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.

<p>I got around to building this -- it worked great most of the time (sparks up to 5-6&quot;). 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? </p><p>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)?</p>
Changing the capacitance is not the answer. Because a Tesla transformer work by resonance, changing the capacitance will disrupt this effect, and you will experience a loss of output, if not a complete failure to produce any discharges. Your coil has a natural point at which it will resonate. Raise or lower the available capacitance and it will not work so well.<br>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--&quot;What's that liquid? What are those things in the liquid?&quot; etc.)<br>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.
<p>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?</p>
Because the Tesla coil produces AC, the charge ceases when the power is shut off. In very large systems, or DC Tesla coil systems, bleeder resistors are important safety components attached to the tank capacitors. The terminals and spark gap do not retain any charge that I have ever heard of.<br><br>Having said that, there is a phenomenon called (I believe) &quot;captive capacitance,&quot; 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.<br><br>This only happened to me using an upright, externally grounded coil. I have never experienced it with a horizontal, bipolar coil.
<p>Hi, nice tutorial!</p><p>I'm building this now and trying to select a power supply. If the sign has &quot;secondary mid point ground&quot; should I avoid that like GFI? Thanks</p>
<p>For example this one: </p><p>http://www.ebay.com/itm/FRANCEFORMER-7530FM-NEON-SIGN-TRANSFORMER-7500V-30ma-REMOVED-FROM-BROKEN-SIGN-/121797739104?hash=item1c5bb60260:g:XlQAAOSw~bFWJDwy</p>
That transformer does not look like it would work. You want an old school style NST, no GFI.
<p>Ok, I've been looking around more, and I found this site: partsforsigns.com</p><p>Do you think any of these would work?</p><p><a href="http://www.partsforsigns.com/Ventex_VT9030CL_120_transformer_p/vt9030cl-120.htm" rel="nofollow">http://www.partsforsigns.com/Ventex_VT9030CL_120_t...</a></p><p><a href="http://www.partsforsigns.com/Allanson_SS935ICH_transformer_p/ss935ich.htm" rel="nofollow">http://www.partsforsigns.com/Allanson_SS935ICH_tra...</a></p><p><a href="http://www.partsforsigns.com/France_15030P5G2U_transformer_p/15030p5g2u.htm" rel="nofollow">http://www.partsforsigns.com/France_15030P5G2U_tra...</a></p><p>I have also found lots of Allanson OBITs, like this one: <a href="http://www.ebay.com/itm/ALLANSON-2721-605-Oil-Burner-Ignition-Transformer-120V-/381024638311?hash=item58b6d69567:g:74EAAOSwKIpWDcOl" rel="nofollow">http://www.ebay.com/itm/ALLANSON-2721-605-Oil-Burn...</a></p><p>Anything special I should consider with an OBIT? Or will it operate just like a NST?</p>
<p>The Ventex and Allanson, definitely not. They have GFI. The France looks like it would work, the OBIT too.</p><p>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. </p><p>Try to find an older model of NST or OBIT.</p>
<p>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?</p>
<p>I know people use OBIT's often, but why doesn't this happen?</p>
<p>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 <a href="http://www.kronjaeger.com/hv/hv/msr/spk/">http://www.kronjaeger.com/hv/hv/msr/spk/</a></p><p>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.</p>
<p>On the other hand . . . </p><p>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. </p>
<p>Hi Mr.Apol,</p><p>My son is in high school. He want's to follow your design to build a Tesla coil.</p><p>1) You have two version shown here. One with horizontal primary/seconday and another with vertical. Which is easy to build?</p><p>2) Once material is gathered, how much time would it take?</p><p>We both have basic skills with tools and building etc.</p><p>Thanks for help</p>
Oh, and it an be built in a weekend, if you have all the materials, transformer, capacitors, etc.<br>
<p>Question on Secondary coil:<br>I plan to use PVC pipe instead of cardboard. I looked for Shellac at HomeDepot. They look expensive ($50+). I looked at Polyurethane varnish (Example: PolyShades Honey Satin Stain and Polyurethane in 1 Step) which is around $13. Will this work? What brand you will recommend? I do not want my secondary to fail due to bad insulation. Thanks for help</p>
<p>Steep prices! A half pint can of Minwax water based Polyurethane ought to be in the neighborhood of $8, if not less, at discount store. (You don't want stain in your varnish. Pigments often contain conductive material, like carbon.) In fact you don't need varnish or shellac to have a working coil. The main function of varnish is to keep the secondary wire in place on the form, and to moisture proof cardboard, if you use it. You can make a perfectly good coil without varnish if you use doublesided tape underneath the wire on a PVC tube. The additional insulation gained is minimal. High frequency electricity laughs at insulation anyway.</p>
<p>Thanks for response</p><p>1) Got the elbows,Tjoints,end caps,tape for carpet etc<br>2) I went to Lowes. I could not find MinWax waterbased polyurethane. I located MinWax oilbased polyurethane. Will this work? Instead associate there showed me MinWax warterbased polycralic. Should I buy this?<br>3) I did not find 1.75 inch PVC tube. Available sizes were 1.5 inch or 2 inch. I bought 1.5 inch white PVC. If this is OK, then<br>- How much lenght I should use?<br>- I am planning to use 26 gauge magnet wire which I have. Should I use same length as used by you or should I use same turns used by you? Let me know if there is a formula which relates diameter and turns together?<br>4) I located Allanson 7500 volts NST on amazon for $130. What is your model/number?</p>
<p>The oil based varnish is fine. It's smellier, so use it in a well-ventilated place. Polyacrylic is fine too. Water-based products are less toxic. If you're using carpet tape you may not need varnish at all.</p><p>2 inch PVC would be better than 1.5 inch. For a winding length of 12 inches you should use at least 14 inches of tubing (inch free inch at either end). 26 gauge wire is fine. You can google &quot;AWG wire turns per inch&quot; and find tables like this: <a href="http://users.silenceisdefeat.net/~lgtngstk/Sites/Wire.html." rel="nofollow">http://users.silenceisdefeat.net/~lgtngstk/Sites/W...</a> It looks like 26 gauge gives you 58 turns per inch. X12 would be 696 turns, a very respectable amount. Often you can buy odd-gauge wire (23 gauge or 27 gauge for example) cheaper than the more popular even-numbered gauges. For a Tesla coil odd gauge wire works perfectly well. 800 to 1,000 feet of wire is more than adequate.</p><p>It isn't necessary to find an exact copy of my transformer. $130 is way too much anyway. Search eBay for &quot;neon sign transformer&quot; under $100. Any voltage between 5,000 and 15,000 at 30 milli-ampos will work well. You'll have to adjust the capacitance to fit the voltage, etc. That's for later.Make sure you don't buy any transformer with Ground Fault Interrupt (GFI) protection. This will prevent the transformer from operating a spark gap Tesla coil at all. You might also look for oil burner ignition transformers (OBIT). They generally come rated at 10,000 volts at 23 milli-amps. I would stay away from microwave oven transformers. They can be lethal if mishandled. </p><p>Paul</p>
<p>Thanks.</p><p>I looked at e-bay. I found following two OBIT from ZORO</p><p>1) Allanson 2721-647 for $41.67</p><p>2) Allanson 2721-630 for $64.41</p><p>Looking at the picture, I think 2721-630 will be easy to connect since it has terminals. But I have no experiencee with OBIT. Let me know your thoughts?</p>
<p>The other OBITs have spring terminals; it's not hard to connect alligator clips to them. Most OBITs will need a power cable grafted on. This isn't hard to do--you can use wire nuts and electrical tape--but be sure to use a cord rated for the OBITs amp draw. That info should be written on the OBIT somewhere. Used equipment is usually a better deal. For example:</p><p><a href="http://www.ebay.com/itm/Allanson-Ignition-Transformer-Interchangeable-type-619-for-Wayne-oil-burner-/400860930822?pt=LH_DefaultDomain_0&hash=item5d552c6306" rel="nofollow">http://www.ebay.com/itm/Allanson-Ignition-Transfor...</a></p><p>This will work as well as the ones you listed. To use an OBIT, you simply put in the array in place of the NST. They have plenty of voltage, but less amperage, so there arcs are less intense, but just as long. Here's a video of one of my other coils in action using an OBIT:</p><p><iframe allowfullscreen="" frameborder="0" height="281" src="//www.youtube.com/embed/kKs7UgkoYtA" width="500"></iframe></p><p>Paul</p>
<p>We are ready to start. I looked on e-bay and found following on e-bay. Will appreicate your feedback on this. Thanks for help</p><p><a href="http://www.ebay.com/itm/ALLANSON-IGNITION-TRANSFORMER-FOR-OIL-FURNACE-TYPE-630-NIB-/161665975833?pt=LH_DefaultDomain_0&hash=item25a40b0619" rel="nofollow">http://www.ebay.com/itm/ALLANSON-IGNITION-TRANSFORMER-FOR-OIL-FURNACE-TYPE-630-NIB-/161665975833?pt=LH_DefaultDomain_0&amp;hash=item25a40b0619</a></p>
<p>Looks OK. The label doesn't mention ground fault interrupt, which is good. 10,000 volts at 23 milliamps will make a perfectly good tabletop Tesla coil power supply. Be ready to graft on a power cord. That won't be hard. Just match wire colors, use wire nuts and tape, etc. You may want to use a power strip with an on/off switch built in to control the coil once you have everything put together. Good luck!</p><p>Paul</p>
<p>Thanks for all the help. Now I have got sufficient information to make purchase etc. I shall be helping my son build the coil. My son plan to start working on this starting next few weeks (homework,tests etc). I shall ask more questions during build time.</p>
<p>Let me get the material. I shall ask you more questions as we start working on it.Thanks for the quick response</p>
Both designs are about equally easy/difficult. The horizontal coil has the advantage of not needing an external ground connection. You can use the vertical coil with the secondary tied into the primary, but that introduces dangerous current to the arcs . . . I suppose I would recommend the horizontal (&quot;Tabletop Tesla Coil&quot;) style.<br><br>Feel free to ask for other help if needed.<br><br>Paul
<p>Hello Mr. Apol</p><p>I want to build your design here for my first TC for a physics class. I've been looking for a NST and have noticed that many say they have ground fault protection, is that the same as GRI? Also I am thinking of using a higher voltage (15k), should I make any adjustments to any parts of your design other thatn the capacitance array?</p><p>Thanks for you time.</p>
<p>What I am seeing on the NSTs is that thay have SGFP.</p>
<p>Secondary ground-fault protection. Is that the same as GFI?</p>
<p>Sounds like it. Avoid! It's intended to prevent arcing, which is what you need the spark gap to do.</p>
<p>Thanks for the feedback. I think I found one on ebay that will work. </p><p><a href="http://www.ebay.com/itm/191505725025?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT" rel="nofollow">http://www.ebay.com/itm/191505725025?_trksid=p2055...</a></p><p>My only worry is there is no connection for for a ground on the 120V connection.</p>
Don't use any NST with GFI; they won't fire your spark gap. If you go up to 15,000 volts you'll need to rebalance your apparatus for best resonance (start in the neighborhood of 12 nF). You can use a program like TeslaMap or JavaTC to predict what capacitance you'll need.<br><br>Paul
<p>can i change the power supply and reduce the voltage as i am doing this for a school projet and 7200V seems very dangerous.</p>
7500 volts at 30 milliamps isn't as fearsome as it sounds, but you can operate a Tesla coil at voltages as low as 3000. Streamer output will be commensurately less. You will also need to re-balance the capacitance of the system to achieve best resonance.<br><br>Frankly, as long as you're in control of your apparatus (and you are a sane, responsible person ;-) ) voltage isn't too much of a problem. Be aware that coil generates RF energy, ozone, and other irritant gases, especially in a small space.<br><br>Paul
<p>exactly what i was thinking about i dont think that a school project should be something that causes air pollution but thanks for your help. </p>
<p>Thanks man, this worked well. Made this after trying one out on my own, didn't work quite as well. But still effective.</p>
I am having trouble. When I connect my spark gap, It works. When I connect the capacitors and the coil, it stops working. I am making an oudin coil,and it is wired like this (see image). What could be wrong with it?
I need more info. What is your voltage source (volts &amp; amps)? How wide is your spark gap? What is the total capacitance of your tank caps? How many turns on the secondary? How many turns on the primary? Are they wound in the same direction? What is your topload and how is it connected? <br> <br>Paul
Here is a toroid I think might work. I am trying to get visible streamers from my coil, but it is fully operational. Note the toroid is hollow.
I am using a 7.5 thousand volt nst. My spark gap is about a 1/2 of a centimeter wide. I have about 2.45 micro-farads in my caps. I will tell you the rest as soon as I get a chance to check it (I am kinda tied up a the moment)
There are approximately 330 winds on the secondary.There are 4-5 winds on the primary. My topload was made of tinfoil squished into a toroid wrapped in HVAC tape. I stabbed a hole in the bottom with scissors. I stripped one of the ends o my primary and stuffed it in. I do not undurstand what you mean by &quot;wound in the same direction&quot;. Please clarify. Also, why does my spark gap not work when I attach the coil and the caps, but remain functional when it is isolated. It is connected in paralled, after all?
More questions, and maybe a few answers. <br> <br>What gauge is the wire in your secondary? What gauge is the wire in your primary? You say you connected the topload to the primary? I hope you mean secondary. Try to find a simpler topload--an old doorknob will do, or even an aluminum drink can (with all openings sealed with aluminum tape, of course). Wadded foil has enormous surface area. In effect you asking your coil to light up a huge topload. <br> <br>Did you strip the enamel insulation from the ends of all connecting wires? (Don't be offended by the question; with clear enameled magnet wire it can be easy to forget to strip the insulation). <br> <br>2.45 microfarads is way too much capacitance. It should be more like 2.45 NANO farads. This alone will prevent your coil from operating! Though the NST will make the spark gap fire, there is way too much capacitance in the system to allow the coil to work. <br> <br>It is often said that if you have a secondary wound in one direction--say, overhand--and a primary wound the other way (underhand), that this interferes with proper resonance. I have tried both configurations, and I can't say it makes a great deal of difference, but any difference in a very small coil makes more relative difference in total performance.
My wire is 24g wire. My primary is 12g I will check my insulation. You said the NST will make the gap fire even if my capacitance is to high, however It wont fire at all when i connect the capacitors, just when it is isolated. Why? I will go fix my topload, now.
the wire gauges are okay, though 330 turns is a little low for really big performance. It should work fine with these gauges though.<br><br>It sounds like you have way too much capacitance. Trying to fire a coil with too high capacitance is like trying to fill a swimming pool with a teaspoon. <br><br>The spark gap merely shows you're getting 7,500 volts across the gap. Once you connect the oversized capacitors and coil, your voltage is getting lost in a way too large environment. <br>
I saw your &quot;Tesla's candlestick&quot; ible', and your cap array. would you be willing to sell some to me? If not, where did you get them. Also, where did you get your NST? <br> <br>Kirkthepyro <br>
See http://www.ebay.com/itm/High-Voltage-K15-4-KOB-2-Doorknob-Capacitor-500pF-20kV-Lot-of-4-/251087660475?pt=LH_DefaultDomain_0&amp;hash=item3a75fda5bb<br><br>for example.
So I should just lower the capacitance? <br>
Would photo-flash capacitors work? I have a ton. they are from old cameras.
If I solder a bleeder resistor across the terminals of the cap, will that work?

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