TeslaMap Bad wire

Tesla coils have killed people.

Please make safety a top priority when working with your Tesla coils.

When working with Tesla coils it's likely you'll be exposed to very high voltages and currents, charged capacitors, exposed wiring, strong electric and magnetic fields, induced currents, fire dangers, chemical and explosion dangers, ozone, ultraviolet light and loud noise.

When running a Tesla coil be sure to have fresh air, hearing protection and do not look directly at the spark gaps. Make sure that people or animals cannot inadvertently enter a dangerous area. Try not to work alone and never work when tired or under the influence of alcohol, drugs or medications. Have a fire extinguisher and safety glasses near. Tesla coils may interfere with pacemakers.

This is a list of guidelines found at The Tesla Coil Mailing List:

The arcs generated by a Tesla coil are dangerous. You may have seen people touching the arcs or shooting arcs out of their fingers, but they are experienced experts using carefully controlled conditions. Without proper precautions the arcs can easily burn or kill you. The "skin effect" will offer some protection, but not complete protection all of the time. You can look, but do not touch!

The NST is especially dangerous because it supplies several thousand volts and you'll be working in close proximity to it. It's easy to accidentally leave it turned on. Except for a very quiet humming, there's no indication it's turned on.

Tesla coils can generate strong RF interference. It's usually not a problem, but it is a potential problem. Generally the FCC does not like RF interference and laws are in place that make it illegal to produce RF interference. The interference can be reduced or eliminated with proper grounding and the use of a Faraday cage.

I've killed several electrical devices that were near my Tesla coil. Out of an abundance of caution I recommend that you unplug everything in the house and move anything electrical away from the area before running your Tesla coil.

It's a good idea to clean off any dust that may have accumulated on your components before operating your Tesla coil. Dust can allow shorts to develop.

A good Tesla coil safety document is available on the site.

All about Circuits - Electrical Safety is a good source of information.

Adjusting Gaps

The widths of all the spark gaps in the Tesla coil will need to be carefully adjusted for optimum performance. The proper width for a spark gap will depend on the electrode size, shape and surface finish. Other factors like humidity and air pressure will also affect the proper gap width. If you're using a variac I recommend setting the gaps while supplying full voltage. I've never used a variac in my Tesla coils. In fact, I don't think they are needed and can cause problems.

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I recommend disconnecting the NST output from the Tesla coil and connecting each spark gap directly to the NST output. This will eliminate voltage spikes and things that made it difficult to set the spark gaps. This also allows you to open the gaps without blowing up your MMC array.

You should start with the safety gaps. To adjust the safety gaps you should reduce the gap width to a very small width (about 0.1 inch or 2mm). Turn on the NST and verify that the gap is shorting. Unplug the NST. Increase the spark gap width an additional 0.01 inch or 0.3mm. Turn on the NST and check the gap. If it's still shorting then increase the gap width an additional 0.01 inch or 0.3mm. Repeat this procedure and continue to increase the gap width until the gap no longer shorts when the NST is turned on. Follow the same procedure for the MMC protection gap and any other safety gaps in the Tesla coil.

Once the safety gaps are properly set you can adjust the main spark gap. This procedure only applies to static spark gaps, not rotary spark gaps. Open the main spark gap width so that it is too wide to short. Connect the main spark gap directly to the NST output. Turn on the NST and verify that the gap is not shorting. Close the main spark gap width about 0.1 inch or 2mm at a time until it begins to short.


Do not open the main spark gap any wider than the proper width (described above). Doing so will make your NST much more susceptible to damaging voltage spikes.

It's possible that corrosion on the spark gaps could alter the optimum gap width. I recommend that you periodically check your spark gaps, clean them and adjust them as necessary.

Adjusting a rotary gap can be a bit more challenging. The gap firing will have to be synchronous with the charging voltage peaks. This means that the rotating electrodes will have to line up just as the voltage reaches it's positive or negative peak. Typically the sync motor is rotated relative to the stationary electrodes until the best performance is achieved. You can also rotate the disk or propeller that's connected to the motor. John Freau has created a sync gap electrical remote phase controller that allows you to electrically adjust the motor phase while the Tesla coil is running. You read more about it on the Tesla's Legacy website, written (I think) by P Tuck. Be sure to use motor "run" caps in his circuit.


Before you run your coil you'll need to tune it. Tuning refers to the process of adjusting the resonate frequencies of the LC tank circuits to the same frequency. The coil must be tuned to produce the longest possible arcs. Usually the inductance of the primary coil is adjusted because it's the easiest component to adjust. I recommend making the tap at the innermost turn permanent, and adjusting the tap near the outer turns of the primary coil. The TeslaMap program can be used to get a good idea of the number of required turns on the primary coil. The typical tuning procedure is to tap the primary at the suggested number of turns and run the coil checking for the arc length. Adjust the tap point 1 turn (in either direction) and run the coil again to check arc length. If the arcs are longer then you're moving in the right direction. Make smaller changes as you get close to the best tap point. Adding a pointed object like a thumb tack to the top load can help the arcs breakout and make arc measurements easier.

At this point you should be ready to run your Tesla coil. There's always a chance that voltage spikes could find there way back into the house wiring. I recommend unplugging all electronic devices in the house before running the Tesla coil. I've always left large appliances (refrigerators, washer, dryer, etc) plugged in and they've never been damaged. Remember to be safe!

Fun Things

So what cool stuff can you do with your Tesla coil? A few ideas come to mind:

Setting various things on the top load can be fun. Try a banana or an extended tape measure.

Setting a grounded target (like a metal pole) close to the top load gives the arcs something to strike. Arcs hitting a grounded target will be brighter, thicker and a little longer.

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Take some pictures. Film cameras seem to work better than digital cameras. Try adjusting the exposure to about 1 seconds. Long exposure times will require a tripod to prevent blurry photos.

Hold or place a fluorescent tube near the top load (but not close enough to get struck). It will light up without any apparent power source.

Make an ion propeller. Make a propeller shaped like an "S" out of aluminum foil. Bend the tips up a bit. Attach the propeller to a thimble or something similar. Set the propeller and thimble on a vertical needle or nail. Be sure the propeller is balanced and spins freely. The force of the arcs shooting out of the propeller tips will cause it to turn rapidly.

If your wife's cat is causing trouble... (just kidding!)


Unfortunately it's not uncommon to have problems with a Tesla coil. This is a list of things you may want to check:

Generally the first thing you should do is double check your wiring and connections.

If the wiring looks good and you throw the switch and nothing happens, then start checking the power supply. Is it plugged in? Is the fuse or circuit breaker blown? Do you have a GFCI circuit in your NST? If you have multiple NSTs in parallel, are they wired in phase? You can easily check all this by disconnecting the outputs of the NST filter from the rest of the Tesla coil and reconnecting the NST outputs to a spark gap with a narrow gap distance. If you get an arc from the NST filter outputs then the power supply section is working.

If you have a variac, you can try removing it from the circuit. I've never used one in my Tesla coils.

If the power supply area looks good then move on to the spark gaps. Be sure to follow the adjustment procedures in the spark gap adjustment sections. If the primary gap (a static type gap) is not firing constantly then you can reduce the gap width a bit and see if that helps. If you have a rotary gap you should make sure the electrodes are lining up just as the voltage reaches it's peak. The adjustment procedure is covered in the spark gap adjustment section. It's normal for the safety gaps to fire every few seconds, but anything more than that and you should reset their widths, also covered in the spark gap adjustment section.

If the main spark gap is shorting correctly but you're not getting any arcs from the toroid then you should place a small, pointed metal object (like a thumb tack) on the side or top of the toroid. This is called a break out point and will help the arcs break out of the top load.

You may want to try moving the primary coil closer to the secondary coil to increase coupling. Usually lowering the secondary coil is the easiest way to adjust the distance, although raising the primary is also an option. Generally the bottom windings on the secondary coil should be just above the inner primary coil winding. If you see arcs running up and down the secondary coil then you have over-coupling (or poor RF grounding) and you should move the primary and secondary coils apart.

You can replace your top load with a smaller top load. This will make it easier for the arcs to break out, but you'll have to re-tune the coil.

If you still are not seeing arcs from the top load, you can place a fluorescent tube near (maybe 1 foot away) from the top load. If it lights up (even dimly) when the Tesla coil is running then your top load is generating an electromagnetic field, but the field is not strong enough to allow arcs to break out. A smaller top load (or more input power) will help the arcs break out easier.

You should check the MMC. Make sure none of the caps have blown, check all the connections and use a capacitance meter if you have one.

You may want to check with the experts at the Tesla Coil Mailing List.

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If you see some arcs from the top of the secondary coil, under the toroid, it means the toroid is too high above the secondary coil. Generally the bottom of the top load should be about even with the top windings on the secondary coil. You can cut about an inch off the top of the secondary, add a smaller toroid below the original toroid or redesign the toroid to sit closer to the secondary coil windings.

You may be tempted to test the Tesla coil without the secondary and top load in place, but it's generally recommended to keep the secondary coil and top load in place.

Be patient and don't give up. Tesla coils can be very finicky. I spend more time fiddling with my Tesla coil then I spend running it, that's half the fun.

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