A compact, high performance, Double-Resonant
Solid State Tesla Coil
13 x 3.5in Secondary, AWG34, 5 foot (4.6x) Sparks, 120kHz
been a lot of hard work and a journey of failures, learning, and
success. After several months of work, I will like to present my second Double
Resonant Solid State Tesla Coil (DRSSTC 2). This project was born from a
natural progression of my foray into Solid State Tesla Coils, beginning
with my first SSTC1 in 2012. After that project, I built
my first DR Tesla Coil variant, which was a small table-top
DRSSTC 1 standing just about a foot tall in total, and capable of
some two feet of sparks. With the development of a MIDI controller, I
was also able to modulate the spark output and play music!
After these two projects, I wanted to work on a much more powerful
DRSSTC running as much power as I could, all within a small footprint. Thus
DRSSTC 2 was created.
The DRSSTC was invented sometime around 2005, and
differs from a traditional SSTC with the addition of a primary tank
capacitor which cancels the impedance presented by the primary coil. The primary current flow is therefore much higher, placing
greater stress on the switching transistors, but also allowing for
greater performance. Regardless, this new breed
of SSTCs still represents perhaps the most efficient of all Tesla Coils
DRSSTC 1 used a half-bridge of TO-247 package IGBTs, running no greater than
300A peak, with an average power draw of around 200W+ depending on the pulse width and frequency. I happened to have some large CM200/400 IGBTs
around, and decided to take my DRSSTCs one step further, running at some 2000VA at 500A of primary current.
This page documents the design, construction, testing
and results of DRSSTC 2 - my highest performing and most impressive
Tesla Coil to date. This project was successfully completed in mid 2013
after about half a year of development, and I hope it will be a valuable
resource for many coilers around the world. [Update: I've had many
people from all around the world report replicating this coil with equal
success! If this page was useful in your project, I'll be happy to share
your project on this page as well! ] Enjoy and be safe!
*note* I do not know of any other DRSSTCs built in
Singapore by any fellow friends in Singapore. DRSSTC 2 may very will be
the first of it's kind in my little sunny country! Thank you for
visiting my page and if you have any questions, wish to share your
projects, or feel that my projects have inspired you in one way or
another, feel free to email me at loneoceans[at]gmail(dot)com.
- CM200DU-24F 1200V 200A Half Bridge IGBT, RTC
disabled, ~450Apk current limit
- 2kV 1uF Aerovox Snubber
- 1875uF 350VDC Bus Capacitor x 2 in series (voltage doubler)
- 10k 7W balancing resistor across primary/MMC at IGBT side
- 0.154uF (measured) 6kVDC MMC made of nine 150nF 2kV CDE 942C capacitors
- Modified UD2.1 driver with Primary Feedback and Phase Lead for ZCS
- Flat Spiral Primary, 0.25" Copper Tubing, 9 turns tapped at 8.6 for
106kHz on the primary
- 13.07 x 3.5" Secondary, AWG 34, ~1855 turns
- 4" x 17" Toroid, Aluminium Ducting (later 4.2 x 19")
- About 120kHz secondary resonant frequency
- 10-150us pulse-width time, controlled via optical input
- Designed for 240VAC input max for 678VDC on the bus (half bridge)
- *A beautiful 4.5" x 20" spun aluminum toroid was purchased for
this coil in late 2013*
Video of DRSSTC 2 playing Dance of the Sugar Plum Fairy (~180VAC input)
For much more videos and images of the coil in action, please scroll down to
Having built my first DRSSTC 1
a few months ago, I decided to embark on a much more powerful and more
impressive, yet still compact DRSSTC. I assume that the reader of this
page has some
familiarity with the general workings of a normal SSTC, and therefore
won't go into much detail here.
Check out my other pages for more descriptions on DRSSTC
General Theory of Operation of a DRSSTC
A DRSSTC differs from a standard SSTC mostly due to an
addition of a primary tank capacitor. In a simplified explanation, the primary and secondary
circuits are in tune - the capacitance due to the tank cap cancels the
inductance from the primary coil, so we have no reactive component in the primary Tesla Coil
circuit. Therefore, the primary current flow is limited only by the DC
resistance of the primary coil, cabling and capacitors, and is therefore much
higher than in SSTCs. My DRSSTC 1 runs on the order of 200-300Apk. In
this design, I plan to run it at 500Apk with some 'brick-type' IGBTs.
addition, the primary current of a conventional SSTC is limited by
streamer loading and primary reactance, allowing many variants to run in
the Continuous Wave (CW) state. This is not possible for DRSSTCs, due to
the huge current ring-up which would otherwise cause catastrophic
failure in the switching transistors. Furthermore, the large currents
flowing in the primary circuit can create voltages much higher than the
supply voltage due to resonant voltage rise. Therefore, DRSSTCs run in
the transient state, on the order of several tens to around three
hundred micro-seconds per pulse, at several hundred pulses per second.
Like my DRSSTC 1, this coil will also allow an optical
interrupter input to control the pulse-width and frequency of the
sparks. The interrupter sends a one bit signal (on or off), telling the
bridge drive circuit to turn on for short periods of time (this is the
pulse width). The coil is then shut-down for several milliseconds. This
results in a reasonably low duty cycle, with a total power not exceeding
3kVA (designed). However, the instantaneous power
of the coil during operation can be on the order of several hundred
In summary, the DRSSTC is actually a 'pulsed' device,
operating at several hundred or thousand Hz (pulse repetition frequency
or PRF), with each pulse lasting around 50 to 300us (pulse width), and
closer to 50-100us for DRSSTC 2. Here
is a basic description of what happens each pulse.
The goal for the primary drive: make a huge
current sinusoid in the primary circuit. When the interrupter sends an
on-signal to the driver circuit, the driver turns on the bridge (half or
full bridge), which places the primary bus voltage across the primary
coil and tank capacitor, like a 'step-input'. This causes the primary
current to rise in a sinusoidal fashion at the primary resonant
frequency. This current rises to a level determined by the surge
impedance of the circuit, where impedance Z = Sqrt(L/C), and V = IZ. As
the primary current goes down on the sinusoid and starts to flow the
other way, we sense this with a primary current sensor, and turn the
bridge the other way to add more energy into the tank circuit - a
resonant drive. This is
like pushing a swing at the resonant frequency.
In DRSSTC 2, a small current transformer senses the
primary current, and allows the drive circuit to determine the polarity.
Using this information, the drive can then switch the bridge
appropriately. Of course, it is important to know the correct polarity
of the bridge, otherwise the bridge will work to oppose the resonance.
When all this works well, we have more and more energy added to the
primary, and the current gets bigger and bigger!
As this happens, the primary coil generates a strong
oscillating magnetic field around the secondary coil. Through induction,
energy is transferred to the secondary coil via magnetic field coupling.
For maximum power transfer, we try to make sure that these two resonant
frequencies match. Eventually, the secondary accumulates enough energy
to charge its capacitor (the toroid), to a high voltage. When the
voltage gets high enough, an electrical streamer/arc forms. When this
happens the Q of the circuit (ability to store energy) drops, as energy
is dissipated into the streamer. This is exactly want we want, since it
makes the streamer longer and brighter. In practice, the streamer length
grows for the next 2 to 4 RF cycles, and does not seem to get any
longer. At this point, we have successfully obtained our goal of making
a big nice spark, and we can turn off the circuit.
Ideally, all the energy left in the primary should be
transferred to the secondary - this can be usually observed in the
primary current which drops drastically when a large streamer forms.
However, if there is any energy left, the anti-parallel diodes in the
IGBT recycles the leftover energy into the main bus capacitors as the
current rings down.
Design Considerations & Schematics
These were my design guidelines for the project:
- Be reasonably portable, but impressive output
- Made well with high quality machining
- Solid enough to be transportable (in an airplane luggage bag, or
through the post without crumbling apart for
- 4 feet of spark output, 5 feet if possible
- Run off 240VAC max input voltage
- Be reliable and robust
- Compatible with existing interrupter from DRSSTC 1 (hence, also music capable)
- Be a great learning experience as my first "brick-coil"
In DRSSTC 1, I used TO-247 packaged 60N65 IGBTs, which
were nice, but pushing any more performance out of 60A IGBTs would be
challenging. The workplace I was building my coil in, MITERS, had a
bunch of Powerex / Mitsubishi CM200 transistors lying around. These are very nice
'brick' transistors (so named for their large shape and size), good for 200A continuous, 400A peak
rated, and come
in a half bridge configuration per module. My original plan was to build
a full-bridge of Toshiba 150A 600V half-brick modules, but one of them turned
out to be damaged (oh well... ebay!), so the project took an abrupt
change and was modified to run a half-bridge, but at twice the original
bus voltage via a voltage doubler.
Typical CM200 'brick' IGBT - from Powerx/Mitsubishi Datasheet
The goal was to design the coil to run off
120 to 240VAC,
with a maximum bus voltage of 678VDC, safely within the margins of the
1200V IGBT. However, it is important to give the IGBTs a large safety
margin, due to stray inductance which can cause severe voltage spikes
Therefore, I designed the coil to have a low a primary bus inductance by
keeping the bus capacitors close to the IGBTs and added a nice snubber
capacitor specifically designed for this purpose.
The goal for this project was to make a very impressive
coil; i.e. a coil with huge output for its size. Also, I wanted a coil
that was easily transportable. Therefore, I designed the coil to have a
footprint of just about a square foot, with a secondary length of
around 13 inches. The output goal would be to be at least 4 feet, and if
possible 5 feet, which would outperform my large
ARSG Tesla Coil 2.
[May 2013 - The coil has performed beyond all
expectations and makes 5-foot sparks!]
[Mar 2014 - Tweaks to my Tesla Coil 2
has now seen it making in excess of 6 foot sparks, but this is again
eclipsed by my newer DRSSTC 3 built in early
Because I was using relatively large 'brick' IGBTs which
do not respond well to fast switching, I decided to design the coil with
a sub-120kHz frequency goal (turned out to be around 108kHz). In
addition, I decided to use a modified driver originally designed by
Steve Ward, which uses Phase Lead for ZCS and had over current detection
(OCD) built in. Phase Lead works by adding a delay to the switching
(tuned by a variable inductor), which switches the bridge slightly
before the current inverts, from the second cycle onward. This needs to
be tuned by hooking up the primary to an oscilloscope. OCD measure the
voltage from the current transformer (across a resistor), and shuts off
the driver when it crosses a threshold voltage via a comparator. I
decided to set the OCD to 500Apk, which should hopefully give me
reliable performance from my IGBTs (which are known to be able to handle
2-3x their current rating in Tesla Coil use). OCD also protects the
transistors in the event of strong ground-strikes (increases current
flow greatly). I later replaced this with a UD2.7.
Components and Construction
Having made many secondary coils now, the construction of this
one was relatively straightforward. I used a standard 3" PVC
pipe (actual outer diameter of 3.5") cut to about 14 inches in length, and machined two
caps for it on the lathe. These were carefully threaded to
accept 3 nylon screws each to hold it in place. The middles were
then threaded with 10-24 threads to allow easy connection to the
toroid and box.
About two thousand turns of red AWG 34 enameled copper wire (from ebay) was wound on the
coil. This took a surprisingly long time of almost 4 to 5 hours
due to the extremely thin AWG34 wire. Ideally I would wind a
larger secondary coil with thicker wire, but the idea for DRSSTC
2 was to make a compact coil, whilst keeping the
secondary frequency as low as possible. The ends were secured
with black vinyl tape. The coil was then carefully covered with
four thin layers of Polyurethane varnish, allowing each thin
coat to dry before the next was applied for a beautiful finish.
Originally I had calculated about 2050 turns would fit in the
13" long secondary coil and this would achieve my 100kHz
operating frequency with a 4 x 18" toroid. But In real life,
because AWG34 is so thin (6.3mil thick, or 6.3 thousandths of an
inch), and based on my experience that insulation thickness is
about 0.4mil each side, I would more realistically be getting
1855 turns or so, dropping the frequency. A catch here for
Enclosure and Box
I knew I wanted the coil to be housed in an elegant, beautiful
yet strong and easily constructed box. For this, I turned to
technology and employed the help of a CNC Laser Cutter to get my box
components precisely cut in clear Acrylic.
The pieces were quickly designed on paper, sketched out in Adobe
Illustrator and sent to print on the laser printer to cut in clear and
smoke-black acrylic. The sides are held together via 80/20 aluminium
square stock, resulting in a very nice, strong box. In addition,
the primary supports were laser-cut and allowed the 1/4" copper
tubing I was using to be simply press-fitted in. This resulted
in a product that not only took a fraction of the time build and
assemble than my previous methods, but also looks beautiful. Hooray for technology and CNC tools!
In retrospect while everything fit super well in the end in the
1 x 1 square foot footprint, I
would probably have done better with a slightly more generously
sized box to make my life easier :-).
For the strike ring, I used a loop of 3/8" copper tubing which
was attached around the primary. The break in the loop prevents
eddy currents from being induced causing unwanted losses (afterall the primary coil is basically a big induction heater when in
operation!) The ends are terminated by
spherical brass knobs soldered into place, just to look fancy.
Also note the grounding plane at the middle of the primary coil.
Slots were later cut into the copper to reduce eddy-current
heating during operation - the plate was getting hot enough to
start melting both the acrylic base and the PVC coil even during
The interrupter as described above sends a 1 bit signal to the
Tesla coil driver. Basically, it turns an LED on or off, and
sends this via fiber optic to the driver.
I developed the interrupter based on Daniel's and Bayley's
open-source MIDI controller. I also added an additional output
which allows the use of both 1mm plastic fiber output or
standard ST Multimode fiber output. The interrupter is simply
based on an Atmel 328p Microprocessor and allows adjustable
frequency, pulse width from 0 to 135us and also accepts a
standard MIDI input with up to two notes of polyphony. All this
is powered via a 9V battery and housed inside an aluminium box
for shielding. This is also the interrupter that runs my
DRSSTC 1. This was later updated with a
completely new MIDI controller of my design (late 2013)
featuring more control and multiple coil capability.
Power Bridge (Full Bridge, which became a Half Bridge)
My original design was centered around two IGBT half-brick
modules I bought on Ebay. These were the Toshiba MG150J2YS50
IGBTs, good for 150A 600V each, and I had planned to use them in
a full-bridge configuration with a single 2300uF 450V capacitor.
These were assembled on a one-piece heat sink with an integrated
bridge rectifier, copper bus-bars and wiring. This turned out to
be very elegant and beautiful, and fit perfectly into my box. In
fact, initial tests of the bridge at 80V seemed to work
half the time, but
produced very unusual results otherwise. I later found out that one of the
IGBTs in the half-bridge wasn't working properly and was in
fact, half dead! Therefore I
had to scrap the bridge and build something else. This nice
looking bridge is currently awaiting repairs with a new set of
Thus, after several hard days of work, I present Brick-Bridge
Half-Bridge which drives the Tesla Coil.
This bridge uses a single 200A 1200V half-bridge
module, with a voltage doubler from the two 1875uF 350V
capacitors. Note the integrator rectifier, snubber capacitor,
onboard Gate-Drive Transformer as well as 1.5KE33 TVS diodes on
the gates and drive resistors. A 10kR 7W bleeder resistor is
added across the output of the inverter to bleed residual MMC
voltage; and a 68nF 3kV capacitor is wired (incorrectly in this
photo) across ground and the negative bus rail for primary
strike protection. The small PCB that holds the TVS and diodes
was later removed for more clearance in the crowded box. Some
parts of this bridge were slightly tweaked (see photos below).
This CM200 IGBT comes with an integrated RTC
module, with limits the current and turns off the IGBT when it
exceeds 400A. In order to make the IGBT suitable for DRSSTC use,
I opened up the IGBT and carefully cut the wires to the RTC,
effectively disabling it. You can see my
DRSSTC 3 page where I
describe this in detail.
27 - 29 April 2013
With the discovery of the damaged 150A Toshiba
IGBTs, I quickly built a new bridge. The modular design allowed
me to quickly get the coil up and running again. Using an
oscilloscope, I was able to adjust the phase-lead for perfect
compensation, and made sure the primary current and bridge
voltage was what I was expecting.
Performance of the
coil before tuning, at 120VAC in (122kHz primary).
Performance of the
coil at 120VAC in on 29th April 2013 (38 inches) after tuning with an
oscilloscope with simulated streamer loading.
It was time for the first full power test at
120VAC input (340V on the bus). Initial runs on 27th April
proved to be somewhat disappointing. I suspected that tuning
might be a problem, so I went back to the lab and tuned my
primary to be in resonance with the secondary (with a 4-foot
wire attached to simulate streamer loading). This brought my
resonant frequency to around 108kHz. Adjusting the primary tap
by half a turn made a huge difference, with the coil achieving a
38" output with 120VAC in! The coil performed
spectacularly but it really wants 240V in. Notice that I removed
the small toroid because it didn't seem to make any difference
in the resonant frequency of the secondary.
Just for recording purposes, here are my measurements I
took when tuning the coil in this configuration. Secondary frequency
with the 17 x 4" Toroid measured to be 122kHz. Attaching a 4 foot wire
to it dropped the frequency to 106kHz. The primary coil at turn 7.9
measured out to be 112khz and at turn 8.8 was 101kHz. In the first photo
above (before turning), I was running at turn ~8, giving me 112khz, As
you can see, I had nice sparks but they were in the ballpark of around 2
to 2.5 feet. I tried at turn 7, making the resonant frequency more like
120kHz, this time with poor results, about 2 feet. Then I tried turn 9,
again with poor sparks around 2 to 2.5 feet. Then I tried turn ~8.6 and
this time the results were spectacular, and I could really see an
increase in spark length. At this turn, the primary frequency should be
around 106kHz, which corresponds well with the secondary frequency with
a 4 foot wire attached. The runs were conducted at 140VAC in, and OCD at
450A was not tripped. I also found that a pulse width of ~135us produced
12 May 2013
This coil was really designed to be run at
higher voltages such as 240VAC. I was unable to get a powerful
enough step-up transformer, so I did a quick test running this
coil off 2 poles of 208VAC (from 3 phase). This time, the coil
made full length sparks at 200bps, ~100us and 500Apk.
The results are
spectacular, with arc lengths of around 5 feet, which
is very impressive for a coil this size! :) At this point, the
coil has already exceeded all my expectations, and I'm glad that
physics, math, simulations and good construction had paid off.
Even with modest pulse-widths of around 100us,
the lab I was running was just at the very limit of containing
the spectacular output of the Tesla Coil. OCD ws set at 525A. Much more results to
come after I obtain a suitable step-up transformer.
Video of DRSSTC 2 in a short test run, 208VAC in.
Right now, the goal is to finalize the Tesla
Coil by improving wiring and component positioning (stuff is
very messy inside after shifting to a different bridge design),
improving grounding, and perhaps adding some lights and a new
spun toroid to the Tesla Coil. I hope the end product will be a
powerful, reliable Tesla Coil which I will be able to use for
future demos. Definitely more to come in the coming months
09 Nov 2013
I managed to finally find a 240V step-up
transformer to run the coil! The results are very impressive and
the coil is basically limited by its secondary height, with
sparks reaching 5 feet making it very difficult to contain
secondary to ground-ring strikes!
On 9th November I set it up for a test run with
the new 2kVA step-up variac. The maximum input voltage was set
just around 200VAC and pulse-width set to just over 100us. I
found that with a single toroid, a lot of sparks tended to hit
the ground strike ring. Hence I ran the coil with a second
smaller toroid below just to lift up the big toroid higher from
This photo shows DRSSTC
2 making sparks happily - check out the nice
banjo effect on the right side. I actually ran into an
interesting phenomenon with the driver locking up forcing the
coil to go semi-CW. This was due to the fact that I had got
ahead of myself and made a
grounding cable (which is connected to the grounded box the
driver was in), which was initially connected to a large metal
counterpoise on the floor (but not really grounded!). The OCD kicked in and
prevented the coil from blowing up, but it was an interesting
observation. My guess is that the large counterpoise acted as a
large antenna forcing the 'ground' to have significant induced
The results are spectacular and the coil runs
great! Here DRSSTC 2 makes what seems like a 53" (134cm) monster spark
to air. This is 4.077x the secondary length!
Finally, I tried to set up the coil to make long
sparks for a spark record. This photo shows the coil making 48"
sparks to the chair, but the maximum recorded was 51" to ground
with 525A OCD.
Anything more than that becomes a bit tricky due to the sparks
wanting to hit the floor instead. The next step is to put a breakout
point at the top of the toroid and try to make it hit a ground
plane suspended from the ceiling, as well as using a beefier
power supply (I suspect the 2kVA transformer was maxing out
limiting power to the coil), as well as running at a higher
I had been working on other projects for a while and
DRSSTC 2 did not see much action in 2014. However, having moved to a new
location, I wanted to try to get the coil back up and running (and to
see if it survived a cross country trip in a post box). There
were a few small refinements I wanted to do for the coil but nothing
major. First, the original junky-looking toroid was poorly made and had sharp
points all over, so I wanted to make a new toroid and slightly
increasing the size to further reduce the resonant frequency. In
addition, the coil previously used a single 25.2V transformer for the fan and
logic power and a modified version of the UD2 driver. This caused some
thermal issues with the driver getting very warm, as well as the fact that it was limited to 120V operation
on the logic-side. So I took the opportunity to do some small changes to
New 19 x 4" Ducting Toroid
I made a new toroid using the proven aluminium-ducting
method I used in my DRSSTC 3.
Instead of buying just a spare piece of ducting, I spent
a few dollars more and got the one with proper aluminium end sockets.
This was a 8' x 4" section from Lowes. I wrestled it carefully into a
donut shape, and then used a self-tapping screw to secure the ends. The
joint was then sealed with aluminium tape. To ensure a good connection
to the top of the secondary coil, I also connected some wires to the
toroid using some ring terminal. The torus is held in place between two
11" acrylic plates separated by about an inch or so, and held in place
by the compressive tension of the torus! I found it easiest to assemble
the middle plate first before carefully squeezing it through the torus.
Finally, a breakout point was added. Much nicer than my previous topload,
I think. :-)
With the new toroid, the new secondary resonant
frequency dropped slightly from 122 to 117.4kHz. Attaching a 2.5 foot
simulated wire dropped it to 109.6kHz, and a 4 foot wire to 103.5kHz
(down from 106kHz). The simulated optimal tuning point is at turn 7.6
(116kHz), though a ~10% de-tuning to the existing turn 8.6 would
probably yield good results.
New Driver and Logic Power
As mentioned, I wanted to swap out the clunky 25.2VAC
transformer powering the fan and the driver. This was causing problems
because the transformer had an output voltage closer to 28VAC or so. The
previous UD2 I used also ran very hot (regulators have to drop the
voltage down from 39VDC) so I replaced the
entire driver with my UD2.7 rev A driver board. I
mounted this over a cheap $8 China 24V 2A power supply I got from eBay, which
accepts 110 - 240VAC input. I also didn't want to run the 24V Nidec Beta
V fan at full power (too noisy), so I added a 25W 33R resistor in series
to the fan. The entire setup is much tidier and now runs much cooler, so
hopefully this will bode well for reliability. The OCD was set at 450A.
Same CM200 Half Bridge
Not much was changed from the original CM200 half
You might note some small differences from the original
photograph, but otherwise nothing has changed. Once the few small
changes were done, the parts were then assembled back into its small
case and everything fits well. One new feature was that I added
an ST-ST fiber optic coupler on the front panel of the box, so the input
ST fiber can be connected directly at the front of the box. This makes
setup much tidier as well since everything is integrated.
Here are the waveforms of the primary current (yellow)
and bridge output (cyan), showing perfect switching and very little
switching spikes :). I tuned this up to about 105A or so. Good to go!
So did the coil work in the end?
Yes it does and continues to run as spectacularly as
before! I found it to be very reliable running at about 180 - 200VAC
input producing 3 to 4 feet of spark at sub 100us pulse widths very
reliably. The coil was displayed at a science fair and ran dozens of
times with its musical display of lighting from a 208V power supply with
voltage adjusted via a 5kVA variac.
I would like to thank the following people, for this coil would not have been completed without them.
The great people at the 4hv forums for giving me great advice and suggestions
Philip and Steve for helping me out with debugging
and giving suggestions on the coil
Inspiration from Philip's DRSSTC V (f0 at 135kHz,
800A OCD, 150-200us, 150-300bps, 400V full bridge)
Friends who helped me get access to the laser-cutter
in school to get some parts laser-cut
All the great tesla coil websites on the internet which gave me valuable advice and construction details
And everyone else who has helped me in one way or another.
More to come soon!
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(c) Gao Guangyan 2022
Contact: loneoceans [at] gmail [dot] com
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Rights Reserved. Design 3.
Removal of any material from this site without permission is strictly
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Disclaimer: Projects and experiments listed here are dangerous and should
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