As these devices first appeared around 1891 there is no rocket science involved. I knew very little of them a while back and I only have both a basic electronics knowledge and academic background. If I can build one anyone can. So this page is just a collection of tips about designing coils that I have picked up from the web or other coilers, and I have found useful.
Also this information is for the 'old-fashioned' type of coil using conventional spark gaps, as I will leave the latest trend to solid state coils to others.


The basic points to consider:

Balance: Everything centres around getting a balance. Any one particular aspect of the design is either affected by something else, or it in turn will affect something else itself. So before you change anything, think of the consequences.

Software: Programs like JavaTC help enormously.

Research: Remember these things are dangerous, so never take one single website as your guide. Viewing various sources lets you see what advice is correct, what is wrong, and what is just plain dangerous. Hopefully the latter does not apply here.

Location: Consider where you're going to run the coil before you build it, so you can size it accordingly. Have I followed my own advice - No!
My 8 inch diameter coil is too big to run indoors, but running it in the garden is not the safest of places as you can see.

Transportation: It is a good idea to design it to breakdown and re-assemble easily.

Cost: The cost for virtually every component goes up significantly with the size of the coil that you are building. You can easily spend £500 just on capacitors alone on medium to big coils, so fully plan it out with software and then price everything up.

Components: The HV power source, NST, OBIT, MOT, PIG etc, and the secondary coil diameter size are totally inter-dependant to one another. If you wind a big secondary you need 'big' everything else!

Capacitor: The capacitor size chosen needs to be capable of being recharged within milli-seconds, so bigger capacitance values demand higher wattage power supplies.
If your using an NST avoid having a capacitance value that is in resonance with the transformer's inductance. NST's, unlike other power sources, can be rather fragile and will not handle a high resonant rise in voltage. (Don't confuse resonance here with the resonance between the primary and secondary coils.) If though you are using a 'PIG' , resonance can be used to your advantage however.

Resonance:The AC equivalent of 'resistance' is reactance. With AC if the reactance of the inductor matches the reactance of the capacitor, the two cancel one another out, this is resonance. Then with little resistance left in the circuit, apart from the wires themselves and the internal resistance of the power supply, the voltage across the capacitor can soar.

NST: NST's in the UK and Europe have a maximum voltage of 10Kv, or 15Kv in the USA.

Power: NST's and OBIT's etc can be run in parallel to double the power. Multiple NST's however cannot be run in series to double the voltage though. Just isolating the cases is not enough, it is more complex than that.

Secondary Diameter: A good guide for low power coils below 500 Watts would be a 2 inch to 3 inch diameter. While 500 to 1000 Watts would be 3 to 4 inches, 1000 to 3000 Watts is 5 or maybe 6 inches, over 3000 Watts and your looking at 8, 10, 12 inches or more!

Aspect ratio: The secondary diameter versus the actual height of the winding itself. The aspect ratio for low to medium power is around 5:1, for other sizes 4 to 4.5:1 seems to be popular. The actual former on which the coil is wound needs around 2 inches unused at each end, so add that measurment on afterwards.

Secondary Turns: Nowadays people aim for around 1200 / 1400 turns or even up to 1600 turns for smaller power coils. The 'golden' figure used to be 900 to 1000 turns, and even today there is debate over this matter.
It's important to remember that these are not like conventional transformers where the number of turns affects the output voltage.

Coupling Coefficient: The physical proximity or coupling coefficient (k) between the two windings affects the number of cycles needed to exchange all the energy between the primary and secondary. A too high a coupling will give the problem of racing sparks, while too low a coupling means more cycles, and may result in more losses in the spark gap.
I used to over-couple my coils but after advice from more experienced coilers I found that sometimes under-coupling can actually produce bigger sparks. Under-coupling can't damage the coil, but over-coupling certainly can!

Normally the bottom of the ACTUAL secondary winding (not the coil former) would be within an inch or so higher of the horizontal plane of the primary.






The best component to get first is probably the HV power source (NST, OBIT, MOT, PIG) so you will then know the power that you have available. The HV power figure is very important because it not only dictates the diameter of the coil that you can build, but also affects the amount of capacitance.

Once you know the power you decide on the secondary diameter and aspect ratio. Next having decided on the number of turns needed you reach for your trusty calculator.
By trying different gauges of wire, try to find a combination that gives you all the requirements your looking for (number of turns, within the height and diameter constraints that the aspect ratio allows).
You will normally find that the inductance value of your secondary design will work out satisfactory and you need not concern yourself with it at this stage.

Once you know your secondary specifications you decide on the 'top load' or Toroid size. Its minor diameter will normally be similar to the secondary diameter, while the toroid's major diameter will be similar to the height of the secondary winding.
A very good guide can be found at the Deep Fried Neon website. Also using free software like JAVATC is undoubtedly the best way of seeing how one component affects another.

As already mentioned, knowing the HV power source specifications (voltages and current) allows you to choose a suitable primary capacitance value. Small coils will use 10 to 20 nano farad (nF) up to very large 10-15KW coils needing up to 150nF. A bigger cap means more energy discharged into the secondary, but you MUST have sufficient power from your transformer source to fully recharge the cap in the short (milli secs) time period available.

The next stage is to work out the number of primary turns you require. It is best to try to aim for around 12 to 14 turns to be available (allows for adjustment) on the primary and aim to have your tap at around 10 turns if your using a static gap.
With a rotary you can use less primary turns if you wish. The reason is that lower turns will mean more current flowing and a static gap with several spark interfaces would loose more power. A rotary uses a much smaller gap and also has less interfaces so it's not so much of a problem.

An 'interface' occurs where the spark leaves a solid and enters air and vice versa. So a single air gap will have two interfaces and two places for losses to occur.

Getting the primary coil's geometry correct so you will tap it in the correct place is best done using the JavaTC program. The inner diameter coil of the primary wants at least an inch clearance around the secondary coil's diameter for small secondaries. While a 6 inch secondary coil may have 1.5 inches or so. My 8 inch has a 2 inch gap. This spacing also affects the 'k' coupling value.
Most people use small diameter copper tubing for the coil. The spacing between each turn being at least 1.25 times the diameter of the tube you're using to avoid 'flashover' between turns and leave room for a clamp onto the tube.
Fortunately if you follow those guidelines you should find the primary ends up with a reasonable inductance value all of its own accord.

If you don't balance things correctly the coil will still run, but the performance will be below that possible from a well designed coil, and it may damage some of your components.

The Very Basic Tesla Circuit Diagram



Tesla coils nowadays have very little real use in main stream science so any building and research is nearly all done by amateurs. The result is that some issues are ongoing and yet to be resolved. A good example was the size of the capacitor needed. At one time it was always considered desirable to use a value that was resonant with the HV supply. This is now recognised to be bad practice with NST's for the reason mentioned above.