Picture of How To Build A Spark Gap Tesla Coil (SGTC)
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Shoot foot-long bolts of lighting through the air, light fluorescent tubes without wires, and power your other high voltage experiments with the aid of this tabletop-sized Tesla coil! Once the parts have arrived, it comes together in about a weekend, and for less than $200 even for those without a big bin of spare parts. I built this coil for fun over a few weekends during 10th grade, juggling eigenvalue problems and European history for drilling and soldering high voltage components.

The key to this coil's performance on such a tight budget is that all of its components are designed to work well together. Using some basic concepts from AC circuit design, the components are matched to perform well without requiring massive amounts of power. Some "coilers" use microwave oven transformers to pump kilowatts of energy through poorly matched circuits, resulting in large losses and mediocre performance. This instructable will show you how to avoid making such mistakes and how to properly design a spark gap Tesla coil.

UPDATE: This Tesla coil is now on sale on eBay for a starting bid of $99.99, less than the cost of the materials! http://www.ebay.com/itm/250-000-Volt-TESLA-COIL-Assembled-2-Foot-Tall-8-12-Sparks-/180826521311?pt=LH_DefaultDomain_0&hash=item2a1a19bedf

For contest entry details on this instructable, see step 10.

UPDATE: New diagrams for primary capacitor, primary coil, and spark gap construction have been added. Click the top left information icon to view them in full size.

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(I named it the "Valentine's Day Tesla Coil" in this video because I finished it over Valentine's Day weekend 2011)

To read more about this project, visit my website: http://xellers.wordpress.com/tesla-coils/sgtc-ii/
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jmood1 month ago

Looks cool, BUT, the use of spark gaps in unshielded projects will create a type pf broadband transmitter, of the same type used back in 1912 when the Marconi Company made literal Spark Gap transmitters for ships, like Titanic.

Spark Gap transmitters and devices that use Spark Gaps and inadvertently transmit, create havoc on RF frequencies over an extremely broad range of frequencies. They are outlawed by the FCC and International Law. I suggest you shield your project VERY heavily. The last guy I saw in the FCC enforcement bulletins that was cited by them paid out $10,000.00. The FCC does not play.

pvcpipefan2 months ago

could i use 628 feet of wire? thats all i can find.

Kovee984 months ago
Im planning on purchasing a machined aluminum toroid. Is there a way to figure out the best size for optimal performance? A ratio or maybe an equation?

What is the function of the top load?

It adds capacitance to the secondary. That means it stores charge on its surface and changes the resonating frequency of the secondary.

LandauMartin5 months ago

Why didn't you use a RC filter (Terry filter)? isn't it necessary to protect your NST?

Xellers (author)  LandauMartin5 months ago
I have never had an NST or other transformer fail in a Tesla coil, but I'm sure the overall mean time between failures would be somewhat lower if I used a Terry filter (mostly likely because of the safety gap and not the actual filter -- it's easy to get the main spark gap spacing wrong and accidentally ring up a near-resonant primary capacitor at 60Hz). I don't believe a filter is absolutely necessary because placing the spark gap in parallel with the primary transformer shorts any conducted noise that might otherwise do damage to the secondary windings, and NSTs are designed to operate continuously driving the low impedance of an ionized discharge tube.

I might be convinced otherwise if someone presented noise measurements or tried to figure out quantitatively what it actually takes to make an NST fail. As it stands, I've seen people build successful Tesla coils with anything ranging from no filtering to Terry filters that cost more than the NST they're trying to protect.
NewCaleb8 months ago

So just to be clear before I start ordering parts, you can not have a NST power supply that complies with UL 2161, or contains any form of ground fault interruption, secondary or other wise. In other words, every single power source I've looked at on eBay. (for ex. this and this) If you could make a note next to or on the product list, it may help other juvenile coilers such as myself. Thanks again for this well written Instructable!

BoatN1 year ago

What length of cable should I look for in the primary coil?

kf7oor BoatN1 year ago

that depends on the size of the coil you are building and the number ot turns you will need. generally it is at least 10 turns so that you have room to tune the coil. i used a 50 foot 1/4 inch copper tubing from lowes, did my ten turns and cut it off there and i had just enough left over for the strike rod.

sciencetor23 years ago
found these capacitors on ebay, they're from a supplier so they should be around for a while.


all the info you would need isn't there, but with a bit of research done after i recieved my order, they appear to be the crimped version of these


which should(probably) be suited for coil use, though the specs are slightly different from the instructable, so you would need to change your MMC configuration. i will probably go with a 12x8 setup(12 per string, 8 strings in paralell) which yields a capacitance of 0.01uF and a voltage rating of 19200v(19.2kV). i will let you know how it goes when i finish my coil.
Hi. I am just wondering whether these capacitors would also work?

could you give a link for the resistors you used. Thanks.
yep :) here you go! happy coiling!

ok, just finished my tesla coil and ran it, these capacitors work great! highly reccomend them for their price, amount, and continuous availability.
kf7oor1 year ago

have you tried downloading a free long exposure iphone app like i did to get better pictures! here are some of my tesla , 2.5" by 10" secondary, and 12000v 30mA NST power supply and a SINGLE .01 uf 20kv pulse cap and a 12inch by 3 inch dryer duct top load. by the way this was my own tesla i built not off this instructable...

seraine2 years ago
I need 17 0.15 uf capacitors, and these appear to be perfect for my Tesla Coil


Can a MMC use polypropylene metallized film capacitors?
Xellers (author)  seraine2 years ago
Yes - those are precisely the kind of cap you'd want to use (unless you can afford CDEs).

Good luck!
-max- Xellers1 year ago
All these capacitors are DC rated, but I see them in a configure which the voltage across them will be AC, due to ringing between the caps and the primary coil. a 2000VDC capacitor will have a rating like 400VAC, so I would think the AC rating is what I avoid exceeding, however it appears everywhere I look where they build an capacitor back, this is overlooked. What rating should I be concerned with?
seraine Xellers2 years ago
Thanks for the speedy reply!
where would you recommend getting the transformer
james346022 years ago
How can I build a quiet spark gap
Always it was loud
Xellers (author)  james346022 years ago
Build a solid state or vacuum tube coil :P
agod3 years ago
Would a circuit board like a pcb or breadboard work to hold the capacitor bank?
i doubt it, as a breadboard would not provide enough insulation to contain such high voltages unless you modified it by removing un-needed rows, and then it may heat up, a pcb could work if it was custom for your bank, a pre punched prototyping board might arc, but i dont know
agod3 years ago
Ä° will be running my tesla coil in a science competition indoors ( in a basketball court) where there is no access to a place to put a large metal spike in the ground so i was wondering if it would be ok to ground it to the ground in an outlet. Ä°t is exactly the same as yours execpt my secondary is 4 inchs wide and i am using 30awg wire. Thanks.
well, provided the electrical system is properly grounded, you CAN, i did... but be sure to filter everything well with a specialized RF filter, as by grounding it to the outlet you can create potentially damaging RF interference that can damage sensitive things like flat screen tvs and computers and such if left unfiltered.
agod3 years ago
my transformer is the same as yours but 60ma. what will i need to change in the cappacitor bank? THanks
Xellers (author)  agod3 years ago
See the step on building the capacitor - I discuss how a value is selected there. You'll need to run the calculations yourself. Good luck!
sciencetor23 years ago
built a working Tesla Coil from this instructable :D made a few modifications due to necessity/improvement/budget/availability, but it works great! (still a bit out of tune though since I have been crazy busy) here is a video of it running


and pictures below.
IMAG0114.jpgtesla pic.pngtesla pic4.pngtesla pic1.pngIMAG0115.jpgIMAG0113.jpgIMAG0117.jpgIMAG0124.jpgIMAG0123.jpgIMAG0121.jpgIMAG0122.jpg
Other quick note: in the video I used a PVC pipe with GTO high voltage insulated wire taped to the end, and the other end of the wire connected to the main ground, as a grounding rod to attract the arcs. while this is all well and good in this particular spark gap coil, i have read several places that other varieties of coils such as vaccum tube tesla coils can be damaged or destroyed by ground strikes such as this.

Also, while this grounding rod worked safely, as a general rule you shouldn't really be attracting the sparks to you, as this can be dangerous or deadly. an improperly constructed, or improperly used grounding rod could cause electrocution, so DO NOT ATTEMPT! I AM NOT RESPONSIBLE FOR INJURY OR DEATH RELATED TO THIS METHOD!
also a quick note on the capacitor bank, due to the unavailability of the exact capacitors in the instructable, the ones I used had a capacitance of .015uF, and a voltage rating of 1.6kV, i posted an ebay link to these earlier, but due to the slightly different capacitance, they must be arranged in 8 rows of 12 capacitors, rather than the 10 rows of 10 directed in order to achieve the 0.01uF necessary for this coil. This configuration also has a max voltage rating of 19.2kV.
kdbacho3 years ago
just to ask two questions. why were there insicions in the speaker wire and is the secondary coil connected to the primary. because i dont know were to put the loose ends
also, the "incisions" in the speaker wire are where the insulation was stripped to connect the test clip, in order to experimentally tune the coil i believe.
Bottom of secondary is connected to a ground, top wire is taped to the top load, or bolted to the toroid if you went fancy
sciencetor23 years ago
sorry to be bothering you so much, but i also cant find any capacitors rated for 1.5kV 0.01uF... where did you get yours?
sciencetor23 years ago
i take it the resistors need to have a max voltage rating of around the same as the capacitors? what was the rating on yours and where did you get them?
sciencetor23 years ago
ok, im back, Found what appears to be a REALLY great deal on a pair of 9kv 30 mA very-used-but-still-working-guarunteed neon transformers, the only problem is they have GFI, but seller claims they have a switch to bypass it, would they be suitable? (id show you the auction but I'm afraid someone will grab them first looking at the comment, email?)
Xellers (author)  sciencetor23 years ago
daniel (dot) kramnik (at) gmail (dot) com

I can't guarantee that I'll be right, but I can try to tell!
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