Tesla coil construction guide

250 watts

By Chris Swinson 2003



In this article I will give some basic steps to construct a small power 250watt Tesla coil. While the overall design is based on a 250watt coil you can run a higher power though it even upto 500watts. There is no real "exact" way to build one. For the same power input of say 250watts there can be many variations and many designs to build one. Bascially they will all do the same thing. Different people have thier own "rules" to build coils. Here I just give my own basic setup. You can improve and change things as you see fit.

This article will provide a rough guide only. It is not intended as a compleate or acurate construction plan. While everything here is based on my own working coils it does not mean they are the best for your design. Build this coil at your own risk. there are many variations on building Tesla Coils. While this page is based upon a working coil it is not necessarly the best designs.

How it works.

The wiring diagram is very simple to follow. From left to right we have the NST. The HV out is connected directly across the capacitor. One end of the capacitor goes to the primary. The other end goes to the primary via the spark gaps. That is bascially it! Once the NST charges the capacitor the voltage accross the gaps rise and fires. 100's of peek amps from the capacitor are shorted out across the primary which provides a strong pulse. this pulse is then passed onto the secondary onto where the high voltage is developed.

The actual operation is more complex and is beyond the scope of this article. many sources can be found for indepth explinations on the Internet.



Secondary spec

The best part of any coil is to build the secondary. Normally this is the hardest thing to wind. Though its not as hard or as time consuming as you may think. You can see from my Secondary construction page that I only have the very basic setup to wind my coils and I have done all my coils the same way. On that page I wind a test coil and it is only to give you a rough idea on how I wind my coils. Don't wind them to the spec on that page!

For a solid basic coil which I have used on 250watts upto 500watts I use around 40 to 50" high , 6.3" dia form. You could use 4.3" though you might want to add a few more inches onto the height since the tube is not as wide you will need to make it a little taller. I wind my basic coil with 0.4mm enamaled copper wire. You need to get a 2.5Kg reel which you can probably obtain at a good price. About 1Kg of 0.4mm wire will be needed though its best to get a bigger roll to have some spare. The form I use is underground waste pipe which is a brownish colour. You can buy this from just about any DIY shop or builders yard. It is a thick pipe at around 5mm wall thickness. You could also use 2.5" dia tube at around 60" high though it may not preform as well as a wider coil. I've tested a lot of coils out and I only use 6.3" dia coils now. It is of course upto you which you use. They will all give good results though in some cases different variations of coil size can effect output.

It is hard to to give exact coil dimentions since it depends greatly on what power input you have. My 50x6.3" coil works well for lower power such as 500watts. It will also work well for 250watts though building a slightly smaller coil may work better. I would expect the same coil be be able to run at 1000watts input with no problems. Please don't use anything else for the coil form but solid waste pipe. Cardboard is not a good idea, its a bad fire risk for one reason. Its also not normally strong enough to support the weight of the wire. To get a basic idea on how I wind my coils got my coil winding page here .

Don't forget to varnish the coil when you done. I use polyurathane varnish or so called "yaut varnish" which is clear and comes in either spray or paint tins. When your done winding you coil fix both ends with some tape to stop it from undoing when you move it. Theres nothing worse than spending hours winding a coil for it to unwind when you move it in seconds! The best way to paint it with varnish is to apply a small coat to start with and leave it for 24 hours to dry before adding another coat. If you add too much in one go then only the outer part of the varnish will dry and leave the inner varnish unset.

I normally give 2 faint coats then 2 larger ones afterwards. It normally takes a week to finish varnishing a coil properly. You can keep adding coats on if you like to totally cover the wire. Its not necessary to do this. The varnish is only really used to hold the wire in place. It does also offer some insulation to the wire which can help coil preformance.

When your coil is wound you may want to unwind the top and bottom of the coil a few turns. I normally do this since it can be hard to wind on and wind off the wire. the first few turns on my coils normally spaced out or overlapped. this is normal and it can be a good idea to wind your coil a inch or 2 longer than you need to make sure you have a good start on your winding. Having spaces or overlaps in the wire is a very bad idea. If you overlap your wire or have spaces in it then start over again. If you have spaces you may well have arcing problems along the sides of the coil.

For the bottom end of my coil I drill a small hole normally 3mm or so and fix a nut and bolt there. I strip off the enammel off the wire and wrap it a couple of times around the bolt and hold in place with some wasers. this will then provide my ground connection. Care must be taken since the wire is thin it can break easy. Its best to have the bolts length inwards not sticking outwards from the coil. If its sticking outwards then it could be too close to the primary coil and cause arcing.

For the top end of my coil I strip off about 20" or so off the wire. This bard length of wire will become my contact for the top connection of the coil. I run some 1" wide srip of sticky tin-foil around the top of the tube. then I taken 3 or 4 strip across the diameter of the tube to over the hole. You may want to fix a circle of metal for the top to make it strong since the foil can break easy.


Here you can see a range of coils I have wound. You can also See the foil on top. It is important to make the foil as neat as possible since sharp edges can spark and even catch fire. A better choice would be to cut a circle of metal out and glue it over the top of the coil with expoxy glue. The wire would then be fixed to a bolt in the center of the metal which can also be used to fix a toroid on op with. the foil is a good quick solution though I wouldn;'t use it for higher power coils since it could be a fire risk. It is however easy to place toroids ontop of the coil or anthing else like a screwdriver as show. Notice the end of the winding is near the top of the coil. Its a bad idea to have the coil winding finish more than 1 or 2 inches from where you will mount your toroid.

You could of course make the coil half the size if you wanted too. Though I have found for the tests I have done that the coil shown was the best "all round" solution for the range of power levels I use. output could be better or worse it depends on your primary and capacitor values. You may want to build a few sizes of coils to see which ones work better for you.

Primary construction.

The primary can be tricky and time consuming to build though it is easy to make. The image below shows my large flat primary winding. The tube used in this was 8mm dia copper pipe. It comes in 25meter lengths which is soon used up as you can see! The primary is about 1 meter across in size. I have 8 supports which are made from 1"thick plywood. The are 1" wide. The spare between turns of the pipe is around 10mm or 0.38". You should be able to just put your small finger in bewteen the turns. Any closer and it will arc between the turns of pipe.

I used around 100 7mm cable clips to fix the tube in place. This may look nice and neat though be warned that the voltage can arc from the tube via the cable clip nails into the wood and arc inside the wood to the the next turn. this is another reason why its good to make sure you keep the spacings fairly wide. I wouldn't make them more than half a inch apart. You may notice that the inside of the primary is cut to match the 6.3dia of the secondary pipe. Its a friction fit inbetween these 8 stips of wood. You must however remember to leave about 1 inch free on the base of the secodnary coil else it wont fit and you will damage the wire.

Also note that my primary does not start at the beginning of the 8 strips of wood. this is to space out the primary from the bottom turns of the secondary form. If they are too close then the primary will arc to the base of the secondary coil since it will be grounded. Its good to leave about 2inches spare here to stop this. You must also remember that your ground connection on the secondary might also be a problem so keep its length at a minimum. You will also want to drill a 10mm hole in the center of the primary to root your secondary ground cable though. You may also want to add some form of feet to raise the primary off the ground to make room for the cables.

The inner end of the primary I place some thick dia wire about 1 foot long into the inner turn of the pipe. this gives a good conduction area for the cable. I sqush a few inches on the start of the pipe to hold the wire in place.

See my primary construction page here for more information.

Primary capacitor

The capacitor is probably the hardest thing to construct out of the whole coil setup. While its design and actualy construction is probably easy ( but time consuming ) it does take a lot of abuse so making sure you get it right first time can save hours of work. See the primary construction page on how to build a simple flat plate capacitor. It took me months to build a capacitor which didn't fail in normal use. While 1mm of plastic seems enough it probably wont be. I used styrene plastic 1 mm thick (2 layers). I wouldn't use anything thinner than 2mm thick for 10KV since it will puncture easily. I also sink my capacitors in transformer oil. this helps to boost the capacitors life and preformance. Though be warned that this stuff smells really foul and you dont want to get it on your hands! The oil when its buring puts off foul fumes which can casue vometing.

You dont have to use transformer oil, though if you dont then you might wish to add another 1mm of insulation to your capacitors to be on the safe side. There is a drawback on adding more insulation and thats the capacitance value will go down half each time you double the size of the insulation. My large 9x9x6" capacitors are only 3nf each. Though I have 2mm insulation under oil and have ran than upto 18KV with no problems. You might be safe with 1mm of insulation with 8Kv though for 10Kv or over use 2mm.

I use normal tin foil from the supermarket. Not the cheapest kinda, buy the extra strong stuff! Cutting tin foil is a nightmare at the best of times eithout having it rip on you everytime you move it. You can of course double up on the foil to make stronger layers if you wish. Cutting 100's of strips of tin foil will drive you loopy so spread it out over a few days.

Once you have all your layers put them together as shown in the primary construcion page above. You may want to build several smaller capacitors then one big one. then should one fail you only loose 1 and not the lot! Its also a good idea to buy a cheap capacitor meter to measure the value. If you follow my design steps then each capacitor will be around 3nf. I have 4 of these which makes 12nf total. I can of course change my connections for 3,6,9,12nf. 6nf and 12nf being about the limits for a 250-500watt powered coil. I doubt you would need anymore capacitance than 20nf. In most cases 10nf will probably be the best solution. Having a large primary means you can tune a wider range of coils without having to alter the capacitor.

The value depends on what transformer you use and how many turns of the primary coil you wish to use. For my coil I use 12nf and I tune in my coils near the outer turn of the primary. If you built a half sized secodnary then you would need less primary turns and/or less capacitance. While values are not exact you need some room for error so having the option of lots of primary turns and smaller capacitors means you can adjust to find best preformance.

Don;t underestimate the stress on these capacitors! the peek voltages on them can be double the transformer voltage. Make sure you have no 2 connections closer than 2 inches else the WILL arc and destroy hours of work. Since Tesla coils run at high frequency it can cause the arc to travel across the plastic as I found out in some tests. Even though I couldn'y get the trasformer to arc more than about 10mm they still managed to arc over 30cm of plastic. These capacitors really do take a beating in tesla coil use.

You can buy high voltage pulse capacitors though these are VERY expensive. I managed to drop on some smaller paper rolled capacitors for about £100 in total. this gave me around 10nf at a high voltage. These work well. You can see below a picture of my 4 caps conected up and placed on a shelf out of the way.

Transformer rating.

The Transformers used normally are Neon Sign Transformers, or NST for short. I have a few of these and I have 2 10KV 27ma types. I wired these in parallel to give me around 60ma current at 10KV. Its not really a good idea to to use 2 NST's in this way though I've not had any problems in doing this. With 1 NST with the coils I suggest I can gain at least 18" arcs. the best Ive had from 1 NST input is about 24". With both of them running im not sure what the arc length would be since the room is only very small. It could probably hit around 40" or so.

I suggest you start off with a small transformer of around 10KV 25ma. Should something fail then its better to fail at a lower power level than a higher power level which could result in costely damage. Make sure you get the "current limited" types. Tesla coil use is not good for the normal types and the NST could may well fail.

Spark gap

The spark gap is what will be used to discharge the capacitors power to the primary. The peek current can be 100's of amps. They will get VERY hot even after a few seconds of running. See here for my rotary spark gap pictures. My basic spark is as showen below.

A rotary gap is normally better in most cases than a "static" spark gap as shown in the picture. This spark gap was built with 12 2" long 1" wide copper tubes. These were bascially the "end to end" joins for joining 2 copper pipes together. They were all seperated with 1 thin card layer , the card actually came from the box which the tin foil came in. They are all friction fitted between 2 strips of wood. I used some rubber matting each side to grip them. Since the rubber can move I glued them into place and let the card strips between the tubes until the glue had set. I drilled though each end of the wood and fixed a large nut and bolt to sandwitch it all together. The wood actually bowed in the middle so I ran a 3rd bolt though to hold the middle pipes in place.

I used 2 smaller battery terminal clips for the wires so I could alter the number of gaps I wanted. The downfalls of this gap are that it needs a constant air blow to cool it down. The copper gets warm and seems to expand a little which shorts out the small gaps between them. A air flow is needed to keep clean cool air flowing though the gaps. While this spark gap worked it only was about half as effective as the rotary spark gap. I dont recommend building a rotary gap for your first coil due to the mecanics involved. My RSG is only a test type and is far from compleation.


The toroid is where the arcs will come off on the top of the coil. You can buy ready made Toroids though they can be expensive and hard to find. a cheaper solution is to buy a meter of ali-ducting and form a circle with it. The type I use is about 8" in dia and corrigated so it can be bent into a circle easily. You can see some of this elsewhere on these pages. The size of the toroid can effwect spark length. The bigger the toroid is then the higher the voltage needed to break out from it will be. A smaller toroid will not rise to a high voltage and will arc a a much lower voltage.

The toroid is like a capacitor. You chanrge it up with voltage and when its full it lets out all that power in one arc. To small and it will arc to soon. too big and it may not even get fully charged and not arc at all. A smaller toroid is better for this coil suggested. I have a made toroid which is about 14"x6" which is a perfect size. The ducting is about 30" in dia though it still works well.

Connections and wiring.

Wiring is very inportant. Keep all wiring as short as possible. Only use sold strand cable. I used 30amps ring-main cable and stripped it down into red and black lengths. You can use this type of cable though it is NOT rated for high voltage use. The insulation can only withstand around 600volts so if you use this cable keep it away from everything. 10Kv NST cable is not expensive. It best to use this for your connections. Please note that the capacitors devilver a lot of power so keep the cable short and thick.

For my primary connection I used a car battery charger glamp. this fits nice round the primary and is easy to move and gives a good connection. For the secondary ground connection use a a thick cable. The thicker the better and the shorter the better. Never connect this ground to radiators or the house ground. Drive a 2 foot metal rod into the garden and use this as your ground connection. My coil is run on the 1st floor of my house and I ran the ground cable up the side of the house to the room. I don't recommend doing this however.

The on/off switch is VERY inportant for when you turn on and off the NST. I used a switch mains rated which was like a large relay with large contacts rated at 5KV contacts at 10amps. While this works fine for turning on and off the coil it does still manged to give a nasty shock from it. the contacts are about 2 inches under the push button and I can get shocks from it. Whatever switch you use make it a good one and make it a momentery action switch. Never use a latching switch it can be very dangerous and foolish to do so. Extend the switch shaft by at least 6" and use this to turn the push switch on. The back EMF from the NST can be rather nasty on turn off. Make sure your no where near the mains connections!

If you use filters or anything than make them high power ones. I had 3 10amps filter and blew them when I turned off the NST power. Having 300V spark gap suppressors across the mains will help to limit this problem. You could also use high power voltage supressors thought hey will need to be high current types. Spark gaps offer the low cost solution though they can blow the mains fuse, which in a sence is good. Never turn on the coil with the main wall switch. The Voltage comming back from the NST can arc though the switch to your finger.


Keep the coil far from any mains equiment and house wiring. Keep it away from computers and TV's and radios. I mnaged to blow the TV a few times without realising that the arial cable runs up the side of the house near to where my coil was running. You CAN pickup voltage spikes with any cables which run near to the coil. Keep it WELL AWAY from any cables and equipment.

When you turn on a coil keep well back from it. Should something fail you dont want to be anywhere near it! Always wear ear protection. The sparks are very loud and it will hurt your ears without protection. Never look at the spark gap while its running. It is very bright and it will hurt your eyes.

Never touch or adjust anything with the power on or even connected. Always unplug the coil from the mains before altering anything.

Remember the capacitors can contain charge even when the power is turned off for sometime. Make a discharge device to short them out before you tough anything. a metal bar on the end of a long broom handle is all is needed to short them out to make sure they are dead before you make your adjustments.


Other good Tesla pages and links.

http://www.richieburnett.co.uk/tesla.shtml Very good site on just about everything Tesla coil related. Also SSTC work!

http://hometown.aol.com/futuret/page1.html John Freau's page ( where I buy my toroids from).