I am often asked - “Please tell me how I can build a Tesla coil”. Many would-be coilers would like to be handed a cookbook recipe and shopping list. If money were no object, that could be done, but that’s rarely the case. A Tesla coil built from factory-new components would be expensive, but most coils are built with the parts that happened to be cheap and available, so no two coils are ever identical.
Tesla coils make nice projects in that component selection is extremely flexible, and most parts can be found cheaply, used, or on the surplus market. But if one substitutes one component, it’s likely that other components must be altered to compensate. And a certain degree of technical know-how is necessary in order to understand what may be altered and what else must be altered to compensate. Readers are free to copy the designs of the coils I have built and described elsewhere on this site verbatim, but more likely your parts will differ from mine, so I won’t present any cookbook recipes.
What follows is a basic outline of the sequence and steps involved in building a coil.
1. Power supply. Choosing the power supply transformer is the first step, as the rest of the coil design follows from the parameters of the power supply. Neon Sign Transformers (NST’s) are most commonly used, although Oil Burner Ignition Transformers (OBIT’s) and Microwave Oven Transformers (MOT’s) are also used. At the high-powered end of the spectrum, utility pole distribution transformers, a.k.a. pole pigs, and potential transformers are used, but coils using these are large and expensive, and not at all recommended for a first time builder.
NST’s come in two basic types. Electronic or solid state transformers are much smaller, lighter, and cheaper, but cannot be used for Tesla Coils because the output is a high frequency, not 60 Hz. There are probably other reasons too. What you want is a “core and coil” type of transformer. If you dropped one on your foot you’d break a bone, so use that as a test. NST’s come in output voltages ranging from 3kV to 15kV and current ranging from 10mA to 120mA. Any voltage and current combination you come across can be used, as long as the other coil components are adjusted accordingly. Higher voltages are generally preferred, but will require a more expensive capacitor. The NST output power (Volts times Amps, or VA) will generally correlate to performance. A coil using a small NST, say 4kV/20mA, will be smaller, and generate shorter sparks than one using a 15kV/60mA NST, but the other components will also be much smaller, cheaper, and easier to build than with a bigger NST.
2. Secondary coil. Design your secondary based on your power supply (transformer) power rating. There are no hard rules here. Start by choosing a coil diameter. A low powered power supply like a 4kV/20mA NST should use something like 1-2” diameter. A 15kV/30 mA NST does well with a 4” diameter secondary, and a 15kV/60mA NST should use something like 6-8” diameter.
The collective experience of coil builders suggests that the aspect ratio (ratio of winding length to diameter) of the secondary coil should be between 3:1 for bigger coils (i.e. pole-pig powered) and 5:1 for smaller (NST-powered) coils. The wire gauge or diameter should be chosen to provide between 800-1600 turns over the length of the coil.
Most people use (and I recommend) PVC pipe as the secondary coil form, because it’s widely available in many different sizes, is inexpensive, structurally sound, and is easy to work with. Cardboard, acrylic, fiberglass, Sonotubes, and other materials are also used, but none offer any functional advantage. Some people will tell you that it’s necessary to bake and moisture-seal PVC pipe prior to using. I think that’s unnecessary and have not done that with any of my recent coils. I do coat the finished secondary coil with several coats of polyurethane, but this is just to keep the wire from shifting, for protecting the wire, and for aesthetics.
3. Top load. The top load is the thing on top of the secondary coil that the sparks jump from. The most efficient shape for the top load is a toroid – a donut-shape. Spheres are sometimes used, but they don’t work as well as toroids. The top load serves two functions – it prevents sparks from jumping from the top of the secondary windings (electric field control), and it provides the necessary capacitance to the secondary coil to match resonance with the primary circuit. Except for setting the correct secondary resonant frequency, there is no formula for the optimum top load size. The needed top load size correlates with the power supply rating – a more powerful coil needs a larger top load. A crude rule of thumb is to make the height of the toroid (minor diameter) about the same size as the secondary diameter, and the outer diameter of the toroid (major diameter) the winding length of the secondary.
The top load shape should have no sharp edges, corners, or bumps, and its exterior surface must be electrically conductive. It doesn’t matter if the top load interior is solid or hollow, conductive or not. Aluminum foil tape (in the HVAC aisle of your hardware store) is often used to cover top load forms to make the exterior conductive. Corrugated aluminum cloths dryer ducting (also HVAC aisle) is commonly used for toroids up to 6” minor diameter, though it will be fragile.
A too-small top load will break out with multiple shorter-than-maximum streamers, even if the shape is smooth. Some may like that. A too-large top load won’t break out at all. Using a breakout point - small bump or nail or piece of Al tape sticking out on the side of the toroid, may be enough to cause breakout to occur if it won't otherwise, and allows one to direct the sparks..
4. Having established the design of the secondary coil and top load, calculate the secondary resonant frequency. There are many computer programs available to do this:
E-Tesla6 is the most accurate, but that level of precision isn’t always necessary. http://hot-streamer.com/TeslaCoils/Programs/E-Tesla6.zip
Wintesla is probably the easiest to use, and is also good for all other coil calculations. http://hot-streamer.com/rscopper/wintesla5.exe
5. Tank capacitor. In Tesla’s time the only feasible capacitor dielectric construction was glass. Plate glass or glass bottle (a.k.a. saltwater caps) capacitors are still used today, but are large, heavy, messy, lossy, compared to plastic dielectric caps. See http://hot-streamer.com/greg/swc.htm.
Rolled polyethylene/aluminum caps were built and used in the 90’s and offered better performance than glass caps, but were still bulky, messy, non-trivial to build, and most report that they eventually leaked and/or failed. See http://www.laushaus.com/tesla/rolledcap.htm.
Commercial polypropylene-dielectric pulse caps (i.e. Maxwell’s) are great, but are very expensive unless you find them on the surplus market.
The only capacitor type that I would recommend to a new hobbyist is the so-called MMC (Multi Mini Capacitor) - a series array of commercially bought, small, polypropylene-dielectric pulse caps, connected so that the series-string has a high enough voltage rating to survive the tank voltages.
The power supply transformer voltage and current ratings determine what the needed capacitor uF and voltage rating are.
It was once thought that the best uF cap size was one that would resonate with the NST’s secondary winding at the mains (i.e. 60Hz) frequency. This cap value is referred to as the mains-resonant value or matched value. More recent experience suggests that a cap value of between 1.5-2.0 times the mains-resonant value performs best and is least likely to damage the transformer should the gap be set too wide, and this would be referred to as an LTR (Larger Than Resonant) cap value. WinTesla can calculate the mains-resonant value of a cap for an NST given its faceplate secondary voltage, amps, and line frequency. It’s up to you to multiply by the 1.5-2.0 factor to arrive at the LTR cap value.
The required capacitor voltage rating is determined by the peak transformer voltage. If one has a transformer with a faceplate rating of 10,000 volts (RMS), the actual peak open-circuit secondary voltage is 1.414 x 10,000, or 14,140V, so the finished MMC cap must have a voltage rating of 14,140V or higher. If each of the caps in the MMC is rated at 2000V, you’ll need at least 8 in series to give the completed cap a voltage rating exceeding 14,140V.
6. Primary coil. We so far know the resonant frequency of the secondary circuit, and we’ve chosen the primary capacitor value. There is only one value of primary inductance that will resonate with the primary cap at the secondary’s frequency, and that’s what the primary coil must be. WinTesla is very good at this. A flat spiral geometry is best, with an inside diameter 2” greater than the secondary coil form. Use either heavy gauge solid (NOT stranded) copper wire, #14 or heavier, or ideally, Ľ” copper refrigeration tubing.
The underlying principal of a Tesla coil requires that the resonant frequency of the secondary coil plus the top load’s capacitance match the resonant frequency of the primary coil plus the tank capacitor. The fine-tuning of the coil is done by experimentally choosing the number of primary turns that provides the best performance, so be sure to build the primary coil with several more turns than calculations suggest, and a means for connecting the wire to any of the outer turns as a means of “tapping”.
7. Spark gap. A static gap is simpler to build than a rotary gap and is recommended for beginners. The jury is still out as to whether a multi-segment gap or a single gap offers better performance. I favor single gaps - they’re easier to build and to adjust, and I believe they have lower losses than multi-gaps, but I can’t prove it. Several things are certain. There must be airflow directed through the gap electrodes for best performance. The electrodes must be such that the point of arcing doesn’t get too hot, so it’s good to design it so the arcing is distributed over more than a single point, and the electrodes must be massive enough that heat is drawn from the arcing area.
The separation between the gap electrodes is critical. There is no formula for setting the distance based on the power supply specs. The gap width MUST be set experimentally, such that with only the gap across the power supply (NST), the gap will _just_ begin to fire when the maximum AC voltage is applied to the NST primary. Increasing the gap separation beyond this point will improve performance, but this generates voltages on the NST secondary higher than the NST was designed to withstand and may destroy the NST.
8. Protection circuits. Protection circuits or filters are often used between the spark gap and one’s NST to protect the NST from hazards. The necessity of an NST protection circuit is unclear. For my table-top mini coil I didn’t use one, mostly because the small NST is not irreplaceable, and also because I wanted the coil to be compact. But for my big coil, my 15/60 NST is an expensive component, so I felt it worthwhile to equip it with a protection circuit. Only one thing is certain - operating with a too-wide spark gap WILL kill an NST.
9. RF ground. The base of the secondary coil, NST case, (and possibly a few other things) are wired to what is usually a dedicated ground, independent of the mains or green-wire ground present in your AC socket. This provides a return path for the RF current in the secondary streamers that does not travel through your mains wiring. An 8-foot electrical ground rod driven into the earth is often used for the RF ground, and this needs to be tied to the coil’s RF ground connection with reasonably heavy cable. The cable length should be kept to a minimum to minimize inductance. The quality of the RF ground and its connection will have little or no impact on coil performance. Rather, it affects how much RF gets coupled through your AC mains wiring, potentially damaging appliances elsewhere in your house. If a ground rod is not possible, some have used a counterpoise - a large metal object whose capacitance to the earth is a suitable ground connection. Sheet metal or sheets of aluminum foil on the ground work. But for very low powered coils, I have just used the mains ground for RF ground.
10. Mix well. Assemble the components, taking care to lay them out so that interconnections are as short and direct as possible. The loop consisting of the cap, gap, and primary coil is called the tank circuit, and its wiring must be heavy and direct. The spark gap should be wired in parallel with the NST secondary, and the cap and primary coil in series go in parallel with that. An alternate way to wire things has the cap in parallel with the NST secondary, but this is NOT recommended, as it subjects the NST to more RF. The primary coil should be elevated at least 6-8” above any large metal objects like the NST or a large cap - more so if the primary is large. The lowermost turn of the secondary should be roughly at the same elevation as the flat primary coil, but the secondary should be mounted in a way that allows its height to be adjustable, to vary the pri-sec coupling.
11. Tune for maximum performance. Resist the temptation to vary the spark gap width, and leave it set as described above. Vary the number of primary coil turns by tapping the outer turn connection, leaving the inner turn fixed. Some coils benefit by varying by a fraction of a turn, but most are not so critical. After the best primary tap is found, try varying the height of the secondary coil relative to the primary to adjust the coupling. You want the coupling to be as high as possible (secondary close to primary). If the coupling is too high, what’s called "racing sparks" will travel on the surface of the secondary coil, and if left that way, will damage the secondary coil. Increase the separation between the pri & sec coils and try again, until the racing sparks stop. Raising the top load above the top of the secondary may also help reduce racing sparks.
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