a third coil
being driven by a conventional Tesla coil! The coupling between the primary
coil and secondary coil in a Magnifier generally starts at 0.4, whereas in a
conventional Tesla coil the coupling factor is only about 0.1 to 0.2. This
higher coupling in Magnifiers creates a high current spark in the gap, which
can be very difficult to quench. A conventional Tesla coil has a tuned
secondary, which achieves large voltages due to a "resonant rise" of
voltage. Magnifiers do not have a tuned secondary at all, and all the voltage that
appears at the top of the secondary is entirely due to pure transformer action.
The Extra coil, on the other hand, is where resonant rise takes place in a
Magnifier. The quenching in the spark gaps in a Magnifier is so fast that the
gaps cannot operate a conventional two-coil system!
Figure 1.
Schematic diagram
with all the required parts for a Tesla Magnifier. Many of the components used in a Magnifier are the
same as used in a conventional coil, but much of the coil and spark gap design
is different.
First, we will discuss each of the
components used in a Tesla magnifier, then we will build and assemble a
Magnifier system capable of producing sparks over ten feet in length.
Power Transformers
The first item we want to examine is the
power transformer. Magnifier power transformers typically have much higher
output voltages than transformers powering conventional Tesla coils.
Conventional Tesla coil transformers are rated from about 5,000 volts up to
about 14,400 volts, whereas magnifiers work best with transformers in the 20kV
to 30 kv range. Also,
conventional Tesla coils can be operated on neon transformers, something I do not
recommend for Magnifiers. Neon transformers do not output high enough voltages,
and are not tough enough to operate Magnifiers reliably. Potential or
distribution transformers should be the only transformers used in Magnifier
service. Unfortunately, high power transformers are more dangerous to operate
than the small neon transformers, they come with a higher price tag, and they
require ballasting and control equipment that neon transformers do not need.
How to use potential and distribution
transformers
Potential transformers
are used by the Power Utility Company to precisely monitor power line voltages. They come in a variety of sizes, usually ranging from
1 to 5 kVA. Output voltages can range from about
4,500 volts to over 34,000 volts. Potential transformers are NOT current
self-limiting. For example, if you take an "unloaded" potential
transformer (one that has no load connected to the secondary), and then connect
its primary directly to a wall outlet, only a couple of amp s will flow through
the primary. This small current is the amount of power that is required to
magnetize the iron core. But, if you connect a load to the secondary, then very
large currents will flow in the primary of the transformer, often hundreds of
amperes! All potential transformers require some method of limiting or
controlling the current, which will try to pass through the transformer once it
has a load connected to it. A large variable transformer, also known as a variac, is usually used to cont rol
power to the transformer. Along with the variac, a
variety of methods of controlling or ballasting current to the transformer are
used, and we will cover these methods in more detail later. Potential
transformers are very robust. They are designed to handle the abuse that comes
with power line service, including the voltage transients induced by lightning
strikes. The secondary windings of potential transformers are generally
designed to withstand voltage spikes of 110,000 volts! Potential transfor mers are far, far better
than neon transformers when it comes to use in medium and medium-large Tesla
coils. Internally, they are often insulated with a high quality silicone rubber
potting insulation, or a very good epoxy. You can find potential transformers
at salvage yards, and at electric utility auctions. Bring a buddy along if you
think you might buy one, as they are very heavy for their size, usually weighing
anywhere from 150 to 250 pounds. You will want to purchase a transformer with a
ratin g of around 20,000 volts and two kVA power rating. You will not need more than a 5 kVA transformer for this project.
Figure 2.
Shown are a 2 kVA
and a 5 kVA potential transformer. They are compared
in size to a typical 360 VA neon transformer.
Again, you need to be aware that potential
transformers require the use of large variacs and
massive resistive and inductive ballasts to properly operate them. That $35 bargain
transformer you buy at the salvage yard will probably cost you upwards of $300
for inductive ballasts and big variacs before you can
use it.
It is very important that the
Tesla coil builder understand a key safety difference between neon transformers
and potential or distribution transformers. A neon transformer will shock you,
and you’ll probably live to coil another day. A potential or distribution
transformer will kill you – there are no second chances. Great respect and
extreme caution are required when working with these large transformers!
Distribution transformers
Distribution transformers, also known as
"pole pigs" are the ultimate Tesla coil power source. Like potential
transformers, they have been designed to operate outdoors in very harsh
conditions, and can take severe electrical abuse without failing. Pole pigs are
filled with transformer oil. Be careful when purchasing old pole pigs, as the
oil may contain PCB’s. PCB (poly-chlorinated biphenyl) is a nasty additive,
which used to be put into the oil to improve its dielectric properties, but
PCB’s were proven to be both carcinogenic and mutagenic.
Federal and state environmental agencies have banned PCB’s totally. It can cost
you several hundred dollars to have a PCB capacitor disposed of in a certified
toxic waste incineration site. Make sure your capacitors and transformers are
PCB free before you buy them, else you might end up with hundreds of pounds of
highly toxic and costly material.
Figure 3.
A variety of distribution transformers,
or "pole pigs" as they are known.
Like potential transformers, pole pigs
require external current limiting. This is accomplished with a combination of
resistive and inductive devices placed in series with the primary of the
transformer. A large variable transformer, also known as a variac,
is usually used to control power to the transformer and the ballasts. Pole pigs
are rated in both voltage and kVA. Typical pole pigs
are 5 and 10 kVA, with big ones going to 25 kVA and up. The input voltage is most often 240 volts,
controlled by v ariacs and ballasts. You can expect
your input current on a pole pig setup to be at least 50 amps, and you might as
well plan on putting in one hundred-amp breaker service to your controller.
Output voltage on pole pigs can be anything from 4,500 volts to over 35,000
volts. Tesla Magnifiers require a potential or distribution transformer
secondary rated at 20,000 to 25,000 volts. This higher voltage will necessitate
the purchase of very good capacitors capable of withstanding the higher
voltages.
Controlling and Ballasting Potential and
Distribution Transformers
As mentioned earlier, potential and
distribution transformers require external current limiting. This is provided
by a combination of resistive and inductive elements placed in series with the
transformer. Usually, part of the inductive ballast consists of a large
variable transformer, better known as a variac. The
resistive ballasts range in value from a few ohms at most, down to a tiny
fraction of an ohm. Power ratings for the resistance will typically be several
thousand watts. Often, nichrome-he ating elements from electric heating furnaces or ovens are
used. Heating coils out of clothes dryers are very good.
Figure 4.
Some heater resistive elements used as
current limiting ballasts for large Tesla coils.
The inductive ballast is most often an
electric welder. The output cables of the welder are shorted together. The
primary of the welder is connected in series with the pole pig. Because the
current going to the pole pig has to go through the welder first, the maximum
current that can go through the transformer is the same maximum current that
the shorted welder would normally pull. A shorted welder will typically pull a
maximum of 60 amperes. By changing the current taps on the secondary of the
weld er, the amount of primary current through the
welder can be controlled. Thus, by changing the current settings on the welder,
you can vary the current in the pole pig from a few amps all the way up to
about 60 amps. Multiple welders may be paralleled for more current, or you may
use a larger welder.
Some coil builders have made their own
inductive ballasts. Large laminated iron cores are wound with 8 gauge or
6-gauge wire, creating an inductance of about 8 millihenries.
Taps can be placed on the turns of the coil so that different inductances can
be chosen from about 8 millihenry down
to about 2 millihenry. Typically, for a 10-kVA
transformer, an inductance of about 3 millihenries
works well when used in series with a resistance of about 0.3 ohms. Some coil
builders prefer connecting the resisti ve ballast in parallel with the inductive ballast instead
of in series. In this case, the resistance will usually be higher in value.
Using inductive ballast improves performance over using pure resistive ballast
because of a "kick" effect on the voltage provided by the inductor.
Figure 5.
Typical methods of
connecting resistive and inductive ballasts to limit current to potential and
distribution transformers.
Coilers have found if they use pure
inductance, with no resistive ballast, that their transformer "bucks"
or suffers from large current surges. When this happens, adding some resistive
ballast smoothes out operation. Most large coils use both resistive and
inductive ballasts, and the ability to finely adjust the best combination of
inductance and resistance in the ballasts is one of the things that can separate
a good coiler from a mediocre coiler!
Variable transformers
Figure 6.
Shown are several styles of variacs. Note the dual and triple transformers on one
common shaft. Dual 120-volt transformers can be wired for 240 volts. The triple
transformers are originally designed to be used with 3 phase, 240-volt power,
and can still be salvaged for single-phase use.
Variacs are rated in both voltage, and current, usually 120
or 240 volts, with current ratings ranging from 15 amps to 50 amps or more. You
can generally run a variac beyond its nameplate
rating by a factor of up to 100% for short periods of time. I have run 50 amp variacs at 85 amps with no problems. You can find large variacs at salvage yards, theatrical equipment auctions,
and in heavy electrical power equipment, especially radio frequency induction
furnaces used to heat treat steel. Expect to pay $50 to $200 for 50 to 100 amp variacs. You will be better off buying 240-volt variacs instead of 120-volt if you are planning on
operating potential or distribution transformers. Keep a lookout for 120-volt variacs where you find two variacs
mounted on one common shaft. These double 120-volt variacs
can be wired to work on 240 volts! Avoid 600-Hertz surplus military variacs like the plague.
Figure 7.
Methods of wiring variacs and ballasts to control current to transformers. Note the method of connecting two 120-volt variacs to control 240 volts.
Line filters
Tesla coils, especially Magnifiers, can
produce a lot of RFI, or Radio Frequency Interference. An Extra coil that has
been tuned to produce maximum spark will radiate very little RFI through space.
The secondary coil, on the other hand, can create an electromagnetic field that
reaches out several yards and is intense enough to damage electronic equipment.
You can still get a lot of RFI and transient voltage back through the
transformer into the house wiring where it can play havoc with other elect ronic equipment. The old, long "Tesla trail" is
littered with dozens of dead radios, computers, VCRs, garage door openers, and
other gadgets ruined by severe RFI and voltage transients that were not kept
out of the house wiring. It is highly recommended that you purchase and install
"line filters" between your wall outlet and Tesla coil power supply.
A line filter can be made from suitable capacitors and inductors, but there are
many suitable line filters available on the salvage and surplus market at
dirt-cheap prices. I usually pay $5 for 50 amp rated filters at ham fests. Even
with a line filter installed, I still always unplug sensitive electronic
equipment throughout the house before I run any Tesla coil. Keep your camcorder
and other sensitive electronic equipment very far away from the
primary/secondary driver circuitry of a Magnifier!
Figure 8.
Several line
filters, ranging in size from 5 amps to 50 amps.
Protecting the transformer
There are three devices used to protect
the transformer from early death. It is recommended that all three be used
together. The first protection is a safety spark gap placed across the
transformer. The gap is set so that normal operating voltage will not jump
across the gap, but should a high voltage transient appear,
the gap will conduct, shorting out the voltage and preventing it from damaging
the insulation inside the transformer. The gaps are connected to ground, so
that any transient voltages go straight to ground, bypassing the transformer
windings. It is highly recommended that safety gaps, chokes, and bypass
caps be used on potential transformers and pole pigs connected to Magnifiers,
because of the additional RF fields and transients created by the driver
circuitry.
Figure 9.
A safety spark gap constructed of
1-inch ball bearings mounted on ceramic insulators and connected to the
transformer. Note that the center ball is grounded.
The second protective device is a bypass
capacitor. This is simply a small high voltage capacitor that goes across the
high voltage windings of the transformer or from the high voltage winding to
ground. Typical values are 250 to 500-pF. The capacitor will appear as an open
circuit to the 60-hertz line frequency, but the cap will appear as a low
resistance path to ground for RF noise, which might damage the transformer.
Figure 10.
Shown are typical bypass capacitors.
These are small ceramic and mica "door knob" capacitors.
The third transformer protection is a
choke. Chokes are coils wound either on a plastic pipe, or on ferrite toroidal cores. The chokes are placed in series with the
wires leaving the high voltage bushings of the transformer. The ferrite core
chokes have the advantage of high inductance in a small space, but a
disadvantage is that careful construction is a must to prevent electrical arc-overs on the windings. Air core chokes must be physically
large in order to have enough inductance to be effective. Be cause the chokes
are not in the "high current" tank circuit, they may be wound with
small gauge wire. A few millihenries are a minimum
value for an effective choke. Many electronic supply houses sell ferrite toroidal cores 2 to 4 inches in diameter, usually for $3 to
$4 each.
Figure 11.
4 inch diameter, 6 inch long air core
choke wound with 28 gauge enamel wire, and a 2 inch ferrite core choke, also
wound with 28 gauge wire.
Figure 12.
Typical transformer
protection circuits.
The third circuit is a "pi" circuit and is very effective.
One of the things that the coiler has to
be aware of when using bypass caps and chokes is that it is possible to
accidentally wind a choke that resonates at the same frequency as the resonant
tank circuit of the Tesla coil. This is bad news. Should this happen, the choke
no longer protects the transformer, indeed, the
resonating choke can actually create high voltage transients that can
damage the transformer! The one sure sign that a choke is resonating at the
coil’s primary frequency is that it will get very hot, so hot, in fact, that it
can smoke or even burn the insulation off the wire! It is always a good idea to
run a Tesla coil that has new chokes for several seconds, then power down the
coil and feel the chokes and see if they are getting hot. If you have a choke
that is getting hot, try changing the inductance by using fewer or more turns,
or rewinding it on a longer or shorter form, or a form with a different
diameter.
Spark gaps
Tesla Magnifier coils use a main spark gap
to commutate the high voltage from the transformer to the tank circuit. It
works something like this: during the first ½ cycle of the AC current, the
transformer moves charge from one capacitor plate to the other. The voltage
appearing across the spark gap at this time is only the peak voltage of the
transformer, which is not quite enough to jump the gap. On the next ½ cycle of
the AC current, when the current flows in the other direction, the transformer
tries to move the charge from the second plate back to the first plate of the capacitor.
But this time, we have two voltages involved; the voltage of the transformer
PLUS the voltage of the charged capacitor. This summed voltage is enough to
jump the spark gap. When the voltage does jump across the gap, the capacitor
discharges its entire charge through the gap into the Tesla coil primary. This
primary current can reach hundreds of amperes in small coils and thousands of
amperes in large coils. Once io
nized, the air across the spark gap becomes very
conductive, and is for all intents a short. The current stored in the capacitor
continues to bounce back and forth between the plates, passing through the
primary coil, again and again, each time giving up energy. This passing
back-and-forth occurs at a frequency determined by the value of capacitance in
the capacitor and by the inductance of the primary coil. These two components
form the resonant tank circuit of the Tesla coil. The alternating current p assing through the primary, when observed on an
oscilloscope, starts out very large and drops every cycle until it reaches
zero, forming a damped sine wave on the oscilloscope’s display.
Figure 13.
The damped wave observed in the primary
circuit.
Once the voltage stored in the primary
circuit has dropped nearly to zero, the spark gap extinguishes, and the
transformer again begins charging the capacitor and the cycle begins anew. Gaps
used with Magnifiers require much faster quenching than conventional coils, due
to the higher coupling between the primary and secondary coils.
Cylindrical gap
While a small conventional Tesla coil can
sometimes be operated with a very simple spark gap, Magnifiers require
ultra-fast quenching. This is best accomplished by using series multi-gap
rotary spark gaps connected to cylindrical series static gaps. This
"series static gap" works very well if you use copper cylinders, 2.5
inches in diameter and 4 inches in length, mounted side-by-side. The large mass
of copper, open on the inside, allows for good air circulation and excellent
heat dissipati on. A copper cylinder gap, using
2.5-inch by 4-inch cylinders is good for power levels up to 8 kilowatts. For
better quenching and higher power, the cylinders can be mounted in one end of a
box, called a plenum chamber, and a vacuum cleaner blower motor mounted at the
other end of the box. The motor is positioned so that air is sucked into the
box through the gaps between the cylinders. This "vacuum quenched"
gap performs exceptionally well with a Magnifier when combined with a series
quenched rotary gap.
Rotary gap
A rotary gap is made of a plastic disk
with evenly spaced screws or tungsten studs placed around its periphery. A
variable speed motor spins the disk at speeds up to a maximum of about
10,000-RPM. The electrodes spin past two stationary electrodes. Every time a
moving electrode comes between the stationary electrodes, the gap can fire. The
high-speed movement of the electrodes allows for excellent spark quenching. The
"flying" electrodes stay very cool, and the air they stir up helps c ool the stationary electrodes a bit. The disks are usually
made out of G-10 epoxy-glass composite material, polycarbonates such as Lexan, or even acrylic. The epoxy-glass material has the
best thermal and tensile strength properties. Acrylic can shatter or melt. The
polycarbonates are very tough, but can still melt.
The rotary spark gap used with a Magnifier
has at least four stationary electrodes, and some designs use as many as eight.
The gaps are connected in series, or a clever combination arrangement that
allows sets of series gaps to be alternately connected together. The more gaps
used, the better the quenching. Some series rotary gaps quench so well that
they do not require the additional quenching provided by a cylindrical static
gap.
Figure 14.
A Lexan disk
is shown with electrodes. The hole in the center of the disk mounts onto the
end of the drive motor shaft.
One problem with high power rotary gaps is
the dissipation of heat from the stationary electrodes. This can be partially
alleviated by using large electrodes, though this in itself can cause problems
because a large diameter electrode increases "dwell" time. One of the
best methods to deal with heat is to mount the electrodes in massive blocks of
brass, which act as heat sinks. Some rotary gaps have their stationary
electrodes cooled with blasts of compressed air, and even pumping cool oil t hrough a hollow electrode is a possibility.
Figure 15.
Rotary spark gap,
showing the Lexan disk, electrodes, and motor.
As a rule, you should use the smallest diameter
electrode material that can withstand the power level at which you run your
coil. The smaller electrodes will insure shorter dwell time and thus better
spark quenching and better coil operation.
Primary coil
The primary coil in a Tesla Magnifier
system should meet several important criteria. It should be of low resistance.
It should be "tappable" for tuning
purposes. It must be well insulated from the secondary. Since Magnifiers
require that the primary be tightly coupled to the secondary, this dictates
that a helical, or cage type of primary be used. Pancake primaries do not
provide the coupling that will be necessary for Magnifier work. This helical
primary coil, with its tight coupl ing and tall physical stance will mean that there will be a
very great tendency for the secondary to arc over to the primary. You will be
required to provide some very, very good insulation between the two coils to
prevent this arcing.
Figure 16.
The Helical primary. Note the polyethylene insulation between the primary
and secondary coils.
Because of the "skin effect"
which is present in currents at RF frequencies, copper tubing makes a superb
primary coil conductor. Magnifier coil builders most often use soft
refrigeration copper tubing, available in 50-foot coils at "Home
Depot" and similar hardware stores. Do not attempt to use rigid copper
pipe! Soft aluminum tubing also works well, though it is seldom available
except as a surplus item. The bare metal tubing must be space wound to prevent
shorts between adjacent turns.
The primary coil support frame is made out
of standard PVC pipe. Evenly spaced slots, the width of the copper tubing, are
cut into the outside of the pipes making up the form. The slots hold the tubing
in place, and maintain a precise spacing between turns.
Taps on turns are usually best
accomplished by using a copper strap that wraps completely around the tubing
and which has a screw and nut to allow tightening. It is important that tap
connections have large surface area and low resistance, as very large currents
will flow through the connections. Poor connections will get very hot, due to
large resistive losses. A large alligator clamp can be used to initially tune
the coil, but should be replaced with a clamp one a good tune has been
achieved.
Secondary coil
The secondary coil is wound on a large
diameter PVC pipe. Because of the phenomenal voltage stress on this coil, the
wire used is high voltage test prod wire. This secondary coil will use four
layers of wire connected in parallel. This arrangement provides relatively high
inductance with low resistance.
Figure 17.
Secondary coil before
insulating.
There are some general rules to follow
when building Magnifier secondary coils.
Capacitors
Capacitors are one of the most important
components of any Tesla coil, and especially so in a Magnifier. The only
capacitors I recommend for Magnifier work are commercial impulse capacitors
rated for Tesla coil service. You can use homemade capacitors, provided that
you use a good combination of series/parallel connected units to provide
adequate voltage protection.
Figure 18.
Two capacitors suitable for Tesla
Magnifier coil use. They are shown in the preferred order, best to worst.
Polypropylene commercial cap, homemade polyethylene capacitor
Let me tell you straight up front that you
need to go buy yourself a rugged commercial polypropylene impulse capacitor
suitable for Tesla coil use, made by Maxwell, NLW, Plastic Capacitors, or any
one of the other major Tesla coil capacitor manufacturers. Get a cap that is
rated for at least 40 kilovolts AC, preferably more. If you can find a good
used capacitor, then good for you! They are very, very difficult to find.
Search surplus yards and junkyards, and electronic surplus stores long eno ugh and you might find one, but it’s not likely. So,
you’ll end up buying one. A 0.015 ufd, 45 kV capacitor is likely to cost you between $400 and $600.
The larger the capacitance, the more it will cost you. The higher the voltage
rating, the more it will cost you. But they are durable, they are lightweight,
they have extremely low losses, and they will make a Tesla coil kick butt! The
number one rule when ordering such a capacitor is to make sure the voltage
rating is sufficient. You should get a vo
ltage rating three times higher than the RMS voltage
that your transformer puts out. If you can afford it, get FOUR times the
transformer voltage. Remember that resonant circuits produce incredible
peak-to-peak voltages, and you don’t want your expensive capacitor to fail
because you skimped on the voltage rating!
I always place a safety gap across each of
my capacitors to protect them against over-voltages produced by kickbacks. You
can make a very simple gap from two pieces of 12 gauge solid copper wire.
Connect a wire to each capacitor terminal, and placed the other end a couple
inches above the cap, with about a one-half inch gap between the ends. Run the
coil, and see whether the gap fires constantly. If so, turn off the coil, and
increase the gap slightly and re-test. Keep increasing the gap until it on ly fires only once every few seconds. This setting will
allow the coil to run normally, but any excess voltage will jump the safety gap
and bypass the capacitor, protecting it. Safety gaps are very loud when they
fire, because the capacitor will discharge into the gap once it conducts. Trust
me, your beautiful hand-built capacitors, or expensive commercial capacitors
are worth the expense and bother of installing a fifty-cent safety gap!
Figure 19.
Safety gap installed
on a commercial capacitor.
All connections from the capacitor to the
spark gaps and to the primary coil should be very heavy gauge cable. How big a
cable should you use? Well, whatever the largest gauge cable you can
successfully connect will be just about right! Copper tubing works very well as
a primary tank connecting cable. Large coax works well, but is a little harder
to connect. Buy some "split bolt" copper connectors from Home Depot
or an electrical supply center – they are wonderful for connecting large gaug e cables together. You can even use small hose clamps,
in a pinch.
IMPORTANT: Keep all cable connections from
the capacitor to the primary coil and spark gap as short and direct as
possible. These components form part of the resonant tank circuit, and long
cables will form "off axis" inductance, which is pure loss, plain and
simple. Careful thought on the tight, neat layout of these components will
result in better performance and longer sparks.
Figure 20.
Safety gap across transformers
Top toroid
capacitance
The final stop for the high voltage,
before it leaps into space, is the top load capacitance, situated at the top of
the Extra coil. Research over the last few years has shown that Tesla coils,
and in particular Magnifier Extra coils, love large
top load capacitances. The top capacitor appears to allow much higher voltages
to form before the spark channel is initiated, and helps to sustain the spark
channel once it is formed.
Figure 21.
Some toroids
made out different materials.
How big should your toroid
be? Well, the size of a toroid is dependent upon two
key parameters: the physical size of the secondary coil and the amount of power
you are putting into the system. Toroids have two
important dimensions. The first is the outside diameter of the toroid. The second dimension is the cross-section or chord.
Cross-section is how "fat" the tire part of the toroid
is. The larger the cross section, the higher the voltage required to
"break out". It is possib le to put such a
large toroid on a given coil that the available
voltage is insufficient to exit the toroid; so no
spark will issue at all. When this happens, you can try to improve the
situation by placing a small bump of aluminum tape on the side of the toroid to allow the spark to exit, or you can increase the
total input power to the system. For the Magnifier shown here, we will build
two rather large toroids and stack them to achieve
the required capacitance that we will need. Some coilers suggest th at if using a single toroid, the toroid diameter should
be at least three times the extra coil length.
Figure 22.
Figure showing toroids
mounted above top of Extra coil
Tesla coil toroids
almost always have a center disk. This allows you to support the large diameter
donut and the disk also forms part of the electrostatic shielding that the top
load provides to the coil. The disk can be made of aluminum, or an insulating
material covered with aluminum tape or aluminum flashing. There is usually a
small hole in the center of the disk, which is used for a rod or bolt mounting
on the toroid so that it will not fall off the coil.
This rod can also be used to support and separate multiple toroids,
as we will do on this Magnifier coil.
For our toroids,
we will use aluminum flexi-duct used for heating and air-conditioning. It is
reasonably priced, but the ribbed aluminum is very soft. We will leave it
compressed when making the toroids. Cost for each 36
inch by 8-inch toroid will be about $60, and we will
require enough material to build three toroids, two
for the Extra coil top-load, and one to act as an electrostatic field-shaper
for the secondary.
Tesla coil grounding system
Magnifiers absolutely, positively, require
a good external ground for safe operation.
Most amateur coilers overlook one of the
most important parts of a Tesla coil system, the grounding system, and in so
doing, they limit coil performance, and they even risk burning the building
down because of an electrical fire!
Figure 23.
Do not connect the secondary to the
wall outlet ground!
DO NOT USE THE
The ground connection on your electrical
outlet is connected to a bare copper wire that is bundled together with the hot
wire and the neutral wire, which all go back to the electrical panel or breaker
box. Usually, this wire is several yards in length. The impedance of the wire
can be very high at Tesla coil frequencies, and any high voltage currents
impressed upon the wire can arc over to the other wires in the "Romex" or conduit and cause a short, resulting in an
electrical fire. I hav e heard of small Tesla coils
causing purple corona to shoot out of nearby electrical outlets because of this
poor practice! The use of a good line filter, and
always connecting the secondary to a good dedicated ground eliminates the
problem.
Well, what constitutes a "good
dedicated ground?" One or more copper clad ground rods driven six feet
into the soil or a metallic cold water pipe are good grounds. The conductor
from the bottom of the coil to this ground connection should be a heavy copper
conductor, or wide aluminum strip; the wider the better. Wide metal strips form
very low impedance conductors for Tesla coil currents. Standard aluminum
flashing works very well for this.
Serious coil builders should install their
own low impedance ground system, designed for Tesla coil use. Choose a spot in
the yard or garden as close as possible to where the coil will regularly be
operated.
Start of lecture:
Before you begin
digging or driving ground rods into your yard, contact your local water, gas,
electric, telephone, and cable TV companies, and tell them you are going to be
doing some serious digging in your yard, and request that they come out and
flag all their buried utility plant on your property. Get the person’s name
that you talk to and write it down. If you fail to have the utilities marked,
and then you drive a ground rod through their buried line, you can (1) get
killed by electrocution, gas explosion, etc., and/or (2) you will be
responsible for the full repair costs, and I can assure you that you don’t want
to pay for a cut utility line! For example, and I’m speaking from personal
experience here, a cut phone cable can cost you a grievous bundle of money! The
utility company can charge you for the trucks (at about $65 per hour, each),
the barricades (maybe $50), the backhoe ($150 per hour), the crew (maybe $50
per hour per employee, and there will be several), their overtime pay (ti me and a half, minimum, maybe double time if it gets to
be night or a weekend), the replacement cable (depends on the size, but a large
one can cost $100 per foot), the buried splice enclosures (as much as $100
each, and two will be required), and on, and on, and you can even be assessed
charges for the estimated lost revenues in long distance calls should you cut a
toll cable! So please, get your utilities marked, and stay away from them.
End of lecture.
Measure off a triangle eight feet to a
side, and drive a six-foot long copper clad ground rod at each corner of the
triangle. It is recommended that you rent a jack hammer and use it to drive the
rods into the ground. If you want a lot of exercise, and don’t want to rent a
jackhammer, you can instead rent or purchase a special length of steel pipe
with handles attached which is used to drive steel fence posts into the ground.
Drive the rods into the ground until the tops of the rods are a couple of inch es below ground level. After your ground rods have been
driven, dig a trench from ground rod to ground rod to ground rod. Lay a heavy
copper strap in the trench, connecting all three rods, and solder the strap to
each of the three ground rods. Bring a length of the copper strap up above the
surface of the ground, so that it sticks up and provides you with an electrical
connection to the buried strap and ground rods. Now, cover over your trenches,
burying the copper strap and the tops of the ground rods, leaving the one
strap-end sticking out of the earth. You may substitute wide aluminum flashing
for the copper, but you are more likely to have a corrosion problem because of
the dissimilar metals. This grounding construct provides 18 feet of deeply
buried copper clad rod and 24 feet of buried copper strap, and supplies a very
good ground connection for coil use.
Figure 24.
Good triangular grounding system
Whenever I operate my coil, I run a length
of aluminum or copper strap from the bottom of the coil, and connect it to the
ground strap sticking out of the ground by clamping the two straps together with a pair of Vise-Grip pliers.
Constructing the Tesla Magnifier coil
NOTE: These Tesla Magnifier coil plans
will produce a working Tesla Magnifier coil, capable of producing sparks in
excess of ten feet in length, providing you build the exact sizes of coils, toroids, and capacitors with the values shown. If you vary
any of the parameters to any significant degree, you may not be able to tune
the coil to resonance. Build the coil as shown and it will work. If you change
the size of the primary, or the secondary, or the Extra coil or the capacitor,
or the toroids, then the coil will not tune to the
expected design frequency, and in fact, you may not be able to tune the Tesla
coil at all without making other compensating changes. Suggestion: build it as
described, tune it for best performance, then you can start changing it
to suit yourself!
Parts list:
The toroid will
require three 8-foot sections of 8-inch diameter aluminum flexi-duct.
240 volt, 50 ampere line filter
240 volt, 50 ampere variac
240 volt, 1 ampere DPDT on-off switch
Two 240 volt, SPDT 50 amp contactors or
one 240-volt 50 amp DPDT contactor
Four 240 pf, 30 kV ceramic doorknob bypass
capacitors
Four 2 inch diameter ferrite choke coil
forms
200 hundred feet, 28 gauge enamel wire for
chokes
Electrical tape
5000 feet of 14-gauge high voltage test prod
wire
20 feet insulated, 8 gauge
stranded wire
0.015 ufd,
impulse capacitor, 45 kVAC voltage rating
100 foot roll of 0.5 inch diameter copper
refrigeration tubing
20 feet of 2-inch diameter PVC pipe
18 pounds of 18 gauge enamel wire, approx.
3600 feet
One 4 foot by 8-foot sheet of 1-inch thick
foil covered Styrofoam core insulating sheathing.
10 copper splice sleeves for 2.5 inch
diameter copper pipe. These copper cylinders will be about four inches long
Base for the cylindrical spark gap, can be
wood, phenolic, or acrylic piece, 5 inches wide, 24
inches long, ¼ to ½ inch thick
Cylinder support pieces, fiberglass rod or
bar stock, ½ inch diameter, 120 inches in length
Fast setting epoxy for gluing fiberglass
bar stock
Rosin core solder, 200-watt soldering iron
1 foot length of 1 inch diameter PVC pipe
42 inch length of 16 inch diameter, thin
wall PVC drain pipe
4 foot by 2 foot piece if ½ inch thick
plywood
Two by fours, total of 50 feet length,
enough to build box frame 4 feet long by 2 feet tall
Decking screws, 1 and ¾ inches long, one
box
Hot glue
PVC pipe, 8 inches in diameter and 26
inches long
J.B. Weld epoxy (requires 24-hour setup)
Large alligator clip for making tap
connections when tuning primary coil.
16 standoff insulators
for bypass caps and choke coils (optional).
5 binding posts for
transformer protection circuit.
Assortment of copper
"split-bolt" connectors for splicing large gauge cable together
240-volt AC meter
100-amp AC current
meter.
240 volt AC power indicator panel lamp
If you use a brush-type AC rotary gap
motor:
200-watt light dimmer control, or a
120-volt, 5-amp variac
If you use a DC rotary gap motor:
120-volt, 5- amp variac
to control power to the DC power supply
DC power supply: suitable low voltage
transformer for motor, full-wave bridge rectifier, 2 MOV’s
for DC power supply,
Assortment of
1,000 watt or larger heating elements
Fan or blower to
cool heater elements
240 volt, 50-amp
"range" outlets and pigtails for ballasts
One
set of hardwood handles for a wheelbarrow
Now that we’ve become familiar with all
the parts of a Tesla Magnifier coil, let’s build one. The plans presented here
will allow you to build a coil powered by a 2 kVA
potential or distribution transformer, and make sparks over ten feet in length!
Figure 25.
The Tesla magnifier coil in operation,
producing sparks in excess of 10 feet in length
Making the box frame
The Tesla magnifier driver is built on an
open box framework, with a ½ inch thick piece of plywood on top. The primary,
secondary, spark gaps mount on the plywood. If you use a commercial capacitor,
it too can mount on the plywood. The plywood top is 24 inches wide and 48
inches long. Plywood sheets, already precut to this size are available at most
lumber supply companies. The box frame supporting the plywood is made from two
by fours, connected together with decking screws, which are also used to attach
the plywood on top (there are areas of the plywood which must NOT have metal
screws, see figure 28). The box frame should support the plywood at least 24
inches from the floor, to prevent the magnetic field produced by the primary from
interacting with the ground or with steel rebar in a concrete floor. After
building the box frame and attaching the plywood, use a good polyurethane
varnish to coat the entire assembly. The coating will help keep moisture out of
the wood. If you have esthetic s ensibilities, you
can stain the wood first.
Figure 26.
The box frame and location of
components
See Figure 28 for the location of the
primary, secondary, gaps, and capacitor on the plywood.
This Magnifier design has handles and an
axle and wheels attached to the box frame so that it may be rolled about. If
you will be using homemade capacitors, you will want to add a capacitor shelf
and straps to the side of the box frame for the capacitor – see Figure 28.
Making the primary coil
The primary coil form is made out of eight
slotted lengths of 2-inch PVC pipe placed in a circle.
Cut a disk out of ¼ inch plywood, 19
inches in diameter. Set this disk aside for later use.
Using a 2-inch hole saw, drill eight
evenly spaced holes in the plywood top of the box frame, as shown in Figure 29.
Make sure that the inner distance between opposite holes is exactly 19 inches.
Cut eight lengths of 2-inch diameter PVC pipe 28 inches long. Number each pipe,
and mark one end of each pipe "TOP". Place the pipes in ascending
number sequence in a simple square frame, aligning all the "TOPS" to
one end. Mark the slots as illustrated in Figure 30. The idea is to have slots
in the pipes into which we will wrap the copper tubing to form a smooth spiral.
Use a radial table saw equipped with a dado blade, and cut twelve slots, each ½
inch wide, spaced 1 inch apart in each pipe, using the marks made in the
previous step as a guide. Make the dado cuts just wide enough to snugly
hold a test piece of copper tubing.
Place the 8 pipes in the 8 holes drilled
in the plywood top. Make sure they are in number sequence, and that the ends of
the pipes marked "TOP" are indeed at the top. Point the slots
outwards on all the pipes. Hot glue the pipes into the holes in the plywood.
Put a five-gallon paint bucket in the
middle of the primary form. Set the 19-inch plywood disk on top of the bucket.
This disk will act as a temporary platform for the roll of copper tubing, and
it also acts as an inner spacer for the tops of the pipes and it will prevent
the pipes from bowing in when the copper tubing is wound around them. Set your
coil of ½ inch copper tubing on the plywood disk on top of the paint bucket,
taking care that you place the copper coil so that the OUTER end of the copper
tubing will unwrap in a clockwise-direction. Attach the OUTER end of the copper
tubing to the BOTTOM slot of pipe #1, using nylon tiewraps
to hold it firmly in place. Start winding the primary using the copper tubing.
Place the tubing into the slots, going around and around the form until you
have wound 12 turns. You may have to use a rubber mallet to seat the tubing
into the slots as you wind. Your primary will use over 50 feet of tubing, so
partway through the winding you will have to use a copper sle eve to solder-splice another piece of tubing onto
the primary. Be sure to setup the new coil of tubing to unwind in the proper
direction! Finish-off the top-turn by tiewrapping the
end of the tubing into the last slot.
Figure 27.
Bolt hole for secondary
See Figure 28. On the coil end of the top
of the box frame, mark a spot in the center of the primary. Use a 1/2-inch
diameter drill-bit and drill a hole at this spot. This hole is for the
secondary coil support bolt.
Figure 28.
Two-by-four support and winding jig
hole placement.
Figure 29.
Location of secondary
ground hole.
A 1-inch wide, 2 inch long hole now needs
to be drilled through the plywood top. Use Figure 31 as a guide for placing
this hole. It is near the end of the box, centered exactly between two of the
primary support posts, and 1 inch inside.
Building the secondary
Cut a length of 16-inch PVC pipe, exactly
42 inches long. The best way to mark the cut line around the circumference of
the pipe is to wrap a flexible plastic ruler completely around the pipe and
mark along one edge. Use a handsaw or fine toothed skill saw and cut the pipe
squarely. Use soap, water, and a piece of Scotchbrite
and scrub the pipe clean. Do not use steel wool, as it will leave tiny bits of
metal embedded in the plastic. Use a moderately warm source of hot air (hair
dryer, room heater, etc.) and bake the tube dry. PVC is somewhat hydroscopic,
so we want to get out as much moisture as we can. After the tube has been baked
dry, coat it inside and out with a good polyurethane coating. Do not use
water-based polyurethane.
Figure 30.
Two-by-four support and winding jig hole
placement.
Cut two lengths of two-by-four about 16
inches long, and shape the ends so that they fit exactly inside the ends of the
16-inch PVC pipe. See Figure 31. Carefully measure and drill a ½ inch hole in
the center of each support. Mount these two-by-fours to the pipe with epoxy and
several ¼ inch diameter plastic or wooden pegs. DO NOT USE METAL SCREWS!
Figure 31.
Hand-crank operated coil winding jig.
It is recommended that you construct a
coil-winding jig something like the one illustrated in Figure 32, or you can
wind the tube by hand. Keep the windings tight, do not kink the wire, and do
not have any "crossovers" in the wire. It is highly recommended that
you stop every six inches of winding length and stick a piece of box tape on
the windings to hold them in place, just in case the wire breaks or if you
accidentally drop the wire. Be sure there are no gaps between any of the turns.
Gaps will propagate from layer to layer and will plague you with continuing
problems. Wind the coil to within one inch of the top end of the tube, and
within 2 inches of the bottom end of the tube. When you have finished winding
the coil, tape the ends of the wire to the tube. Run a bead of hot glue al the
way around the ends of the windings, securing the wire to the tube.
Start the second layer of wire at the same
end of the tube where the first layer began. Wind the second and following
layers in the same direction as the first layer. You should be able to lay the
second layer of wire right on top of the first layer, using the turns of the
first layer as a winding aid. Finish each layer of wire with a bead of hot glue
around the ends of the winding to hold the windings securely in place.
After the four layers of high-voltage
test-prod wire have been wound, connect the bottom ends of the four wires
together and solder them to a ground plate mounted as described below.
First, cut and shape a ½ inch wide, 2-inch
long piece of copper strap to fit the curvature of the coil form. Next, drill a
hole in the copper, and fasten a flathead, ½ inch long screw through it and
secure with a nut. Epoxy this strap and screw to the side of the tube, below
the bottom of the winding, with a generous dollop of J.B. Weld epoxy (J.B. is
used because of the soldering heat, which will be applied later). After the
J.B. Weld has cured (approx. 24 hours), solder the bottom end of the seconda ry coil to this copper
ground plate.
The screw is used to connect the large
copper or aluminum ground strap, which runs to the dedicated ground rod. See
Figure 33 for details on this bottom coil ground connection. Do not attach the
top end of the secondary wires to anything at this point of construction.
Cut a 25-foot length of 120-mil thick
polyethylene, 42.5 inches wide, which should be long enough to wrap completely
around your secondary coil four times, forming an insulating barrier four
layers thick. If you have to use more than one piece of polyethylene, be sure
to overlap the two pieces of poly about four inches. You can use three or four
short pieces of clear Scotch box tape to tape the overlap together. Do not use
a single long piece of box tape from the top of the coil to the bottom! Doing
so will produce an arc-path, which can short out the coil. This important tip
was learned the hard way!
Figure 32.
Ground plate details
Building the Extra coil
Cut a length of 10-inch diameter PVC pipe,
exactly 26 inches long. The best way to mark the cut line around the
circumference of the pipe is to wrap a flexible plastic ruler completely around
the pipe and mark along one edge. Use a handsaw or fine toothed skill saw and
cut the pipe squarely. Use soap, water, and a piece of Scotchbrite
and scrub the pipe clean. Do not use steel wool, as it will leave tiny bits of
metal embedded in the plastic! Use a moderately warm source of hot air (hair
dryer, room h eater, etc.) and bake the tube dry. PVC is somewhat hydroscopic,
so we want to get out as much moisture as we can.
Figure 33.
Details of the Extra
coil, with the three toroid centering pins.
Cut three 24-inch lengths of 1-inch
diameter PVC pipe. Drill two holes at one end of each pipe, the first is one
inch from the end of the pipe, and the other hole is three inches from the end.
Place three marks on your 10-inch PVC pipe 120 degrees apart. Using these three reference marks, drill six holes, one inch and
three inches from the edge of the pipe as shown in Figure 34.
Counter-sink these holes. Do not place any hardware in these holes yet.
Coat the 10-inch pipe it inside and
outside with a good polyurethane coating. Do not use water-based polyurethane.
After the tube has dried thoroughly, we can begin to wind the 18-gauge enamel
wire on the tube.
Wind the 10-inch diameter Extra coil from
the bottom of the tube toward the top of the coil. Start one-inch from the
bottom of the pipe and stop when you reach the holes which are three inches
from the top of the pipe.
It is recommended that you use a
coil-winding jig something like the one illustrated in Figure 31, or you can
wind the tube by hand. Keep the windings tight, do not kink the wire, and do
not have any "crossovers" in the wire. It is highly recommended that
you stop every six inches of winding length and stick a piece of box tape on
the windings to hold them in place, just in case the wire breaks or if you
should accidentally drop the wire.
When your winding
reaches the holes that are three inches from the top of the pipe, stop winding. Using the NYLON screws and nuts, attach the three
1-inch diameter, 24-inch long PVC pipes to the INSIDE TOP of the 10-inch pipe.
The screws must be flush-mount NYLON SCREWS AND NYLON NUTS. Refer to Figure 34.
Under no circumstances use metal screws or nuts.
Hand wind the last two inches of wire over
the flush nylon screws, and finish off with a bead of hot glue around the end
of the winding.
Special note: Do not coat the Extra
coil with polyurethane or any other coating. You may place a bead of hot glue
around the top and bottom of the windings to hold them in place. Coatings
of any kind over your windings can cause your Extra coil to fail.
Building the toroids
Figure 34.
Toroid details
On the three toroids,
we will use aluminum flexi-duct. This duct comes in lengths about 32 inches
long. Leave the duct in this compressed state; it is much stronger than if it
is stretched out. When the duct is stretched out, the duct becomes very soft
and is very easily damaged, even by regular handling.
Making foam disks and covering them
with aluminum
Note: it is very important to exactly
align parts that have been sprayed with contact adhesive. Once the parts touch,
they cannot be adjusted!
Cut six 19-inch diameter disks out of the
Styrofoam core panel. Spray one side of two disks with spray contact adhesive
and let them dry for ten minutes. Carefully align the two foam disks and stick
them together, forming one thick disk. Do this to the other two pairs of disks,
forming a total of three thick disks.
We will now cut aluminum disks to cover
both sides of the foam disks.
Cut six 19-inch diameter aluminum disks
out of the aluminum flashing. Spray one side of an aluminum disk and one side
of a foam disk with contact adhesive, and let them sit ten minutes, then stick
them together.
Cover the other side of the foam disk with
another aluminum disk using the same technique. Do this to all three foam
disks. When you finish, you will have three aluminum-covered disks 19-inches in
diameter, one for each toroid.
Connect three lengths of 8-inch diameter
aluminum duct together with aluminum tape, forming a long straight tube. Do not
rub the tape down yet! The duct will require some bending when it is
formed around the disk. Wrap the duct around the disk and cut the ends
to fit exactly. The two ends of the duct can be taped together with aluminum
tape. Test the toroid for a good fit around the disk.
Once the duct has been cut and fitted properly, take a screwdriver handle and
rub the tape down smooth, leaving no protruding wrinkles. Finish up by aligning
the disk in the middle of the toroid and hot gluing
it into place. Run some lengths of aluminum tape all the way across the toroid and disk to insure everything is electrically tied
together. See figure 34 for details.
After you have made the toroid, you will need to drill three 1-inch diameter holes
in the center of the disk. These holes should be 120 degrees apart, and must
exactly match the positions of your three support pins at the top of the Extra
coil. The toroid disk will slide down over the PVC
support pins, which stick out of the top of the Extra coil. The pins through
the disk will prevent the toroid from ever falling
off the top of the coil. A row of small holes can be drilled through the
support pins, and a plastic cotter key put in place to lower or raise the toroid. You may also cut and use short cylinders cut from
10 inch PVC pipe to set the height and spacing of the two toroids
above the Extra coil.
The third toroid
you will build mounts on top of the secondary coil. You may use some hot glue
to hold it in place. Attach the four wires from the top of the secondary to the
toroid, using a small piece of aluminum tape.
Building the transmission line and
Extra coil stand
The transmission line is very easy to
build. Cut a length of 10-inch to 15-inch wide aluminum flashing 12-feet in
length. Staple two 11-foot lengths of ½ inch by ½ inch pine strip down the
sides of the aluminum to act as stiffeners.
The Extra coil stand is made from a 5-foot
length of 10-inch PVC pipe securely fastened to a large square base made from a
4-foot by 4-foot sheet of ½ inch thick plywood. The goal is to make a sturdy
stand on which you can set the Extra coil and the toroids.
Remember that you have large diameter toroids balanced
on top of a small-diameter coil, and this configuration is by nature very
unbalanced, and the whole mess will want to topple over!
At the base of the PVC pipe stand, you can
cut a 10-inch diameter plywood disk and screw it inside the end of the pipe and
bolt it to the center of the
The capacitor
Hopefully you followed my advice, and are
using a commercial capacitor. If so, it will probably fit on the sheet of
plywood with no problem. If you are using a homemade cap, or a very large
commercial cap, then place the cap on a plastic or wooden stand beside the box
frame, as close to the designated spot as possible. Refer to Figure 28 for
parts placement. The goal is to place the capacitor and spark gap close to the
primary, where the connecting wires are as short as possible. Note: To preve nt
accidental arcs, do not place either the capacitor or spark gap assembly closer
than about four inches to the primary.
Building the static gap
These gaps are easy to build, and are not
expensive. Start by cutting a base for the cylindrical spark gap out of wood, phenolic, or acrylic. It should be eight inches wide, and
thirty inches long.
Figure 35.
Initial placement of
fiberglass rods for static gap.
Be sure to wear a respirator when
cutting and grinding the fiberglass rods! The dust is extremely damaging to
your lungs, and is only slightly less dangerous than asbestos.
Use an abrasive cut-off wheel in a Dremel moto-tool, and cut 22
pieces of ¼ inch diameter fiberglass rod about 1/8 inch longer than your copper
cylinders. The exact length of the rods will depend on the exact length of your
cylinders, which should be about four inches in length. Cut four more
fiberglass rods to a length of 24 inches. Using fast-set epoxy, glue two of the
24-inch long rods and two of the short rods together on the stand as shown in
Figure 36. Let the epoxy cure.
Place two strips of double-sided sticky
foam tape on the top of the gap stand, down the length of the two 24-inch rods,
about ½ inch inside the rods. See Figure 36 for clarification. The purpose of
the sticky tape is to temporarily hold the remaining short rods in place so
that the gaps spacing can be set.
Figure 36.
Placement of
double-sided sticky foam tape on static gap.
The remaining rods are arranged in pairs.
Place them on the sticky tape as shown in Figure 37, using the approximate
spacing shown. Once all the rods are in place, set all ten copper cylinders on
top of the pairs of the rods. Adjust the spacing between the rods so that the
cylinders are perfectly parallel, and each pair of cylinders separated with a
1/32-inch gap. You can use 3 or 4 stacked business cards to make a handy feeler
gauge that is just about the right thickness. The sticky tape will hold the
rods in place, but will still allow you to change the spacing between them
until all your cylinders have perfect gap settings between them. Once you are
satisfied that the gaps are all even, then carefully lift the cylinders off of
the rods. Mix up some J.B. Weld epoxy, and glue the rods to the stand so they
will no longer move. After the epoxy has set up for about half an hour, set
your cylinders back on top of the rods and verify that the rods are exactly
where they need to be. The epoxy will stil l be pliable enough at this time that you can make slight
adjustments, if necessary. Be sure to take the cylinders off the rods again, so
that the rods will not be slowly moved by the weight of the copper cylinders!
It will take 24 hours for the epoxy to fully cure. Once the epoxy has hardened,
your gap will be perfectly aligned every time you set your cylinders on the
rods. As you use the gap, the copper cylinders will discolor because of the
heat. Discoloration is not a problem, but over a period of time, a black oxide
scale will build up on the outside of the copper cylinders at the area of the
narrow gaps. If this should happen, simply rotate the cylinders until fresh
copper is in the gap area. This neat trick will allow you to run your gaps many
times before they need to be re-polished!
You need to solder a length of the 8 gauge
stranded wire to the inside of the cylinders at each end of the row so that you
can connect this static gap to the rotary gap, and to the rest of the Tesla
coil circuit. You will need a heavy soldering iron; I have found that a
200-watt unit works well for this. You can use an acetylene torch, but a touch
of skill and a good rosin flux will be necessary.
Building the rotary gap
A rotary gap incorporates a plastic disk spinning
at very high speeds. The disk can shatter and throw the rotating screws and
shards of sharp plastic at high velocity in all directions. It is recommended
that you either build the rotary gap into a box or place a Lexan
"scatter shield" around the gap to protect yourself and bystanders
from shrapnel, should anything ever go wrong.
Because the disk will be spinning very
fast, as much as 10,000 RPM, the disk needs to be cut and machined to a fair
degree of precision. This is best done in a machine shop on a lathe. The disk
can be cut and "faced" on both sides. Once the screws have all been
mounted and ground to length, it is possible to have the disk dynamically
balanced; though this is an expensive proposition. I do not personally know anyone
who has spent the money to balance a rotary gap disk. If the disk is cut an d trued-up on a lathe, and if identical hardware is used
and accurately placed around the disk, then balance is almost always very good.
I have build excellent rotary disks using
no tool fancier than a nice drill press. If you have a good drill press
available, go ahead and make your own disk. If you do not have a drill press
available, then spend $25 or $30 to have your plastic disk trued up on a lathe.
Buy the machinist a few beers after work, and he’ll likely do it for free.
Figure 37.
Layout of rotary disk
Use Figure 38 as a guide for the layout
and cutting your disk. I recommend eight rotary electrodes made out of common
10-24 screws and nuts. The disk should be made out of G-10 epoxy-glass
composite, perhaps the strongest material available for this application. If
you cannot locate G-10, or cannot afford it, then use ¼ inch thick Lexan sheet. Lexan is virtually
indestructible, but has a relatively low melting point. As a last resort, you
can use acrylic, but it cracks easily, and I’m always nervous when I’m near a
spinning disk make of acrylic.
G-10 is a glass composite, and is very
hard. I have worn all the teeth off a metal-cutting saber saw blade almost
instantly trying to cut this material. Use carbide or diamond toothed saw
blades.
Gathering the materials for the rotary
First, you need to find a suitable AC/DC
motor. Let me warn you that most motors will be unsuitable for rotary gap use,
because they do not have carbon brushes in them. No brushes, no speed control. For example, the motor in a cheap box fan
will not work, because it only runs at a fixed speed. The best motors are
115-volt brush-type motors, like the ones that once were used in sewing
machines. They will operate on either AC or DC, and because they are brush type
motors, their speed is adjustable. B est
of all, you can control their speed using nothing but a cheap $5 light dimmer
control, eliminating the need for power supplies or variacs!
As an example of what you can do, I once pulled the brush-type motor out of a
rotary skill saw and made a rotary gap with it. The shaft turned at 5,000 RPM,
and the end of the shaft was already drilled and tapped with a bolt. To use it,
all I had to do was take the motor from the saw, unbolt the saw blade, and bolt
in my plastic disk. It was very powerful; the mo tor
speed was controlled with a light dimmer control, but gosh was it loud!
Figure 38.
Simple controllers for AC/DC motors, and a DC power supply for a DC motor.
After AC/DC brush-type motors, the next
best rotary gap motors are low voltage (6 to 50 volt) DC motors. These can be
found at surplus electronic catalog suppliers, etc. They were often used in the
old IBM mainframe tape drives. You will need to build a DC power supply to
convert the 120 volts AC to the appropriate DC voltage for your motor. A simple
step-down transformer and bridge rectifier is all that is needed here. The
easiest and most robust way to control the DC motor speed is by controlling the
120 volts from the wall outlet to the power supply through a small variac. Do not attempt to control a power supply
transformer with a light dimmer control! Always use a variac
to control power to a transformer. See Figure 39 for a diagram of a suitable
power supply and controller for a DC motor.
When you are looking for a suitable rotary
motor, it is important to try and find a motor that already has a mounting
screw/bolt/flange/hub-thingy on the end of the shaft that allows you to easily
bolt your disk onto the motor. If the motor does not already have this feature,
then you’re gonna have to make or pay someone to make
a suitable shaft-adapter that will allow you to bolt your disk to the shaft.
Once you have the motor with some kind of disk mounting arrangement, then you
can start to make the disk.
Start making the disk by using a compass
and marking off a 9-inch circle on your disk material. Use a jig- saw or saber
saw and cut as accurate a circle as is humanly possible. If you are using the
G-10 fiberglass board, you’ll have to use special blades because of the hard
glass fibers. The plastics are much easier on saw blades, but they can be hard
to cut without melting the plastic. The key to cutting plastic is to use the
correct blade, adjust the blade speed and cut at just the right forward sp eed to prevent the plastic from melting back together
behind the blade. Sometimes I’ve been lucky in that department, sometimes not.
I have one friend who cuts plastic outdoors while cooling the blade of the saw
with a trickle of running water from his garden hose, but I consider doing that
tantamount to attempting electrical suicide! Maybe a friend could instead direct
a cooling blast of air from an air-compressor tank?
Figure 39.
Method of truing up
edge of rotary disk.
After the disk has been cut out, drill a
hole exactly in the center of the disk. This hole should match the mounting
hardware on the end of your drive motor. If you have access to a lathe, chuck
the disk up and true off the outer edge as well as the front and back face of
the disk. If you don’t have a lathe handy, but do have a drill press available
or a router and router table, then you can true up the outside edge in a
different way. Refer to Figure 40. Mount a bolt in the center hole of the d isk, using a couple of flat washers and a nut. Make sure
the bolt you choose is a very snug fit in the hole – you don’t want any slop
here! Put the bolt through a board, so that you can slowly spin the disk. Clamp
the board down so it does not move. You are going to bring a milling bit or a
router bit up against the edge of the disk, just where the bit is cutting away
material. Lock the router or drill down so it cannot move anymore. Slowly turn
the disk by hand, until you’ve turned the disk 360 degrees ,
slowly milling the edge as you go. After the first go-round, the high spots on
the edge of the disk should now be milled off. If the disk is not yet perfectly
round, move the router bit closer to the disk until it starts cutting the
plastic, and rotate the disk slowly around again. After two or three passes,
your disk should be almost as perfectly round as it would be if it had been
turned in a lathe. The key here is to make sure that your mountings are tight,
and that the disk turns freely with no wo
bble or slop at the center pivot. I suppose one could
bolt the disk to a steel bearing assembly and clamp that down and rotate the
disk, but I’ve never had a bearing assembly handy.
After truing up the disk, you want to
carefully rule off the circle into eighths. On each of your eight lines,
exactly ½ inch from the edge of the disk, make a mark. After all 8 marks are
made, drill holes at these marks, using a drill bit suitable for
#10 hardware.
Figure 40.
Detail of flying electrode
Figure 40.
Take eight #10 screws, 1
and ½ inches long, and cut the heads off with your Dremel
moto-tool and cut-off wheel. Mount the screws on the
disk, using lock washers and nuts on both sides of the disk. See Figure 40. Try
to perfectly center the screw, so that an equal amount of screw sticks out of
both sides of the disk. On acrylic, do not over-tighten the nuts, or the
acrylic will crack. If the acrylic cracks, throw the disk away and start over,
because the disk will be unsafe to use!
Figure 41.
Detail of stationary electrodes.
The stationary electrodes should be
tungsten tipped steel electrodes, ½ inch diameter, about 2-inches long. They
are available at any large welding supply center. They are the "high
freq." electrodes used inside TIG welders. Mount them in blocks of phenolic or G-10 fiberglass at least ½ inch thick. The
mountings need to be very rigid so that the clearances between these stationary
electrodes and the flying electrodes remain precisely as set. See Figure 42 for
clarification.
The Magnifier rotary spark gap uses two
sets of stationary electrodes, forming a total of four air gaps. The
stationary electrode pairs are mounted across the disk from one another, so
that each of the two sets of gaps fires at the same time. The two sets of gaps
are wired in series. See Figure 42 for clarification on the construction and
connection of the electrodes.
Connect the first set of stationary
electrodes to the second set of electrodes using a short, curved piece of
copper tubing as shown in Figure 42 and Figure 43. The remaining two electrode
connections on the rotary gap are used to connect the rotary to the primary
coil and to the copper static gap.
Figure 42.
Complete rotary spark gap.
You will need to build a rigid motor mount
for the rotary out of wood, phenolic, or acrylic.
Because no one else in the whole world will be using a motor exactly like the
one you have, you’ll have to make up the design of the mount for your motor
yourself. Refer to Figure 43 for a sample rotary design.
Here are some key things to keep in mind
when assembling the motor mount and stationary electrodes:
· Place the entire rotary gap in a protective box, or install
a Lexan plastic scatter shield to protect you from
flying shrapnel should the rotary disk ever fail.
A rotary spark gap of this design, using a
9-inch disk, with acrylic motor stand and high-temp phenolic
electrode mounts has operated 20,000 volts at 10+ kilowatts for over a year
with no problems.
Building the transformer protection
circuit
First, you will have to wind your choke
coils. If you decide to use an air-core choke, wind it on a
piece of four-inch diameter PVC pipe, at least 8 inches long. Close-wind it with 28-gauge or similar enamel wire. Make
sure you have no crossovers or kinks in your wire. Coat the final coil with a
couple coats of polyurethane to hold the windings in place.
If you decide to wind a choke on a ferrite
core, it is wise to wrap the core with a layer of electrical tape first to
prevent arcing to the core. End the first layer of wire at least ½ inch away
from the starting point of the layer, also to prevent arcs. If you wind more
than one layer of wire, be sure to insulate with electrical tape between each
layer of wire. Winding ferrite donut cores is simplified if you will fill a
small bobbin with wire, which you can shuttle through the hole in the core as yo u wind it. I usually use a
Popsicle stick wrapped with a quantity of wire as a bobbin when I make my
chokes.
After you have built your chokes, cut a piece
of Lexan or acrylic sheet about 12 inches-square.
Mount the four bypass capacitors and four choke coils as shown in the circuit
diagram in Figure 13c. To prevent arcing between parts, do not mount the parts
too close together. You can mount the parts on the plastic sheet with standoff
insulators, or you can glue them in place with hot glue. All high voltage
solder connections should be smooth and round, to reduce corona losses. You may
also use heat-shri nk
tubing over your connections.
I recommend mounting a set of two input
and two output binding posts to facilitate connecting the transformer leads as
well as connecting the tank circuit leads. You will also need a fifth binding
post for your ground connection.
Build a transformer safety gap as shown in
Figure 21.
Figure 43.
Controller parts layout
Figure 44.
Potential transformer controller schematic
Building the remote power controller box
Because this coil can produce long sparks,
the power controller must be connected to the transformers and rotary gap by
means of long extension cords so that the operator will not be struck by the
sparks. A remote power controller box will need to be built. In this box, you
will mount your line filter and variacs to control
the power going into your Tesla coil. The power controller box is built as
shown Figures 44 and 45. The power controller is built into a box, 24 inches
tall, 24 inches wid e, and 18 inches deep. It has a
metal front panel (which is grounded, by the way,) upon which are mounted a
power switch and indicator lamp, a rotary gap light dimmer control or variac, and the main power control variac.
An AC voltmeter and AC amp meter may also be installed on the panel. Two AC
receptacles are mounted on the back of the controller. One receptacle is for an
extension cord for the rotary gap motor (or power supply) and the other is for
an extension cord for the main power transformers.
You should mount your power switch, variac, dimmer control, etc., on a metal panel, which is
grounded. Use the green wire in the main power cord, which runs to the wall
outlet as the main panel ground. If you fail to ground your power panel, it will very likely pick up radiated energy from the Tesla coil
and give you a nasty shock!
Mount your variacs,
dimmer control, switch, lamp, and meters on the metal panel. Wire them together
as shown in Figures 44 & 45. You absolutely must use a grounded, three-wire
power cable to connect this panel to a 240-volt receptacle!
Your power controller will have to
incorporate some 240 volt, 50 amp "range" power receptacles into
which you can plug the welder and heater element ballasts. These plugs are
paralleled with each other, but they are connected in series with the main
power variac. See Figure 45 for specifics on the
wiring.
For the tests described below, it is
assumed that you already have the ballasts plugged into the control panel.
Since the ballasts will be in series with the load, failure to plug them in
will prevent the transformer from receiving power!
Figure 40.
Figure 45.
An adjustable ballast which uses nichrome
wire.
Building an
adjustable resistive ballast: Appliance repair shops sell replacement nichrome heater element wire kits. The nichrome
wire is coiled up like a very long spring. Take some three-inch tall ceramic
standoff insulators and mount two 36-inch lengths of the wire side-by-side. In
the middle, strap them together with a slideable
copper clamp. Your two electrical connections will be on one end of the wires.
You can change the resistance of the load by changing the location of the slideable cl amp. This ballast
will allow you set the resistance of the ballast very precisely. You will want
to mount a fan or blower to cool the nichrome wire.
If you mount the insulators and wire on plywood, you will want to place a sheet
of aluminum flashing between the nichrome and wood to
act as a heat shield to prevent a fire. See Figure 46 for details.
Once you have wired the control panel,
plug a 100-watt incandescent lamp into the main power outlet on the back of the
controller and test the on/off switch, power lamp, and main power variac. Monitor the output voltage and current on your
meters. You also need to plug an incandescent lamp into the AC rotary gap
outlet and test the rotary motor controller as well. If you are using a DC
motor and associated power supply, use a voltmeter to measure and verify that
the DC output voltage of the DC power s upply is
within reason and that it varies from zero to full rated voltage when
controlled by the rotary gap variac. This power controller
should be operated on a fused or circuit breaker protected wall outlet with a
current rating of no higher than 30 amps.
Bringing it all together
Let’s review just where we are, at this
point. You built a box frame upon which to mount the Tesla coil components. You
have wound a four-layer secondary coil with high-voltage test-prod wire, and
built a nice toroid to place on top. You have wound a
helical copper tubing primary coil, which is separated from the secondary with
heavy polyethylene insulation. You wound either four air core chokes, or four
ferrite core chokes and mounted them along with some bypass capacitors on a
plastic plate and connected them to five binding posts. You built a high-power
copper cylindrical static gap. You have also built a series four-gap rotary
spark gap.
The time has finally arrived to connect
all these components together and to test them in the Tesla Magnifier coil!
Step 1.
Connect one end of the static gap to one
of the stationary electrodes of the rotary spark gap. The remaining stationary
electrode and the other end of the static gap are now the two connections,
which will be referred to as "the spark gap" below.
Step 2.
The primary and secondary should have already
been mounted on the box frame. Connect the bottom of the secondary to the
dedicated ground, using a 2-inch wide aluminum or copper strap. Keep this
ground run as short as possible.
Step 3.
Connect the TOP connection of the
primary to the dedicated GROUND. This is the same connection that the BOTTOM of
the secondary will be connected. The top winding of the primary should be the
ground connection, so that any secondary strikes to the top turn of the primary
will go straight to ground and not into the capacitor or transformer.
Step 4.
Mount the Extra coil on top of the
5-foot tall PVC pipe pedestal. Insert the four locking pins into the base of
the coil so it cannot fall. Place the two toroids on
top of the Extra coil, separating them with a 10-inch long PVC pipe spacer.
Step 5.
Run a 12-foot length of 10-inch wide
aluminum flashing from the toroid on top of the
secondary to the BOTTOM of the Extra coil.
Step 6.
Connect a 15-inch length of 8-gauge
wire to the end of the capacitor nearest the primary. Connect the alligator
clip to the other end of the wire, and then clamp the alligator clamp onto the
12th turn of the primary.
Step 7.
Connect the capacitor safety gap to the
top of the capacitor. See Figure 21. Connect the transformer safety gap to the
transformer, as in Figure 21.
Step 8.
The remaining end of the capacitor
connects to the top end of the "spark gap" with a piece of 8-gauge
wire. Because this is part of the tank circuit, it is important to keep this
wire as short as possible.
Step 11.
On the "transformer
protector", connect the binding post marked "POWER-OUT" to the
one remaining spark gap connection. Connect the "GROUND-OUT" binding
post to the dedicated ground (the buried ground connection that connects to bottom
of the secondary.)
On the "transformer
protector", connect a length of high voltage wire from the binding post
marked "POWER-IN" to the output terminal of the potential
transformer. Connect the "GROUND-IN" on the protection circuit to the
ground connection of the potential transformer.
Step 12.
Connect the transformer primary
extension cable to the main power receptacle on the back of the power
controller box. Make sure that ½ of your total ballasts are plugged into the
controller to supply a moderate amount of power to the transformer (once the
coil is tuned, you can add the remaining ballasts to increase power.)
Figure 40.
Figure 46.
Controller, Tesla coil, and the cable
locations
Step 13.
Connect the rotary gap extension power
cord to the rotary gap receptacle on the back of the controller box.
Step 14.
Make sure the power switch on the
controller is "off", and the variacs and
controls are turned to minimum.
Plug the main power cable from the
power controller box into the 240-volt wall outlet.
Step 15.
On the controller box, turn on the
power switch. Verify the panel light is "on."
Step 16.
Test the rotary gap control by slowly
turning up the control. The rotary gap motor should slowly come up to speed,
approximately 80% of the maximum voltage on the control. Listen for unusual
vibration or signs of other mechanical problems with the rotary gap. Turn the
rotary gap off, after performing this vibration test.
Step 17.
Make sure power is turned off. We want
to set the safety gap across the transformer and the safety gap across the
capacitor at this time. With power turned off, adjust the gaps fairly close
(1/2 inch). Run up the spark gap to about 50% speed. Bring up the main power variac until the rotary gap begins to fire. The safety gap
on the transformer should fire very frequently. Turn off power and open the
gaps slightly, and re-test. You want the safety gap setting to be such that the
safety gaps on th e
transformer and capacitor fire once every several seconds. This setting will
allow normal voltages to pass unimpeded, but will bypass excessively high
voltages to ground, protecting the transformer and capacitor.
Step 18.
We will now test the main power
circuits. Turn on the rotary spark gap and bring it up to about 80% speed.
Step 19.
Slowly bring up power on the main power
variac. At some point, usually at about 80% power,
the rotary spark gap should begin to fire. The sparks should be bright white,
with a very sharp report. If they appear as a yellow flame, then your capacitor
may not be connected properly. At this relatively low power setting, and if in
tune, sparks should just begin to break out of the toroid
on the top of the Extra coil.
Note: always make sure power is turned
off before making adjustments to spark gaps or the primary! The voltages and
currents in the primary circuit can be lethal.
Also, it is a good idea to discharge
the capacitor prior to changing the primary taps or working on the coil.
Step 20.
If you see no sparks, increase the variac setting about 1/8 turn. If you still see no sparks
from the secondary, note the power setting, then turn the variac
down to zero, and turn the rotary control down to zero, then turn off the power
switch. Move the primary tap either in or out one turn, and retest, using Steps
11 through 13 as a guide. If you still get no sparks, create a
"breakout-point" on the toroid, by adding a
small (1/4 inch) bump of aluminum tape to the side of the to roid.
If the sparks get shorter, when adjusting
the primary tap for best tune, try choosing a tap in the opposite direction.
Tune the primary number of turns for maximum spark length at this reduced power
setting.
NOTE: These Tesla Magnifier coil plans
will produce a working Tesla Magnifier coil, capable of producing sparks in
excess of ten feet in length, providing you build the exact sizes of coils, toroids, and capacitors with the values shown. If you vary
any of the parameters to any significant degree, you may not be able to tune
the coil to resonance. Build the coil as shown and it will work. If you change
the size of the primary, or the secondary, or the Extra coil or the capacitor,
or the toroids, th en the
coil will not tune to the expected design frequency, and in fact, you may not
be able to tune the Tesla coil at all without making other compensating
changes. Suggestion: build it as described, tune it for best performance, then
you can start changing it to suit yourself!
Troubleshooting - If you have NO sparks
Note: If, after trying all primary
turns, you still get no sparks, carefully do the following:
If you have substituted a known good
tank capacitor, and have verified the circuit is wired correctly, and the spark
gap is producing bright, loud sparks, and you still have no toroid
sparks, then the primary is still not tuned to the secondary. It could be that
the toroid is too small or too large, and has made
the secondary resonant frequency above or below what can be tuned to using the
existing primary coil and tank capacitor. Many times, you can try a toroid of a different size and can re-tune the primary to
resonance. You can also try a larger or smaller tank capacitor value, or even
add more turns to the primary.
Tuning tips
Good luck with the construction of your
Tesla coil, and remember: safe coiling!
End of Conventional Tesla Coil
Construction File