~Transistorized Tesla Coil~
Preface
You may hear youself saying: "What is a Tesla Coil?" Well, the short anwer is that it's a specific type of transformer named after its inventor, Nikola Tesla. A more traditional Tesla Coil is pictured on my homepage. More specifically, the Tesla Coil is a type of step-up transformer that uses a resonant high frequency, on the order 10 kilohertz or more, to produce a voltage gain higher than its turns ratio. What does this mean? Well in many instances, as you can see in the picture, this allows mains voltage to produce spectacular lightning-like discharges, at least when done correctly and enough raw power is fed into it.
Constructing one of the devices described above requires many hours of work, many hundreds of dollars, and quite often the desire to punish ones self. So what are we to do? This is where our transistorized (solid state) Tesla Coil comes to the rescue. While it will not produce the spectacular lightning-like discharges a traditional high power unit, the general design concepts are the same, and so it makes the perfect first coil, or maybe your only coil.
What follows is an article from the August 1997 issue of Electronics Now. It is a 12 volt MOSFET transistor driven Tesla Coil capable of lighting fluorescent and neon light bulbs (flickerflame) without contact. This design, in my mind, is far superior to most transistorized Tesla Coil designs found elsewhere on the web. For one thing, the frequency control is independant and completely adjustable. Also, it uses the high power IRF540 MOSFET transistors which are rated at 40A apiece. That makes the power handling of the circuit 80 amps! Another circuit I built using a pair of 2N3055s kept blowing its transistors. I recommend this project to anyone who is planning on building a full scale Tesla Coil eventually. Many of the operating principles are the same only on a smaller and more controlled scale without the potential dangers of neon sign transformers, wall current, large capacitors, etc. I have already built a working model, so the circuit is basically proven. I have added some comments noting any possible snags and design improvements {{in double braces}} that I ran into during construction that are not mentioned here.
This unit is great for small scale demonstrations. It shows how high-frequency/high-voltage electricity can travel through the air and other objects normally considered as insulators. I have two flybacks that I use with this circuit. One of them is the kind with no internal diode that is described in this article. The other one came from a broken color TV and produces high voltage DC pulses due to its internal diode. I just wound the primary on the same way as on the B&W flyback, and it is large enough to operate without being submerged in mineral oil. You could also put a multiplier across the HV terminal and ground on the B&W flyback to get 50-100Kv DC and up, depending on how high of a voltage you want to produce. These are especially fun because you can create your own static electricity! Just be careful, because when you are near it, everything metal that you touch will give you a shock over and over which can become very irritating.
Finally the pictures!
In the picture, you can see that I have used a small heatsink. Actually, this heatsink doesn't even get warm and turned out to be overkill. You will notice that I used a 15x15 prefboard, although I would recommend something a bit larger as the component spacing is quite tight. (there are several components that are mounted on top of one another!) The funny red stuff in the pictures is high temp RTV, available in your local automotive department. It has very high dielectric strength, which means it blocks high voltage corona really well. The red lead is the high voltage and the yellow is ground. Notice how I have soldered the wires directly to the screw heads.
The article seems to imply that all B&W television flybacks do not have the internal high voltage diode. Unfortunately, this is not the case. In fact, most of the B&W flyback transformers you will run into will have this diode. The ones you want to look for are similar in shape to the one I have pictured. You can usually tell if a flyback has the internal diode by weather the coil is taller than it is wide. Usually they can be found in the old tube driven TVs. I would not recommend purchasing the unit listed in the parts source at the bottom of the article. This unit is overpriced and underpowered. I was unable to attain stable operation even with it submerged in the oil. It just wanted to melt itself. The ones to look for have the potting around the coil covered in plastic. The one I had was blue. They are easy to blow, so try to keep the amp draw of the circuit around 7A.
Here is one possible circuit board layout viewed from the solder side. The red dots are wiring points and the blue lines are on the opposite side of the board.
Diagram of a six stage Cockcroft Walton cascade multiplier:
^^^^^^^^The resistors are there so that dangerous currents are not formed to protect the diodes and humans, which are low current at such a high voltage.
^^^^^^^^Six more stages can be added for higher voltages.
Please remember that any time you have HV capacitors, you are dealing with potentially deadly electricity, even with power off!
Each diode and capacitor should be rated for at least 10Kv, and make sure to keep the voltage under the PIV of the diodes by running the flyback out of resonance.
I could go on and on, but most of that has already been written somewhere else. Just visit the links, they have all the information you need. Wihout further ado - here are the plans.
Electronics Now, August 1997 v68 n8 p31 (7)
Build the poor man's plasma globe.
Abstract: Tips on how to construct a plasma-globe display that
creates a low-power, high-voltage electrical discharge are provided. This
device requires items such as a high-voltage power supply, a flyback
transformer and an ordinary light bulb.
Full Text: COPYRIGHT 1997 Gernsback Publications Inc.
This plasma globe uses an ordinary, decorative light bulb and the power supply
is great for other high-voltage experiments!
Man has been fascinated by high-voltage electricity ever since our distant
ancestors became smart enough to realize that lightning was dangerous. However,
it was thousands of years until man discovered that lightning was electrical in
nature, and later was able to produce the effect on his own. Today,
experimenting with high voltage is one of the most popular activities among
electronics hobbyists. It's fascinating to create your own miniature lightning,
plasma globe storms, and other high-voltage effects.
The plasma-globe display described here is based on a solid-state power supply
that produces a low-power, high-voltage, high-frequency electrical discharge.
The main step-up transformer is simply a TV-flyback transformer with a new
pnmary winding. To prevent overheating, the reworked transformer is submerged
in mineral oil {{baby oil}}. The plasma globe itself is an ordinary 100-watt clear-globe
light bulb.
Many other high-voltage experiments with bizarre effects can be conducted using
the power supply. It is powered from a 12-volt DC supply, so you don't need to
get involved with AC-line current. This is one of those projects that will make
you feel just a bit like a mad scientist.
The Circuit. Figure 1 is a schematic of the high-voltage power supply. It is
simply a step-up transformer driven by an AC signal. Input power is supplied to
the circuit through 10-amp fuse F1 and switch S1. The circuit requires an input
of 12-to 14-volts DC at 5 to 7 amperes. Since the power input is DC instead of
AC, the transformer's input signal is generated by IC1, a Silicon General
SG3525A pulse-width-modulator circuit. That component has two outputs that are
180 [degrees] out of phase. The amount of time that both outputs are off (the
"dead" time) is set by R1.
The output frequency of ICl is made variable by potentiometer R3, with R2
setting the upper limit. That way, the operating frequency can be tuned to the
frequency needed by T1 and any particular load connected to it. If you are
thinking of connecting a voltage multiplier to the output as an expenment,
varying the frequency will run the circuit out of resonance. That will give you
a variable high-voltage DC supply.
The outputs from IC1 are amplified by Q1 and Q2, a pair of MOS-FET
transistors
in a push-pull configuration. Since the transistors are driving a
highly
inductive load {{the step-up transformer}}, L1 and Cl decouple the
transistors
from IC 1, keeping the RF energy generated by the transistors away from
the IC.
Any parasitic oscillation that appears at the gates of Q1 and Q2 is
eliminated
by R4 and R5. (R6 and C3 form a snubber network to keep RF energy from
appearing at the drains of Q1 and Q2. A 10 OHM 3 watt resistor can be
hard to find and you can substitute 3 30 OHM 1 watt resistors in
parallel)
The step-up transformer, T1, is a standard TV flyback transformer to
which a
new primary winding is added during construction. The secondary winding
is part
of the original transformer. A snubber network consisting of R6 and C3
controls
any energy caused by the transformer's leakage inductance. Otherwise,
high-voltage spikes would quickly break down the transistors. The
center tap of
T1's primary is RF-grounded by C4 and C5, bypassing any high
frequencies that
appear at that point {{You will want to use a physically large or low
ESR capacitor for C4. Smaller ones will overheat. Or you can use a
higher voltage rating for C4 and/or a larger capacitance for C5}} The
high-voltage output of the power supply is the result of T1's secondary
coil resonating at around 50 to 70 kHz.
Construction. The Plasma Globe is one of those projects in which the
vast
majority of your time will be spent more in mechanical construction
than in
actual electronic assembly. If you do not have access to the necessary
tools to
make the base, individual components and a kit of all parts is
available from
the source given in the Parts List. While the parts themselves are not
exotic,
you might find yourself spending a lot of time and effort shopping at
many
different stores for the various items needed. Some of the more unusual
items
required include a TV flyback transformer, PVC drain-pipe parts for the
tank, and mineral oil {{baby oil}} to fill it. There is no PC board for
the Plasma Globe because many of the traces wouldn't be able to handle
the current. Instead, a 2-inch square piece of perforated construction
board and point-to-point wiring is used. The parts-placement diagram in
Fig. 2 is just one suggested layout design that you can follow if you
choose. Parts placement is not exactly critical, except for Q1 and Q2
if you either buy a pre-drilled base from the source given in the Parts
List or make one from the plans in this article. Holes in the base are
used to mount the transistors. That way, the base can be used as a
heatsink for
the transistors. Bolting the transistors to the base is also a simple
way to
mount the circuit board to the unit. If you're not using an aluminum
base, or
choose another method of mounting the circuit board to the base, you'll
need to
come up with some sort of heatsink arrangement for the transistors. If
you are
using one of those bases, make sure that the holes on the transistor
tabs line up with the holes drilled on the base before you solder
anything to the transistor leads. {{use care when handling MOSFET
transistors because they are static sensitive. Use an anti-static wrist
strap or touch a grounded surface and handle them by their tabs}}
Wire-wrap connections can be used for the connections that are not {{thick}} drawn in color - those connections must be made with 20-gauge wire. However, instead of using
wire wrap, it might be easier to simply bend the component leads over and
solder them to one another wherever they are supposed to interconnect. Be sure
to insulate any connections that cross.
Six 20-gauge wire leads must be soldered to the finished board. Figure
2 {{omitted}} indicates their positions and lengths. You'll trim them
down to fit when it comes time for the final wiring.
Flyback Transformer. Any black-and-white flyback transformer will do
for the
Plasma Globe {{use a FBT from a color TV for a DC voltage}}. The
easiest and cheapest way to get a transformer is to find one surplus
{{garbage pick}}. A specific part number is not
important-black-and-white flyback transformers are somewhat generic in
design. A suitable flyback is available from the source given in the
Parts List if you have difficulty finding one or are not interested in
buying a complete kit.
The modifications to the flyback transformer are detailed in Fig. 3.
{{omitted}} There is usually some sort of circuit board that has
several pins in it. Those pins are connected to the transformer's
primary windings. The primary windings are usually made from enameled
magnet wire. There will also be a heavier, insulated wire. That wire is
the ground connection of the high-voltage secondary winding. Since a
new primary will be wound onto the flyback, any connections to the
original primary can be discarded. However, the ground connection for
the secondary winding is needed, so it is important to identify that
wire first.
Unscrew the two nuts that hold the circuit board in place and remove the base.
Clip off the wires from the circuit board. Verify which wire is the secondary
return wire by measuring the resistance to the secondary high-voltage wire that
sticks out of the top of the transformer. There will be a low resistance
(several hundred Kohms) between the two wires. Clip off the wires from the old
primary as close to the body of the transformer as possible.
The metal bracket that holds the transformer core together is now
removed. The
core halves are brittle, so be careful when doing the following steps.
One end
of the bracket passes through the body of the transformer and the other
is more
or less free - a bit of glue holds it in place. Bend the glued side of
the
bracket away from the core halves so that you can twist it back and
forth. Wiggle it until it slides out of the transistor body; you might
even have to
"unscrew" it. Once the bracket is freed up enough, the bottom core
half should slide out of the core. The metal bracket and any spacers
that might
be located between the core halves are no longer needed, so they may be
discarded. Set the transformer winding and the core halves aside for
the moment
in a safe place.
The new primary will be wound onto a bobbin. The bobbin can be a piece of
rolled-up cardboard or plastic, plastic tubing, or any similar arrangement. The
bobbin should be about 1 1/4 inches in length with an outside diameter of 5/8
inch and an inside diameter of 1/2 inch. Take two lengths of 18-gauge magnet
wire and mark the ends of one wire "A" and "C." Mark the
ends of the other wire "B" and "D."
Holding the "A" side of the first wire and the "B" side of
the second wire together, parallel wind 15 turns onto the bobbin. That type of
winding is called a bifilar winding - the first wire (winding A-C in
[ILLUSTRATION FOR FIGURE 1 OMITTED]) will be loops 1, 3, 5, etc. and the second
wire (winding B-D) will be loops 2, 4, 6, etc. There will be 30 loops of wire
on the bobbin - 15 for each winding. Wrap the bobbin with electrical tape to
hold the windings in place {{this may be a two-person job}}. Leave about 5 inches
of wire for the leads. Scrape the enamel coating from the ends of the wires and
tin the ends. Connect wires "B" and "C" together.
New shims for the core halves are made from a non-conductive material that is
0.02-inches thick. A business card is usually about 0.012-inches thick, so two
layers of business-card stock placed between the core halves on each side of
the transformer should do the trick {{you could also use cutouts from the
anti-static bags the transistors came in, which are just the right thickness}}.
Check the shim thickness with a caliper if you can, as the thickness is
somewhat critical. An alternative is to purchase some sheet plastic of the
proper thickness from a hobby shop or craft store.
The transformer core halves, shims, primary winding, and secondary
winding are
reassembled as shown in Fig. 3. {{omitted}} A nylon wire tie can be
used to hold the modified transformer together. Do not use the original
metal bracket as it only helps to overheat the transformer in its
modified form. You can extend the
ground return and output leads later on if necessary.
Mounting Base. If you bought the complete kit, then you already have a
ready-to-use chassis. If you want to make your own, you can follow the
dimensions given in Fig. 4 {{omitted}} and make one out of sheet
aluminum. The cutout on top of the base can be made by punching 1-inch
holes spaced 1 3/4-inches center-to-center. Cut and file the remaining
material.
The following holes should be drilled to match the size of the hardware you're
using. Drill a hole for the chassis ground in the position indicated. That hole
should be a clearance diameter for a 6-32 machine screw. Drill holes for
potentiometer R3 and switch S1 on the front side and holes for the fuse holder
and power-wire bushing on the rear side. Holes for the mounting tabs of Q1 and
Q2 are drilled to the same center-to-center spacing as the transistors. They
should be centered side-to-side within the rear parcel and 1 inch from the top.
If you followed the parts layout in Fig. 2, the holes will be 1 1/2 inches
apart, The holes for Q1 and Q2 must be de-burred to prevent the transistors
from shorting through the mica insulators.
Mount a 3-inch flat-bottom PVC cap onto the top of the base. The cap should be
centered on the chassis-ground hole and the elongated slot in the top of the
base. Glue or double-sided tape can hold it in place for now. Using the
chassis-ground hole in the base as a guide, drill the same size hole through
the PVC cap. Using detail "C" in Fig. 5 as a guide, place a 6-32
screw with a solder lug on each side through the ground hole and PVC cap and
tighten in place with a nut. Drill three additional holes in the PVC cap for
the transformer-wire lugs. Those holes should be equally spaced within the
elongated slot in the base. Each hole receives a screw and pair of lugs in the
same way the chassis-mounting screw was installed. Bend the lugs on each side
of all four screws up at a right angle and seal the holes over the screw heads
and nuts on both sides with epoxy or hot-melt glue {{NO! Don't use hot glue! It
leaks like a mother at the least! Use RTV or something similar and make sure
that the screw threads are completely covered with it. I don't know about the
epoxy, use it at your own risk.}} Do not get glue {{See above - NO!}} on the parts
of the lugs where you will apply solder.
Main Assembly. Mount the circuit board to the underside of the chassis as shown
in Fig. 6. The mounting screws for Q1 and Q2 hold the board in place. Be sure
to use nylon screws {{if you can't find nylon screws, it is safe to use regular
metal ones, as long as they don't short, of course!}}, and put mica insulators
between the transistor tabs and the chassis before mounting. Place some
double-sided tape under the board to prevent any accidental shorts to the
chassis. Use Fig. 6 as a guide when making the final wire connections on the
underside of the chassis. Use 18-gauge wire for the power-input leads. Those
wires are indicated in color.
Turn the assembly upright and solder the transformer primary leads to
the top
lug as shown in Fig. 5. {{omitted}} The transformer ground lead is
connected to the chassis-ground lug. It can be lengthened with 18-gauge
wire if necessary.
Insulate the splice with heat-shrink tubing. Re-check all soldering and
wiring,
and check the transistor tabs with an ohmmeter to make sure that they
are not
shorted to the chassis {{should read infinity Megaohms}. Do not cement
the PVC
pipe to the PVC cap just yet, because once you do so, the transformer
can't be
accessed without a hacksaw if testing shows that there is a problem.
If you want to run it for more than just a few seconds at a time full tilt, the
transformer must be submerged in mineral oil {{baby oil}} both for its cooling and
insulating effects.
Testing and Final Assembly. Since the circuit draws 5 to 7 amps when
tuned to
the resonant frequency of T1, a 12- to 14-volt DC power supply with a
current
capacity of at least 7 amps is needed for the Plasma Globe. That's
quite a bit
of current, so an ordinary power supply just won't do. A car battery
will work,
but that's hardly convenient. You can build a power supply, but you'll
need a
very large transformer {{old stereo}}, an exotic high-current regulator
{{just use an appropriately sized full wave bridge rectifier}}, and
some beefy capacitors - an expensive proposition {{not really}} and a
fullblown project by itself.
It's much cheaper (more expensive) and easier to buy a ready-made power
supply. A fixed 12-volt, 7-amp bench-top supply is available from the
source given in the Parts List. Such a power supply can be useful on a
test bench for years to come, which might make such a purchase a wise
investment. Another alternative is a 10-amp car-battery charger, which
is also useful off the test bench. {{also, high quality regulated 12
volt, high current power supplies are available ready-made in most mail
order catalogs at decent prices. Be careful if you use a regulated
supply. The high frequency oscillations can overheat weaker
capacitors.}}
Turn potentiometer R3 fully counterclockwise and make sure that switch S1 is in
the off position. Connect one end of a test lead to the chassis ground and
place the other end about an inch from the bare-output wire of T1. It is very
important to keep your body away from the output. Any discharge sparks can
hurt, maim, or even kill {{an exaggeration - you can draw arcs to your fingers quite harmlessly. The heat will merely burn you. Use leather gloves or
metal scissors, etc...
I've recently read that the electricity does indeed go through deep
tissue, especially nerves. Frequently drawing arcs to yourself can
cause numbness and nerve damage.}} Connect the 12-volt power supply
with an ammeter in
series and turn on switch S1. The standby current should be about 1
amp. Slowly
turn R3 clockwise {{or counterclockwise depending on how you have it
wired. You
can tell if you keep blowing fuses. Watch for the fuse to bend and use
those
lightning fast reflexes to turn the power off and correct}} and note
that the
current rises to about 2 amps with some corona visible at the output.
At that
current level, the circuit can be used continuously without risk of
overheating
the transformer. Continue turning R3 clockwise and note a sharp jump in
current
to around 7 amps. The output terminal's high-voltage discharge will
come to life. Do not run it for more than a few seconds at that level
or the transformer will likely overheat.
If everything is working, you're ready to seal the PVC enclosure tube.
Slide
the tube down over the transformer into the base cap to make sure
everything
fits properly. If the output lead is too short to reach the top of the
tube,
lengthen it with 18-gauge wire and insulate the splice with heat-shrink
tubing.
Remove the PVC tube, apply PVC plumbing cement to the bottom of the
tube, and
slide it back into the base cap. In a few seconds, a liquid-tight seal
that
will never come apart {{easily}} will form. The tube can then be filled
with mineral oil {{baby oil}} to the top of the transformer.
Drill a small hole in the center of the top cap. Install a screw, solder lug,
and nut as shown in detail "B" in Fig. 5. {{omitted}} The arrangement is similar
to the connections at the bottom to the tube. The high-voltage wire from T1 is
soldered to the lug. The top cap does not have to be cemented onto the tube as
long as the Plasma Globe is always kept in an upright position.
At The Top. The plasma globe itself is simply a decorative 100-watt clear-globe
light bulb attached to the output. Those types of bulbs are usually found in
specialty lighting stores. Detail "A" in Fig. 5 {{omitted}} shows one way to
connect a light-bulb socket to the output. Use a socket that plugs right into
an AC outlet. Bend the prong that goes to the center contact of the bulb. The
other prong is cut off flush with the bottom of the socket. The center prong is
bent 90 degrees. Drill and tap a hole through the bent prong so that it can be
attached to the small screw in the top cap. Whenever possible, use clear
burned-out light bulbs - they work just as well as good ones. {{Not necessarily.
New ones give better display.}}
Many interesting experiments can be carried out with the Poor Man's Plasma
Globe. You can test different incandescent light bulbs for various effects, try
lighting up fluorescent tubes, or even connect a mini-Jacob's Ladder to the
output. If you try the Jacob's Ladder, don't expect much in the way of
dramatics. You need a lot more than 7 amps to climb a big ladder.
The one important thing to remember with any high-voltage experiment is safety!
Nikola Tesla would routinely adjust his high-voltage equipment with one hand in
his pocket in order to prevent any accidental shock from zapping his heart. His
death at the ripe old age of 86 is testimony to the fact that one can never be
too cautious around electricity - high-voltage or otherwise.
Warning! This article deals with and involves subject matter and the use of
materials and substances that may be hazardous to health and life. Do not
attempt to implement or use the information contained herein unless you are
experienced and skilled with respect to such subject matter, materials and
substances. Neither the publisher nor the author make any representations as to
the accuracy of the information contained herein and disclaim any liability for
damages or injuries, whether caused by inaccuracies of the information,
misinterpretations of the directions, misapplication of the information or
otherwise.
PARTS LIST FOR THE POOR MAN'S PLASMA GLOBE
RESISTORS
(All resistors are 1/4-watt, 5% units unless otherwise indicated.)
R1 - 10-ohms R2 - 1000-ohms R3 - 10,000-ohms, potentiometer, panel-mount
R4, R5 - 51-ohms R6 - 10-ohms, 3-watt, non-inductive (not wire-wound) {{three
30-ohm, 1-watt, in paralel can be substituted}}
CAPACITORS
Cl - 100-[[micro]farad], 25-WVDC, electrolytic C2 - 3300-pF(.0033-[[micro]farad]), 50-WVDC, polyester C3 - .01-[[micro]farad],
ceramic-disc C4 - - 2200-[[micro]farad], 25-WVDC {{low ESR}}, electrolytic C5 -
0.47-[[micro]farad], 250-WVDC, polyester
SEMICONDUCTORS
IC1 - SG3525A (ECG1701A) pulse-width modulator, integrated circuit (Silicon
General) Q1, Q2 - IRF540 MOSFET transistor
ADDITIONAL PARTS AND MATERIALS
S1 - SPST 7-amp switch T1 - Television flyback transformer (see text)
L1 - l-mH
choke coil Fl - 10-amp fuse Fuse holder, 6-32 x 3/8 nylon screws and
nuts, 6-32
x 1/2 inch screws and nuts, #6 solder lugs, mica insulators for Q1 and
Q2, wire
strain relief, perforated construction board, 20-gauge wire. 18-gauge
wire, 6
feet of 18-gauge magnet wire, metal chassis, 3-inch flat PVC end cap,
3-inch
PVC top cap (3-inch curved PVC or soft plastic), 6-inch length of
3-inch diameter PVC tube. mineral oil (baby oil containing mineral oil
may be substituted and is often less expensive) hardware, etc.
Note: The following items are available from Information Unlimited. P0 Box 716,
Amherst. NH 03031, Tel: 800-221-1705, 603-673-4730, Fax: 603-672-5406, web:
http://www.amazing1.com: Complete kit of all parts including flyback, PVC
parts, hardware, and pre-formed, pre-drilled aluminum base (TCL5-K), $49.50
(rip off city!); Flyback transformer (TCL/SUPPLY), $14.50; Modified flyback
(TCLFLY1O), $24.50; 12-volt, 7-amp DC power supply. $39.50; Complete kit plus
power supply (COMBOTCL), $79.50. Please add $5 shipping and handling on orders
up to $25, $7.50 on orders up to $50, and $10 on orders up to $100. NH
residents must add appropriate sales tax.
Mag.Coll.: 90D4528.
Article A19909422