Build this silent, solid state high-voltage coil to amuse yourself and those around you, but don't leave it next to your computer.
BUILD TIME: 2 HOURS
DIFFICULTY RATING: Intermediate
The devices most commonly known as Tesla coils are high-voltage transformers of a curious design where one end of the secondary (larger) coil is apparently unconnected and is in fact capacitively coupled through air to ground, completing the circuit when the voltage is high enough to ionise air and begin conducting that way. The result is spectacular corona discharges, high-voltage arcs, and the familiar smell of Ozone.
Our mini build is far from powerful enough for any of that but will still yield plenty of fun, particularly from the novelty value of its small size and solid state nature. Traditional Tesla coils are big, heavy, and noisy. This one is battery powered, and the batteries are the heaviest part of the whole system. Besides that, you'll need a resistor, transistor, LED, and some wire.
The circuit is based on the Slayer Exciter circuit, commonly attributed to Dr. Stiffler and GBluer, members of Internet-based electronics communities. The Slayer Exciter has since grown to have its own community with modifications, improvements, variations, and up- and down-scaling all common. It is one of the most accessible high-voltage coil designs around in terms of materials, skills, and difficulty. While it is often referred to in common language as a Tesla coil, it is not.
The Tesla coil is a specific design of a high-voltage air-cored transformer invented by its namesake, Nikola Tesla. It is a form of resonant transformer which produces very low current outputs running at very high frequencies. Generally, Tesla coils use capacitors and spark gaps to achieve their oscillation. While solid state drivers are increasingly becoming accepted as Tesla coils, the much simpler Slayer Exciter is really only ever called a Tesla coil out of convenience.
One of the key features of the Slayer Exciter is that it is self-resonating. Real Tesla coils have to be tuned and are somewhat difficult for the inexperienced to build effectively. The resonance of the coils must be known and matched to the spark gap or solid state driver constructed at just that frequency. This is no easy feat for many people, and in some cases, getting the maths a little wrong doesn't just translate to reduced or less spectacular results, but rather, no results. The Slayer Exciter and its derivatives overcome this by being designed in such a way that the rising and falling voltage in the secondary coil controls the driver circuit, causing the circuit to resonate at its own natural frequency.
Another key feature of the Slayer Exciter is that it can be made very small, and powered by batteries. This is important to us, as it eliminates the intermediate transformer used in many Tesla coil designs. In those cases, a transformer takes mains power up to several kilovolts before being sent to the spark gap and Primary/secondary coils of the Tesla coil proper. No matter which way you go about that, it's dangerous for the untrained and it is illegal in Australia, unless you're a qualified electrician, to make any mains connection that involves anything more than just plugging something into a power point.
That includes things you build yourself and put a plug on. There is a commonly held and long-standing belief that anything that plugs into a mains power point is an appliance and not fixed wiring, and so is not illegal for an unlicensed person to be involved in. Over the last couple of years, the relevant authorities have asserted publicly that this is a myth. So, if it's mains, don't touch it. Battery power is the way to go no matter what type of high voltage coil you're building. You could, of course, use a step-down power supply, but these circuits can produce goings-on on the supply line that aren't healthy for plugpacks.
When designing your Slayer Exciter coil, some care must be taken in certain aspects. We chose to power our design with eight AA batteries, rather than the 9V battery common in Internet designs. This is because 9V batteries don't last that long, and even the extra 3V helped the design a little. Some designs use two 9V batteries in series but again, storage capacity was not to our liking. There is nothing stopping you from exploring that avenue.
The greatest consideration is transistor choice. The transistors used in Slayer Exciters invariably get hot. The temptation to use a bigger transistor is natural, but with a primary coil winding which is virtually a short circuit, this doesn't actually achieve much. You won't get to the point where the load (the primary coil) draws less than the transistor can handle. While MOSFET designs are around, we wanted to stay relatively true to the original Slayer Exciter design. The key instead is to use the resistor on the base of the transistor to make sure the collector-emitter connection is not fully saturated and thus is current-limited, but this still produces quite a lot of heat. We tried a BC337 initially, with its 800mA continuous current, but it got too hot. We even short circuited one in a moment of clumsiness and delaminated it, which some readers may have seen recently posted to social media.
Some designs use a BC547, so you can make these work with small transistors if other design aspects match. We don't know how long those designs lasted though. Do not be tempted to just parallel transistors. Because no two devices are identical, one will have a higher gain or lower internal resistance, however slight, than any other, and take more of the load. It will therefore heat more and fail earlier, beginning a domino effect. In the end, we chose a 2N2222A for our baby build. Even though it has the same dissipation and resistance figures as the BC337, it didn't heat up as much.
The other main consideration is size. Our initial concept prototype was wound around the plastic barrel of a pen. We have since graduated to wooden dowel and plastic tube. You can build a Slayer Exciter at almost any scale, but they are one of the least efficient designs of high-voltage coil, and that on its own is a relative concept because all Tesla coil designs and related derivatives are inefficient. The design suits small builds, so probably keep it desk-top sized.
The secondary coil is air cored, so choose materials that do not conduct either electricity or magnetically. It also needs to be wound with small wire. 0.25mm enamelled copper wire (ECW) was our choice, which is the finest available over the counter. It is also about the finest it can be without breaking easily. If you want finer, you could dismantle a relay or some such with a coil of finer wire. Some designs form on a ferrite rod, but discussions among the community discourage this. The concern is that the ferrite will alter the magnetic flux but the density required to saturate the core, in this case, is far higher than the current available unless you exceed a certain point which will differ between designs.
One of ours, with twenty primary turns and a decent current, came close, but we didn't keep this design for other reasons. That said, our baby build worked better with a ferrite core than when we tried it without. The hypothesise we have is that with such fine wire, the ferrite was actually aiding the magnetic flux of the primary rather than affecting the secondary. We don't know for sure, and haven't sat down to examine the theory and maths enough to work it out.
The primary coil may be between two and around twenty-five turns, depending on the dynamics of your secondary. Most are between four and twelve. Whatever you end up with, it needs to be fairly big wire. We explored with 0.5mm ECW in initial designs but ended up using 1.25mm ECW for our final designs. If you have thicker wire, use it. This is not because of the current being carried, but because at high frequencies electrons tend to move at the surface of a conductor. Solid wire is better than stranded wire but is not always available.
Finally, many designs online use an LED as a diode in the circuit. Our prototypes used a regular 1N4148 instead. The diode's primary function here is as a diode, not as a light emitter (though it will emit light). LED PN junctions are famously weak when reversed biased, and cheaper ones are known to fail when run on AC. While they're not recommended as rectifiers, LEDs don't really get stressed in this circuit, and so we eventually reverted to an LED to give a visual indicator of a functioning circuit. The choice is up to you, and in fact some designs can be found with no diode at all.
WHAT DO YOU DO WITH A SLAYER EXCITER?
Honestly, nothing practical. They're just for fun. The concepts Nikola Tesla was exploring were intended for commercial purposes, but none came to fruition. The Tesla Coil as it is known today was part of those experiments, and has continued to gain interest mainly for its curiosity value and the spectacular displays possible. The Slayer Exciter is nowhere near a lightning machine in its output, but some can create small plasma streams and photographable corona discharges.
The high-voltage charge at the end of the coil does radiate, and this is what lights up neon and fluorescent lights brought close to it. The principle behind the Slayer Exciter is similar to that used in ozone generators as well, which are used for sanitising and also as negative ion generators which de-charge dust particles, causing them to fall out of the air. While modern ozone and negative ion generators are built a little differently and more efficiently, they're not too far removed.
Most of all, however, your Slayer is for playing with. Bigger versions can create sparks or corona streamers that jump to metal tools held near them. During our development of these builds, we explored a little photographic light painting by moving coloured neon globes around the coil during a long exposure in dark conditions. You can attach a flat plate top load to bigger builds and make a plasma motor from an appropriately bent piece of wire. Search on Youtube for 'what to do with your Slayer Exciter' and you'll find some inspiration.
The most fun you'll have with your Slayer Exciter, however, is if you get addicted. You'll want bigger and better, or want to get the best performance out of a smaller one. You'll modify, tune primaries, increase power supplies and find ways to cool transistors. So, brace yourself, because you may end up investigating a lot of time in this!
Nikola Tesla: Genius Or Nutcase?
Nikola Tesla, a Serbian-born American engineer responsible for the high-voltage coil that bears his name, was a pioneer in the field of electricity. Whether his ideas turned out to be correct or not, he still made significant and valuable contributions to science. Even ideas that turn out not to be correct add to the overall body of knowledge, and sometimes indicate directions that lead to successes.
Tesla worked closely with Westinghouse to develop the concepts of Alternating Current transmission, which most of the world uses for power transmission today. Many of the ideas he had about radio turned out to be incorrect, but the knowledge gained from his experiments and his examination, albeit incorrectly critical, of Marconi's work yielded plenty of knowledge. His full bio is well worth a read, but bear in mind sites like Wikipedia are not peer-reviewed.
We explored different sizes of Slayer Exciter, so we decided to present two of them for you to compare. One is as small as we could make it and is a little fiddly. It did end up our favourite, however. The other is a larger, but still palm-sized, build which will be easier in terms of physically manipulating and handling the materials involved. We started with our larger builds during prototyping for this reason, and we'll present that first.
Having said that, the bigger builds were less impressive. The performance of the smaller build actually outshone, literally, the bigger ones. While the bigger build did give slightly brighter results in the 200mm fluorescent tube we used to test it, and the CFL, the smaller build was able to achieve find corona discharges from the top. This quickly filled the area with that high-voltage smell of ozone.
WARNING: Your transistor will get hot. Monitor it and be prepared to only operate the device for short times.
|tools & materials
|Hot Melt Glue
|File or Sandpaper
|Double Sided Tape
|Dowel or Plastic Tube for coil forming
tools & materials
|COMMON Parts Required:
|1 x 24kΩ Resistor *
|1 x LED
|1.25mm Enamelled Copper Wire
|1 x Strip Prototyping Board
|1 x 8xAA Battery Pack
|S5034 + P0455
|8 x AA Batteries *
|1m Hookup Wire
|BiG Brother BUILD:
|1 x BD139 Transistor
|1 x Self-Adhesive Heatsink *
|0.25mm Enamelled Copper Wire
|7 x PCB Pins *
|1 X 3-Way Terminal Block
|BABY STEPS BUILD:
|1 x 2N2222A NPN Transistor
|Relay to dismantle for fine ECW
|5 x PCB Pins *
|12 x Ferrite Beads *
* Quantity shown, may be sold in packs.
Designs for the secondary coil range from 200 to 400 turns in general, although there are no direct limits. We built test coils with 200, 300, 400, and 600 turns. We did so by measuring lengths of 9.5mm Tasmanian Oak dowel, and drilling holes at strategic points. The first hole is for passing the ECW through before winding, and marks the bottom of the coil. After that, with 0.25mm ECW, there are four turns per millimeter. So, we measured 50mm for the 200 turn coil, 75mm for the 300, 100mm for the 400 turns, and 150mm for the 600 turns. The other hole is for the ECW to pass through on completion. Inserting the beginning of the wire then taping it to the dowel that is in the drill chuck helps keep it secure.
Winding the coil properly is essential. The turns have to be side by side, not overlapping (though a mistake here and there won't end the project), and tight. You can do this by hand but we put our dowels in the chuck of a cordless drill, and held the ECW with cotton gloved fingers (which stops friction burns and cuts over time or if the drill spins too fast), and carefully triggered the drill to run at a slowish speed. In this way, making neat coils was easy, painless, and fast. The ECW gets passed through the holes top and bottom, and can be held in place with tape for neatness.
While our initial prototypes were a combination of air wired and protoboard, we chose protoboard for the final product. There isn't much going on in regards to the board, so follow the Fritzing and photos. Points to note are that we used PCB pins for the connections for the resistor and transistor, and terminal blocks for the coil connections. This enabled different coils to be tried with the same circuit, both primary and secondary. The PCB pins enable easy changing of resistors during experimentation, and replacement of the transistor if it evolved into charcoal, without the need to turn the board and its connected coils over to de-solder from underneath. For the bigger build, we used a BD139 transistor with heatsink attached.
The next step is the most critical of the build. The primary coil must be wound in the correct direction, opposite the secondary. In other words, if your secondary is wound from bottom to top in a clockwise direction, then the primary must be wound from bottom to top in an anticlockwise direction. The reverse would apply depending on which way your core went in the drill: Your secondary may be anticlockwise. Primaries range from three to twenty turns depending on transistor, power supply, secondary turns, gauge of wire, and likely other factors.
The primary coil must begin close to the bottom of the secondary. Starting the winding at the bottom means you can easily add and subtract turns as needed from the top when adjusting. We used hot melt glue to anchor the first few turns, then added a few more at a time until we had the greatest results from a neon globe held next to the top of the coil. Instead of having to screw and unscrew one of the terminal blocks to achieve this, we held our wire end FIRMLY to the screw top.
Test the effectiveness of the primary winding by holding a neon globe next to the top of the secondary. The brighter it glows, the more suitable the primary winding. This will mean adding or subtracting turns from the primary until the brightest glow is achieved, as there is a magic zone tapering away as too few or too many turns are used.
Adding a top load is optional. If one is added, it increases capacitance to earth by storing a greater charge. The best shape is a toroid but on a device this inefficient, an imperfect sphere will do fine. We rolled one from successive pieces of aluminium foil, and it's a bit rough. You could also add a steel ball from a bearing, a small foam or plastic ball painted with aluminium paint (real metal paint, not metal effect), or any other sphere you can think of. We inserted a pin into the top of the dowel, wrapped the end of the ECW (with the enamel filed off) around it, and pushed the ball of foil into that. If you use glue, be careful that it does not insulate the wire from the sphere surface.
If you've skipped straight to the baby build, it's worth reading the steps for the bigger build, as there is some information there that you'll need to know. The other differences are the ferrite-cored construction of the secondary and the use of a 2N222A transistor instead of a BD139. The circuit board layout also differs because of that.
Glue the ferrite beads together with cyanoacrylate glue so that you have a rod. Such small diameter rods exist whole but are hard to come by. Wrap the whole thing with double sided tape.
The fine wire needed for the secondary coil came from a broken-open relay. It is still ECW but much finer than the 0.25mm commonly available as the minimum size off the retail shelf. If you can, find the end that already has solder on it. If not, heat one end until the enamel melts and tin it. Filing or sanding such fine wire is not viable.
Wind the ECW from the relay on to the rod carefully, leaving the solder end with enough length for later. Make the coils as neat and close together as possible without overlapping. This will be much harder with this wire than the bigger build, and the drill method will not be suitable. The wire will also snap much more easily if too much tension is applied.
Cut a suitably sized piece of Veroboard or similar, and populate it with the few components needed. The layout is different from the bigger build, and we have gone with PCB pins for the coil connections this time.
Glue your secondary onto a suitable base. You can glue it to the Vero holding the circuit but we found winding the primary was easier if the two were separate. Wind on several turns of 1.25mm ECW and temporarily connect the ends via hookup wire to the circuit. See the bigger build for information on coil direction.
Now you can apply power and hold a neon globe near the top of the secondary coil. Add more layers to the primary, or take them away, to get the brightest glow from the neon. After this is achieved, you can make the primary coil connections permanent.
The top load is optional for the baby build as well, and at this scale, a ball from a ball bearing or a foil ball are really the only options. Many baby builds have no top load, and we chose to go without one on this build. Now you're ready to experiment!
IS IT SAFE?
That's up to you and your choices. The device operates at high voltages but very low currents. It also operates at high frequency. At high frequencies, current flows near the surface of any conductor, including human skin. This means that accidental contact with the high voltage end is unlikely to route current across the heart, and the current involved is in the order of micro- or even nano-amps anyway. However, under exactly the right circumstances, a 9V battery can kill, and case studies can be found where this has happened.
Additionally, the device radiates quite a lot of RF energy, and so should be kept away from sensitive equipment, including pacemakers. This is also not unusual, and labels on commercial products sometimes mention keeping the device away from pacemakers. This is not unique to high-voltage devices. Such a small output will only radiate near the device, so being any more than an arm's length away should be fine, and even then it only matters if you have existing health issues.
The only practical, real-world risks come from the ability of the high-voltage end to cause burns. If you touch it, it will hurt in the immediate area of contact. The burns are local, and usually the size of a pinhead. If you stay in contact long enough, the burn size will increase, but very soon there will be enough burnt skin that there is no longer a conduction path (the water-based ionic solutions in the human skin that conduct electricity having evaporated by that point).
So, while strictly speaking there are risks with building and using this device, it's because there is no such thing as 'perfectly safe'. You're at far greater risk of harm from the soldering iron or sharp blade used to make this thing, and an even greater risk from the car you likely used to go out and get the parts. Just be sensible. One thing worth noting is that ozone in quantity can be a health risk. Make sure you have ventilation in the room.
The biggest danger from a Slayer Exciter on this scale is that it can be very addictive, and you'll want bigger and better. That will lead to Tesla coils, which are electrically dangerous but also cost a lot in time and money. You have been warned!
The theory of a Tesla coil or slayer circuit might be simple or complicated, depending on your existing knowledge and your learning style. Some people can follow and visualise this easily, others not so, and it's got nothing to do with intelligence. Just like some people have a great sense of timing and pitch, and others do not. What's going on in a Tesla Coil or its derivatives is a little unconventional and so may not be obvious.
We always research a DIYODE article to see what others have done or come up with. This is both for fact checking, making sure our existing knowledge stacks up, and also to avoid being too similar. That happens a bit with electronics, and the simpler something is, the harder it is to be original, even when you think of something entirely yourself.
That said, we want to acknowledge SuchBuild, as his screen name is, for his great posts on both YouTube and Instructables about the theory behind Slayer Exciters and Tesla Coils. The analogy he used and the great explanation he provided are hard to improve on, so some of ours will echo his, especially his analogy about pushing a swing. This real-world example that so many people can relate to and understand is hard to beat.
Tesla coils and their family seem at first to make no sense. One end of the coil just ends in the air, either as a loose wire or as a metal shape. How does electricity flow? Doesn't electricity always need a circuit? Yes, it does, and it has one. You just can't see it at first. The key is that there is actually a capacitance between the tip of the secondary, and ground. It is variably called a parasitic or stray capacitance.
Let's start with power first applied . At this point, there is a current path through the resistor to the base of the transistor. It cannot go to ground, because the diode blocks it. It cannot go to the coil, because at present, there is no way for current to flow there.
So, the path through the base and emitter of the transistor is the easiest path to ground, and the transistor turns on. This allows current to flow through the primary coil, which induces a magnetic field around the secondary. This magnetic field induces a current which is opposite the one that created it, so current is induced up the secondary coil . The parasitic capacitance to ground allows this to occur, as the charge builds up in it just like it would in a physical capacitor.
This rising of current up the secondary causes a deficiency at the bottom of the coil. Suddenly, the bottom of the coil is at a negative potential, which gives the current flowing from the resistor an easier path than through the base/emitter of the transistor. Therefore, current stops flowing through there and the transistor turns off . Because the bottom of the coil is at a lower-than-ground potential, some of the current that has just passed through the primary coil also flows to the secondary, through the LED, which lights.
With the transistor off, current stops flowing through the primary, removing the force pushing the current up the secondary. Now, the charge built up discharges back down the coil. However, there is enough potential in the charge for it to be pushed past the ground potential and to a high enough positive potential to activate the base of the transistor . The cycle is now repeating itself.
Here's where it gets interesting, and where we owe the most thanks to SuchBuild for the sheer simplicity of his explanation. Although the base of the transistor is now turning on, there is now a negative potential at the top of the coil, which wants to be equal again. The parasitic capacitance is a big part of this. Current is already moving back up the coil when the transistor turns on, because there is a tiny delay in any semiconductor. With the current already surging back up the secondary coil, the magnetic force from the primary pushes it a little more. With every oscillation of the coil, this push adds a little more.
This happens just like pushing someone on a swing (thanks SuchBuild). If you push just as someone starts to swing downward again, you're adding to the gravitational force and making them swing a little higher with every push. You could not hope to just push them that high from a standing start in one go.
Eventually, the charge gets enough potential from this back-and-forth motion to have a very high voltage, enough to be shooting electrons off into the air and lighting up neon bulbs and fluorescent tubes. We even got small corona streamers of one of our smaller builds, which was very cool indeed. Actually, it was very hot and smelled heavily of ozone.
This up-and-down electron movement is only possible at a certain frequency which differs with coil diameter, wire size, number of turns, spacing of turns (there is capacitance as well as inductance between each turn), top load, humidity on the day, presence of a ground path, and other factors. This is the resonant frequency of a coil, and the fact that the Slayer Exciter circuit tunes itself inherently is what makes it so easy for the beginner. In a traditional Tesla coil, even more recent solid state ones, the resonant frequency of the secondary has to be calculated, and the driver circuit and primary coil constructed and tuned to match. The fine details involved make this very hard indeed.
Now, this is the condensed version. SuchBuild has provided a deeper version with explanations of regular Tesla coil operation, including how spark gaps work, online. It really is worth reading if you have any interest in pursuing Slayer Exciter or Tesla coils: https://www.instructables.com/SLAYER-EXCITERS-TESLA-COILS-everything-you-need-to/
WHERE TO FROM HERE?
Slayer Exciter circuits really only scale up to a certain point. Transistors quickly overheat. Our 600-turn design in retrospect probably underperformed more because the air gap was too big for an effective capacitance, rather than by being not driven with enough current to resonate. Giving one that tall enough current with a Slayer Exciter would be hard, so maybe a ground-connected wire that could be elevated slightly until capacitance was achieved would have worked.
Besides that, we explored increasing voltage to 18V and got some corona discharge from our smaller coils. We would also like to investigate smoother options for top loads, because the many fine sharp edges on the foil top load probably leaked more charge than the increased capacitance offset. We got our best results with no top load. Sadly you can't really upscale a Slayer Exciter effectively. They have a limit, and using huge coils, room-sized heatsinks, and big transistors would not give significantly better results. Strictly speaking, the results would be better, but not proportionally, nowhere near. It tends to just be a lot of effort for not much joy.
From here, we're not going to give any advice or suggestions to upsize. We will just say this: Traditional Tesla coils run on higher voltages and currents, produce UV light and Ozone (which can be dangerous when breathed above a certain concentration or quantity), and are not for the inexperienced.
As always, these resources are not necessarily the work of, or per-reviewed by, experts, even though some claim to be. Always ready any information, including our own work, with critical thinking, and compare as many sources as you can.