Tabletop Tesla Coil
Intro: Tabletop Tesla Coil
Nikola Tesla's famous air core transformer--aka the Tesla Coil--was developed in the 1890s as a source of high voltage, high frequency electricity. In those days it was a valuable component in the emerging field of wireless telegraphy, and Tesla had ambitions to charge the earth and upper atmosphere in order to supply electrical power throughout the world without costly and cumbersome power lines. His concept did not work out for various reasons, and the Tesla coil survives today as a project for hobbyists. The design below is based on popular designs published for amateur builders circa 1910 or so. I have modernized the frame by using PVC, but the horizontal coil was very popular with early radio experimenters in that long ago time.
STEP 1: The Frame
The frame of the tabletop Tesla is made entirely of half-inch (12.7 mm) PVC pipe. There's no point giving you exact dimensions of the stringers and risers because they depend on how big your secondary coil is. You can scale the frame up or down as you wish. I used a 12 inch long cardboard tube for mine, making the footprint of my coil 14 inches by 11.25 inches.
Study the pictures and you will see how the frame is made. The configuration as made requires the following joints:
(4) 90 degree elbows;
(8) "T" joints
(2) end caps
All the rest is straight half-inch tubing, cut to length. NO CEMENT WAS USED TO JOIN THE PARTS and none should be used. The friction fit of the tubing is reasonably strong, and leaving the joints unglued allows you to take the frame apart to work on the coil, make adjustments or repairs, etc.
The center uprights consist of three T joints each, stacked vertically. Short lengths of tubing connect these. If you make a bigger coil these length will have to be adjusted accordingly.
The cross piece that runs underneath and parallel to the secondary coil has to be drilled for the primary supports. Find the center point of the cross piece and drill two holes so that the primary form in centered on the secondary. Again, I can't tell you exactly where, because it depends on what you use for your coil forms. But center it and it will be fine. See the page on the Primary for more details of the mounting method.
The secondary is supported by plumbing caps and tubing adapters inserted into the cardboard tube. The tube I used is 1.75 inches in diameter (it's a thick-walled cardboard paper towel tube). I had to experiment at the home center to find off the shelf PVC plumbing pieces that would fit, but I found ones that slip in closely. Again, no glue was used. You want to be able to remove the secondary for maintenance or replacement.
In the two topmost T joints insert plugs to support the brass terminal posts. These can be anything non-conductive--cork, rubber, etc. I found wine corks fit nicely. Push them down equally on each side until they are level with the top edge of the secondary form. Above them fit a length of PVC tubing. Exact height is not too important; they should be tall enough to keep the terminals away from the active coils to avoid arcing. Mine are four inches tall each.
Drill 3/16ths holes in the center of two PVC pipe caps for the terminals. Drill small holes--about 1/8th inch--in the support tubes opposite the ends of the secondary coil to feed the secondary's wires through. See the step about the Terminals for final connections.
STEP 2: The Secondary Coil
The secondary coil in a Tesla coil is generally a long coil of fine wire. It is not connected to the primary coil, but picks up current from the primary by induction.
For this tabletop model I used a heavy duty cardboard cylinder formerly used in a roll of commercial paper towels. It's about 3/16ths of an inch thick, 1.75 inches in diameter, and 12 inches long. You can also use PVC tubing, or any other type of insulating plastic. Avoid colored tubing. Sometimes the dye is conductive, which will cause your project to fail. Avoid black tubing. It often has carbon in it, and carbon conducts electricity.
If you use cardboard as I did, coat it well inside and out with several coats of polyurethane varnish or shellac. These help stabilize the cardboard and greatly improve its insulating properties.
Once your secondary form is hard and dry, apply two strips of double-sided carpet tape to the outside, 180 degrees apart. This is very sticky tape used to hold carpets down. On a 12 inch tube, I used two strips of tape, 11 inches long each. This will help hold your secondary wire in place as you wind it.
The wire I used is 24 gauge magnet wire. It is solid copper with a coat of enamel insulation. Start winding half an inch from the end of the tube by taping down the wire (temporarily) and winding slowly and carefully, keeping the wire smooth and snug. Do not cross the wire at any time, or your work will come to naught. It's boring, but I wound the whole secondary by hand in about 80 minutes while watching TV. I wouldn't bother with winding jigs unless you plan to make several coils.
You can use other fine gauges of wire--22, 26, 28, 30, etc.--and the wire you choose will affect the resonance of your coil. I had 24 gauge on hand, so that's what I used. There are 515.9 turns on this coil, for a total of 249.7 feet of wire.
Once your secondary is wound, paint it several times with clear polyurethane or shellac. Let it dry thoroughly between coats. Some coil makers put on many, many coats of varnish. This both insulates and glues the wire to the form. I put on four coats of shellac. That seemed like enough.
Carefully remove the anchoring strips of tape from each end. I used a bead of hot glue to fix the wire to the form. Leave about eight inches of wire free beyond the glue. These will connect to your terminals.
(The secondary form is supported by PVC plumbing caps inserted inside. I spent quite some time at the home center trying out different bits of plumbing connectors and caps to find ones that fit the cardboard tube nicely, but I eventually found perfect pieces. )
Feed the wires through the holes bored in the upright supports. Scrape off the enamel insulation (I used fine sandpaper for this) from the last three inches of the wire. Push the tubing and PVC cap up, and attach the bare wire ends to each terminal. Solder if you want. I used aluminum HVAC tape, which is strong and conductive. Push the supports back into top T joints. The ends of the terminals will be supported by the cork or other stopper previously inserted into the frame.
Before you push the PVC caps in place, you may want some friction pads to keep the terminals from swinging around two easily. I cut a rubber stopper in half lengthwise and put one piece in each upright. With the caps in place, the terminal rods are jammed against the rubber, which allows fairly good adjustment of the terminals.
See the Terminals page for details of making them.
STEP 3: The Primary Coil
The primary form is a so-called 2 inch PVC pipe coupling. It actually measures 2.75 inches in diameter. The first thing to do is mark and drill holes for mounting it on the frame. Drilling in-line holes in a round object can be tricky without a jig or clamp to hold the work in place. I used the edge of an open drawer (a natural 90 degree jig) and marked two spots about 3/8ths of an inch from each end of the coupling. Use a 1/4 bit and drill carefully. Next. line up the cross piece that runs beneath the secondary coil. Using the primary coupling as a guide, center the two tubes and use the holes you already drilled to mark the cross piece. After spoiling several piece of PVC I learned it was better to drill the four holes one at a time; that is, drill the top two holes, then align your bit by looking down the inside of the tube to drill the holes through the other side.
You will need 2 1/4-20 nylon bolts (2 inches long in my model) and six nylon hex nuts. Insert the bolts through the holes in the primary form. Put one nut on each and tighten. Put a second nut on each bolt but don't tighten yet.
Now let's make the coil. Stick a 2 inch wide strip of double-sided carpet tape around the middle of the PVC coupling. On my model I used 12 gauge stranded wire with a heavy nylon insulation. This is sold on bulk reels in most hardware stores. At first I put on 9 turns. Later, on the advice of TeslaMap, I reduced this to 5. I would start out with 5 or 6 turns and work on tuning the coil later. The carpet tape will help hold the 12 gauge wire in place, but the insulation and thickness of the wire resists conforming, so once you have the primary wrapped, anchor the wire in place with a couple of layers of electrical tape.
Slip the primary around the secondary. Let it hang loose for a while. Push the plugs into the secondary to support it and snug up the various PVC joints. Make sure the cross piece is in place underneath the secondary and loose primary. Align the holes in the cross piece with the bolts. Attach the last pair of nylon nuts to the underside of the cross piece and tighten them just until they are flush with ends of the bolts. Now tighten the middle pair of nuts down against the cross piece. The idea is to support the primary from below at just the right height to center it around the secondary. Screw the bottom pair of nuts on each bolt up or down as needed to align the coil. If needed, you can also twist the cross piece forward or back to help center the primary. Study the photo; it should make things clearer.
Once you have the primary centered, use nylon zip ties to tie the primary wire to the cross piece. This will support the wire against pulling and help hold the coiled wire in place.
STEP 4: The Terminals
The twin terminals are made from 3/16ths inch brass rods. Cut two equal lengths--mine are 18 inches long--and bend them at the same point about 2/3rds of the way. To make sure they bend at the same place I taped the rods together securely and bent them at the same time over a length of steel pipe. Don't use clamps or pliers or you may mar the brass, Gouges and burrs are not good for electrical terminals. You will have excessive corona leakage at such spots.
Insert the terminal ends through the holes in the PVC caps. Wind a generous amount of stripped secondary lead around each rod and secure them with aluminum HVAC tape.
The electrodes at the tips of the rods are solid brass balls sold in home centers and lamp shops as pull chain knobs. They are 0.7 inches in diameter (18 mm) and bored through for the pull chain. With a 3/16 bit, open the holes enough to accept the brass rods. Wrap 1/2 inch wide strips of aluminum HVAC tape around the rods, about 1/2 an inch back from the end until you have a good friction fit for the brass balls. If you're good with a torch and solder, you can probably sweat the electrodes on, of course.
The rod ends rest on the corks or plugs you put in the frame earlier. To create friction dampers for the rods so they don't swing around too freely, cut a rubber stopper or cork in half lengthwise and press these pieces (one half per post) into the PVC terminal posts. With the caps in place, there should be enough pressure on the rod and stopper pieces to control the motion of the terminals.
STEP 5: The Spark Gap
The spark gap is made entirely of brass hardware and HDPE (polyethylene) plastic. The base is a HDPE cutting board 8 by 6 inches. My first spark gap was a simple two electrode model, but it quickly overheated in use, so I put together the four post model you see here. You will need 4 90 degree brass brackets( 2 x 2 inches), 2 0.75 inch 8-32 brass bolts, 2 2.5 inch 8-32 brass bolts, two 8-32 brass nuts, 4 8-32 brass thumb nuts, and four 0.4 inch (10 mm) solid brass balls, drilled and tapped for 8-32 threads. These balls are sold as lamp repair parts in many hardware stores and home centers. You will also need 6 short machine screws.
The two center posts are mounted on the HDPE base with a single machine screw in the rearmost bracket hole. The screws should be short enough they do not pass all the way through the plastic base. If they do you will have arcing through them when the coil is energized--not a good idea. Using one screw in each bracket allows you to pivot them slightly to adjust the spark gap.
A 0.75 inch long bolt is put through the top hole of each of the center brackets and a nut is turned on tight. Add a brass ball to each post.
The other two posts are adjustable as to length and spacing. Place one at right angles to the center posts and screw it down with two machine screws. Put on thumb nut on one of the long brass bolts. A high voltage wire from the power supply will attach here, and the wire going on to the capacitors. Put the second thumb nut on after the bolt has passed through the bracket. Tighten, then add the ball electrode. Repeat this assembly for the last post, which is mounted at a 45 degree angle to the rest. (This angle is strictly a matter of convenience due to the size of the base. If you have a larger base, you can set the last post perpendicular to the center posts). The other lead from the power supply goes here, and the HV line going on to the primary coil.
Adjust the four electrodes so that they line up and have about 1/16th inch gap between them to start with--about 1mm or slightly less. Adjust as necessary to get good output.
In operation the multi gap still gets warm. When the air is ionized, it adversely affects the rate at which the spark gaps fires. The result is fluctuating power levels and poor output. The gap can be cooled by a stream of air from a vacuum cleaner with its hose reversed, a hair dryer set on unheated air, or even a converted computer cooling fan. Quenching the gap will greatly improve the performance of your coil.
Do not look at the spark gap when it is operating. It gives off intense UV light, like a welder, so avoid looking at it or else shield it in some way. When the coil is operating there is lethal voltage going through the spark gap--do not touch it!
Update 6-25-09: Spark Gap Mark III. I rebuilt the four post spark gap with much larger components. (See pictures). Instead of the little 10 mm brass balls, I installed four brass knobs designed as drawer or cabinet pulls. The two egg shaped knobs are lacquered brass; the spherical knobs in the center are satin nickel plated brass. The brackets and supporting bolts are the same as before
The larger knobs work very well and handle the heat of spark gap operation better. They are superior to the smaller electrodes, and and being threaded for the same 8-32 bolts, they are easily exchanged.
Update 8-3-10: For even better performance, try making a pressurized spark gap, as described here: http://www.stephenhobley.com/blog/2008/09/10/petes-quick-hyperbaric-spark-gap-tm/. I used smaller components than shown--half-inch copper pipe and a half-inch sized PVC junction box--and it works very well when connected up to a small vacuum cleaner. See this video for example.
STEP 6: The Capacitors
Two things matter most in choosing capacitors for a Tesla coil--capacitance and voltage rating. I'm not going into the theoretical requirements here because there are many authoritative texts who do that much better than I ever could. Suffice it say you need at least double the DC voltage rating of a capacitor for it to survive life in a Tesla circuit, and more is even better. With a 7.5 kV power source, I would want at least a 15kV rating, and more is better.
To figure capacitance required by the coil, a design program like TeslaMap is invaluable. For various practical reasons I ended up with two 4nF, 20kV ham radio capacitors I got off eBay. I linked them in series, doubling the voltage rating to 40kV. I measured the capacitance of the two caps in series and got 2.75 nF. I tried using one of the caps alone and it burned out. Perhaps it was defective, or perhaps the voltage rating of 20kV just wasn't enough.
(Since starting I have burned out two more of the ham radio polystyrene capacitors. They apparently don't stand up to the severe stress of Tesla circuits very long. I don't recommend them.)
I next tried my homemade Leyden jar array.
Update 6/16/09: I reworked my soda can Leyden jar battery (12 jars linked in parallel) by joining them with strips of HVAC tape and binding the whole array tightly together to insure contact between the individual jars. This upped the capacitance to 5.76 nF, which works very well with the bipolar coil as built. The Leyden battery takes the power from the spark gap and NST with no signs of over-voltage, heating, etc. Homemade proves better than factory made, in this case!
Update 9/14/09: New photos added of my homemade Leyden jar array. This battery has 18 jars, for a total capacitance of 8.05 nF. It works very well with all of my tabletop coils, upright and bipolar.
Update 10/7/09: I have added a complete Instructable on how to make the Leyden jar battery for a small Tesla coil.
STEP 7: The Power Supply
For power I use an Allanson 7,500 volt neon sign transformer. It is current limited. As Mitch Tilbury writes in The Ultimate Tesla Coil Design and Construction Guide:
"Small Tesla coils that typically use NSTs will not require circuit protection as the NST is current limited and will draw less line current than the 15-A [15 amps] rating that is typical for building electrical outlets." (page 275)
I installed two RF chokes on the lines running to the coil. These are snap-on types sold at Radio Shack. They are there to block or reduce RF interference.
The Allanson unit is handy in that it has a pull-chain switch with a long string attached. You can easily stand two or three feet away and turn the apparatus on and off from a safe position. I mounted the NST on a pine plaque, well-shellacked, and put four hard rubber feet on the plaque to raise it and give it a more finished appearance.
STEP 8: How It Works
I was fortunate the coil worked the very first time I tried it. I was frankly startled at how easily and well it performed. After a few tests, I burned out a capacitor or two and learned my first model simple spark gap wasn't adequate. I have since run the coil with a bank of 12 or 14 handmade Leyden jar capacitors with varying success. I have also run it successfully with ceramic doorknob caps made in the former Soviet Union (check eBay!).
At its best, the bipolar coil makes loud, thick arcs when the electrodes are closest together (a gap of 2.5 inches). With an insulated wand I have pushed the electrodes as far apart as six inches and had a continuous arc. Beyond six inches I get lively brush discharges--purplish coronas around both brass electrodes. An ordinary clear incandescent light bulb, mounted on a insulated rod, is filled with violent, fiery streamers when placed between the terminals. Fluorescent tubes glow several inches away, though oddly enough they glow brightest at about four o' clock low instead of up close to the terminals.
Cooling the spark gap with a stream of air intensifies the arcing, making it louder and brighter. Merely switching to the multi-point spark gap array had much the same affect.
So that's my bipolar Tesla coil. I will continue to experiment with capacitors and spark gaps and report my results in updates to this page.
290 Comments
scott6991 6 years ago
you say doorknob caps
like this ? http://www.ebay.com/itm/0027uF-40000V-Doorknob-HV-...
i have seven of these i pulled from an old military radio thing several years ago not the one on the ebay add but similar mine are .0027uF 25kv rated are these worthy and stout enough to use i have a 7500 volt nst i can arc all day just a climbing spark toy right now but id love to build a tesla coil
leeleebeans230 7 years ago
i am making it for a science project
Mr. Apol 7 years ago
JasonP155 7 years ago
Have you ever considered fluorescent tubes for your caps? I have built a few just by filling either 2 or 4 foot tubes with conc brine and covering in Al foil. When I connect them across my 7.5kV 30 mA NST they make the arcs i draw scream harder than any bottle based capacitor I've tried. I haven't gotten around to calculating the actual capacitance they develop but I can tell i'm on to something. Having a strong dielectric like glass no thicker than 1mm should mean high C and high WV. No one else has tried it to my knowledge. I'm trying to build a coil of my own but i'm having trouble finding the time, space and parts. I'd love to hear from someone who has a complete coil to humor me on this.
Mr. Apol 7 years ago
Any liquid filled capacitor is going to be very lossy too. That's why most coilers use commercially made caps. Their insulation is superior, so you lose less power through corona.
Still, it's an interesting idea. I once made caps out of old incandescent bulbs (no toxic gas there, just argon). They did work, but the dielectric stress of Tesla coil pulsations made them crack. Fortunately I was using dry filler--copper coated BBs--instead of liquid.
woody558 8 years ago
I got around to building this -- it worked great most of the time (sparks up to 5-6"). However, twice it has automatically shut off during operation (after only 10 seconds or so) and stopped working. When I replaced the capacitors, it worked again. My capacitors should be high enough voltage and capacitance (8 nF rated at 20,000 V -- My power supply is 10,000 V at 60Hz). I do not notice any physical damage on the outside of the capacitors, but maybe the dielectric broke down? Might it help to try a higher capacitance rating? Deepfriedneon.com recommended 6 nF for a 10,000 V supply, but maybe something more like 10-15 nF would be better?
I also had some noticeable arcing between capacitors -- probably because the leads are too short and reasonably close together. A lot of electrical tape has helped, as well as narrowing the spark gap. But narrowing the spark gap results in less intense sparks from the secondary. Have you found a good way to balance this (big enough sparks without overloading the capacitors)?
Mr. Apol 8 years ago
Using double the DC voltage rating generally works, but also critical is the kind of cap you're using. My early experiments were with Chinese made polyethylene caps with very high voltage ratings (30KV), and I was using only a 7.5 KV NST. I still popped caps like popcorn because the style of caps I was using could not stand up to the pulses inherent in Tesla coil operation. Hardcore coilers use pulse rated caps. I've always done well with ceramic doorknob caps. I buy ex-Soviet caps made for high power radio and radar, and I've never broken one yet. I also have taken to submerging my tank caps in dielectric (insulating) liquids to prevent arcing and to reduce losses from corona. I've used both castor oil and glycerine with success. Many high power coilers buy actual transformer oil for this purpose. Castor oil and glycerine both have high dielectric strength, and have worked well for me. (Plus, it puzzles people watching my coils operate--"What's that liquid? What are those things in the liquid?" etc.)
Narrowing the spark gap will ease the burden on the caps too, though as you say, it also lessens output. You might try narrowing them anyway and finding a way to cool them. A stream of air on a working spark gap can greatly improve the coil's performance.
woody558 8 years ago
How do I ensure that the Tesla coil doesn't retain a charge in the terminals or spark gap after I turn off the transformer? Should I have a discharge loop of wire (held on an insulating rod) or maybe a resistor across the capacitors? Or is this not an issue?
Mr. Apol 8 years ago
Having said that, there is a phenomenon called (I believe) "captive capacitance," in which the secondary coil of wire retains a static charge. The exact reasons for this I don't know, but I have occasionally received a very mild static shock handling a Tesla coil secondary after its been shut off. It was no worse than the shock you get scuffing your feet on a carpet, then touching a doorknob.
This only happened to me using an upright, externally grounded coil. I have never experienced it with a horizontal, bipolar coil.
woody558 8 years ago
Hi, nice tutorial!
I'm building this now and trying to select a power supply. If the sign has "secondary mid point ground" should I avoid that like GFI? Thanks
woody558 8 years ago
For example this one:
http://www.ebay.com/itm/FRANCEFORMER-7530FM-NEON-SIGN-TRANSFORMER-7500V-30ma-REMOVED-FROM-BROKEN-SIGN-/121797739104?hash=item1c5bb60260:g:XlQAAOSw~bFWJDwy
Mr. Apol 8 years ago
woody558 8 years ago
Ok, I've been looking around more, and I found this site: partsforsigns.com
Do you think any of these would work?
http://www.partsforsigns.com/Ventex_VT9030CL_120_t...
http://www.partsforsigns.com/Allanson_SS935ICH_tra...
http://www.partsforsigns.com/France_15030P5G2U_tra...
I have also found lots of Allanson OBITs, like this one: http://www.ebay.com/itm/ALLANSON-2721-605-Oil-Burn...
Anything special I should consider with an OBIT? Or will it operate just like a NST?
Mr. Apol 8 years ago
The Ventex and Allanson, definitely not. They have GFI. The France looks like it would work, the OBIT too.
OBITs are pretty much alike regardless of brand. They're almost all rated at 10K volts at 23 mA. They are a good choice for a tabletop coil. Some OBITS have GFI, but the Allanson unit listed doesn't mention it.
Try to find an older model of NST or OBIT.
woody558 8 years ago
One concern I thought of today with the OBIT has to do with the terminals. If the terminals are exposed will electricity just arc between them and render the tesla coil circuit unusable?
woody558 8 years ago
I know people use OBIT's often, but why doesn't this happen?
Mr. Apol 8 years ago
The distance between terminals is too great. In general it takes about 25,000 volts to leap across an inch of dry, ordinary pressure air. (actually 25 KV per 2.30 cm). See the table at http://www.kronjaeger.com/hv/hv/msr/spk/
Now, if you get feedback voltage coming back from the coil to the OBIT, you could get arcing across the terminals. That's why high powered coils have safety gaps, to forestall this. A low power tabletop coil like mine doesn't really need one.
Mr. Apol 8 years ago
On the other hand . . .
Midpoint grounded is not the same as GFI, but non-GFI NSTs are usually big and heavy. This unit might be OK, though I am not familiar with the model.
DJayant 9 years ago
Hi Mr.Apol,
My son is in high school. He want's to follow your design to build a Tesla coil.
1) You have two version shown here. One with horizontal primary/seconday and another with vertical. Which is easy to build?
2) Once material is gathered, how much time would it take?
We both have basic skills with tools and building etc.
Thanks for help
Mr. Apol 9 years ago