8.1 Tuning


by: Mike Hammer

Several people have asked me via email and on the IRC Tesla channel about how to go about tuning a coil and the steps to take.

So I thought I would share some info on the subject with the list.

Once a Tesla coil is set up and ready to fire, certain steps should be taken before power is applied. Don't expect to throw full power into a new coil or a newly rearranged system. If you do you may damage components. The coil must be brought into tune and up to full power gradually.

The use of a variac is highly recommended for controlling the input voltage to whatever step up transformer you may be using. (neon, obit) Never throw full power to a coil until you are sure the system is in proper tune.

Set the primary circuit up first. Double check all of your connections. Run your grounds and double check those. Never take any connection for granted. Set the secondary into the primary and attach the RF ground. Use a small discharge terminal at first. A small spherical terminal works well at this point. A smaller terminal will break down and produce spark at a lower power level than a larger terminal. This allows you to search for the proper tune with less stress placed on components. If you dont have a smaller terminal and only have a large toroid place a point discharger on the toroid. Sparks will easily break out from such a point. Often times a well placed thumbtack will work well.


I can't overemphasize this point. Don't run a lot of gap. This will limit the peak voltages in the primary to low levels while things are out of tune. As things are brought closer to resonance the gap can be opened a little at a time to increase the primary energy level.

Apply a bit of power until the spark gap fires. If no spark breakout is seen from the discharge terminal don't worry. Use a flourescent bulb as an indicator. As the coil gets closer to resonance the farther away and brighter the bulb will glow. Turn off the coil and move the tap connection on the primary coil a half turn and check the bulb brightness. Then move it a full turn in the opposite direction and note the brightness of the bulb again. This way you can determine the direction the tap needs to be moved to locate the proper tune.

Open the main and safety gaps up in steps. Apply the same voltage input (low power) and check for discharge. Once spark has begun to break out of the discharge terminal use it as a visual indicator of tune.

If you find the best tune point is with very few primary turns then your primary cap is too large. Adding a larger toroid to your secondary will lower its resonant frequency and allow you to tap in more primary turns also.

If you get tuned all the way out to your last primary turn and you still haven't found the resonant point then your primary cap may be too small or your toroid too large. This often happens with beginner coils wound with far too many turns of small guage wire. The large number of secondary turns causes the secondary resonant frequency to be very low and beyond the range of your primary circuit. Cutting down the secondary to no more than 900-1000 turns will help here.

A system in proper tune should not break down the safety gap often. The safety gap once set properly should not be too active. Once the coil is in good tune the safety gap electrodes may be opened to the point where it only fires occasionally.

If your safety gap is firing continuously then you may be out of tune or you may be running too wide a main gap. Try closing down your main gap slightly and retuning for best output.

Hope this is helpful.

As always comments, criticism and large sums of money welcome.

The only thing I would like to add to that is you should always work towards having as many primary turns tapped in as possible. Higher primary inductances improve gap operation and more fully envelope the secondary with its magnetic field.

One thing you can do to allow for more primary turns include adding a larger toroid to the secondary. This will give several benefits. The lower resonant frequency will allow for more primary turns and the larger capacitance represented by the toroid will give longer hotter sparks. Plus lower frequencies give longer sparks due to the lowering of several loss factors like corona loss and loss due to skin effect.