These devices first appeared around 1891, so there is no rocket science involved! You just need care and forward planing, which hopefully these notes may help you with.
These are the basic points about Tesla coil design that I picked up and found useful.
There is of course a great deal more than this which I am slowly getting to grips with. The best course of action is to join a newsgroup where you will find people who have years of empirical experience under their belt.

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

Software: Programs like those found Here and (my favourite) JavaTC help enormously. In fact I will go as far as saying that using JavaTC should be compulsory before building any new coil. Take time to learn how to use it and it will pay off. A particularly handy feature is that you can use the entry boxes like a calculator.
As an example, if the lower end of the winding is 21.375 inches above ground, and the winding itself is 20.6 inches high, you can simply enter "21.375+20.6" into the box that asks for the top of the winding's height. This will automatically change into 41.975 inches when you move onto the next data input box.
Also join a discussion group where there are people who have gained years of empirical knowledge and will willingly share this with you.

Research: As a complete novice we found the web is a mine of information. But remember these things can seriously harm you, so bearing that in mind don't 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.

Location: Consider where you going to run the coil before you build it, and size it accordingly. The bigger the coil (in output power and therefore in physical size) the bigger the space that is required to run it. My 8 inch diameter coil is too big to run indoors, so it has to be in the garden, not the best of places in the UK winter.

Transportation: Unless you can roll the coil out into the garden complete, you may have to design it to breakdown and re-assemble easily. Big coils can get surprisingly heavy when the whole thing is assembled! I can only assemble and run the coil if someone does it for me because of health issues, so in my case it had to come apart easily. But making them so they dismantle is an excellent idea for everyone.

Cost: The cost for virtually every component goes up significantly with the size of the coil that you are building. Double the size of the coil and cost might rise threefold or more! So fully plan it out with software and price everything up. I know it is boring but its worth it in the long run.

Components: The HV power source, NST, OBIT, MOT, PIG etc, and the secondary coil diameter size are totally inter-dependant to one another. So be realistic if you already have one component and not the other.

Capacitor: The total capacitance value needed is also linked to the HV primary transformer size mentioned above. Avoid having a value that is in resonance with the transformer if it is a NST you're using. (This is the only part of a Tesla where you don't want resonance)

NST: NST's in the UK and Europe have a maximum voltage of 10,000 volts. As mentioned above, avoid a resonant cap value.

Power: NST's and OBIT's etc can be run in parallel to double the current and thereby also doubling the power. 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.

Aspect ratio: This relates to the diameter versus the height of the secondary coil, as this must be within certain limits which are mainly dependant on the power available. If the height is not enough then you will get too many damaging strikes down to the primary. If it is too high the coil will be inefficient.
So starting with the diameter a good guide for low power coils below 500 Watts would be a 2 inch to 3 inch diameter.
500 to 1000 Watts would be 3 to 4 inches, 1000 to 1500 Watts is 5 to 6 inches, over 1500 Watts and your looking at 8, 10, 12 inches or more!
The aspect ratio people use for low to medium power coils (1 kw and less) is around 5:1, for medium 4.5:1 seems to be popular. This means a 5 inch coil at 1200 Watts would be 20 inches tall. While large power coils could be 4:1
These are rough figures and open to some variation.

Secondary Turns: Generally people aim for around 1000 turns in larger coils (6 inches and more) and anything up to 1500 turns for smaller power coils. The wound height of the coil is directly linked to the diameter of the winding wire that is used however.


Coupling Coefficient: The number of cycles needed to exchange energy between the primary and secondary depends on the coupling coefficient (k). A low coupling means more cycles and more losses and also the tuning can become more difficult. A too high coupling however will give the problem of racing sparks and the spark gap will have a harder task quenching. Getting it right is by trial and error.
Normally the bottom of the secondary winding, not the coil former, would be within an inch or so of the horizontal plane of the primary.






The best component to get first is always the HV power source (NST, OBIT, MOT, Pole mounted distribution transformer etc) so you will then know the power that you have available to plan your design with.

The HV power figure is very important because it not only dictates the physical size of the coil that you can build, but also the amount of capacitance needed in the primary circuit.
So using a suitable aspect ratio for the power you have available, you can work out the coils height & diameter and number of turns needed. Grab your calculator and by trying different gauges of wire, 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).
Once you know all your secondary specifications you can next decide on a size for your Top load or Toroid. The minor diameter will normally be similar to the secondary diameter, while the toroid's major diameter will be a bit less than the secondary coil height. All this then allows you to work out the resonant frequency of the secondary.
A very good guide explaining the whole design procedure can be found at the Deep Fried Neon website. Using the site above or using free design 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. This now allows you to work out the number of primary turns you require, so that when combined with the primary capacitance value it will give you a resonant frequency which matches that of the secondary. It is best to try to aim for around 10 turns on the primary as this gives a reasonable inductance value. Too low an inductance will result in bigger losses in the spark gap.

You can see how much of a balancing act you have to deal with now. 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.

The Very Basic Tesla Circuit Diagram








My Own Tesla's Circuit Diagram, with filters and safety protection


The reason for the 16v DC transformer at the bottom of the diagram is to provide power to the two switching relays. This means that the key switch, housed in a separate small box on a long lead, is only operating at a safe 16 volts DC. The two relays are capable of switching 20 amps each, but are only handling around 15 amps each at the very most.

The RFI filter would normally be used in the incoming line of a piece of equipment to protect it from any radio frequency interference that may have got onto the incoming power. In this case however it is protecting the incoming power or house mains circuit from any interference that the coil is sending back.
Normally the filters are labelled 'In' and 'Out' or 'Line' and 'Equipment' or something similar, but it is generally considered that you now need to connect them in reverse. In other words the side that was designed to connect to the incoming mains feed, now needs to connect to the coil instead.

However some people say this is incorrect, see here for a well respected and experienced coiler's views.
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 considered desirable to use a value that was resonant with the HV supply. This is now recognised to be bad practice and is usually avoided. especially when using an NST.