I'm currently working on my first Tesla coil, and to my dissapointment, after completing it, the Coil does not function. I cannot figure out what went wrong.

I have been told that an MOT wont produce a high enough voltage, but even when the spark gap is almost no gap, I still get no sparks.

I am using the schematic found HERE (except with 2 capacitors in parallel)

I am also using 2 different gauge enameled wire for the primary coil, so I don't know if that is a problem! Here are the photos of it, and thanks for the help in advance!

Picture of Tesla Coil Transformer Spark Gap Capacitors

  • \$\begingroup\$ What was your test strategy? Have you been able to test any pieces of it, and demonstrate that they are working? \$\endgroup\$ – gbulmer Sep 8 '14 at 23:55
  • \$\begingroup\$ Um, let me get back to you on that, I know the power is working, and the transformer is good, but I'll make sure the other parts are connected. \$\endgroup\$ – Cameron Sep 8 '14 at 23:57
  • \$\begingroup\$ It might be safer to test parts that are not connected! How do you know the power is working and the transformer good? \$\endgroup\$ – gbulmer Sep 8 '14 at 23:59
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    \$\begingroup\$ My recommendation: STOP! You clearly don't have the experience or understanding necessary to build a working Tesla coil. There are so many ways this will fail, some catastrophically and even lethally. Pick another project. Tesla coils are complicated, and you're not going to be able to make one with your current understanding. Best case: It will continue to not work. Worst case: It will KILL YOU! \$\endgroup\$ – DerStrom8 Sep 9 '14 at 22:42

That is a disaster waiting to happen. Here are just a couple of the problems:

  • You can't use galvanized steel bolts for your spark gap. They'll quickly overheat, erode, and the wooden base will probably catch fire. Also, the zinc plating will burn off in a cloud of toxic fumes. Try a Richard Quick gap or a sucker gap.
  • You can't use microwave oven capacitors as your primary capacitor. They can't handle the high peak discharge currents and will explode, showering the room with hot oil.
  • The hookup wire you've used to interconnect the tank components is too thin.
  • The primary (the ~6 turn coil made from copper tubing) is probably tuned incorrectly. You need a movable tap.
  • The primary-secondary coupling is too high.
  • A single MOT has too low of a voltage. It's not impossible to build a working Tesla coil with a single MOT, but it's quite challenging and from the pictures it is very obvious that you don't have the experience needed.

I am very worried for your safety. You should do more research, starting with this safety guide: http://www.pupman.com/safety.htm You should start off with a different high voltage transformer - a 12 kV, 30 mA neon sign transformer (NST) will be both much safer and easier to get working. MOTs take no prisoners.

  • \$\begingroup\$ To add to using the wrong capacitors, at least the top one appears to be just shorted across, so they're not hooked up correctly at all. \$\endgroup\$ – Connor Wolf Sep 9 '14 at 5:04

I strongly agree with what was said above,

  1. The capacitors from microwave ovens don't like being used for anything other than a microwave ! They will almost always expload in tesla coils !

  2. Your secondary needs to be redone completely to see any efficiency ! Re-wrap it around a WHITE PVC pipe (as grey or black pipe contains carbon making it very conductive at high voltages.) with 20-24 gauge enamelled copper magnet wire. Pick ONE gauge and stick with it ! Otherwise you will have massive amounts of corona discharge when your coil gets going.

  3. You will want a ballast for your microwave transformer otherwise you run the risk of damaging other electrical circuits in your house/blowing a breaker or fuse.

  4. One transformer alone will not cut the power requirement... A MOT (microwave oven transformer) only produces ~2-2.5 KV. You will want to either run another MOT in series with your first transformer or I would recommend looking into a DC resonant charging circuit if you still want to stick to a single MOT design.

  5. That spark gap needs to go ! Replace the steel bolts and spend the money on a quenched (or sucker) tungsten Gap ! It stands a far less chance if catching your house on fire/melting/eroding.
    If you still want to make your own gap or are not willing to spend a little then look into a rotary spark gap design as they not only are safer but you could also control the power/ frequency of the coil !

  • \$\begingroup\$ I've successfully used half inch stainless steel bolts for years on my coil. I rounded the threaded end on a grinder and adapted an insulative, high temp box to hold the bolts in line and to precisely adjust the gap. They do on occasion require cleaning with emery cloth, but work fine. And much less expensive than tungsten. \$\endgroup\$ – docscience Mar 16 '15 at 4:39
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    \$\begingroup\$ Also get some doorknob capacitors on EBay which are rated 15000 V plus. These are used in industrial lasers for semiconductor manufacturing. Gang them together in series and parallel to get the capacitance and operating voltage you need. \$\endgroup\$ – docscience Mar 16 '15 at 4:42

Yes, the previous respondents have pretty well covered the multiple issues that are causing your non-performance. Your main two problems are your transformer and your capacitor, AND your spark gap! Ok, that's three.

As has already been mentioned, a microwave oven transformer can provide a very lethal dose of amperage, to the tune of around 500 to 1000 mA, but only around 2000 volts, which is just too low voltage to reliably fire a spark gap driven Tesla coil. An old, iron cored style 12,000 volt, 30 mA neon sign transformer - NST (without GFI protection) is your best bet to get your coil up and running.

Microwave oven capacitors are only designed for maintaining a constant voltage and are used to double the 2000 volts from the transformer, along with a diode, to about 4000 volts DC to power the magnetron. They are NOT designed for the rapid pulse charge/discharge cycling of a Tesla coil circuit and as previously mentioned, they will quickly suffer a spectacular death if used as the primary capacitor of a Tesla coil circuit. You should look into some Cornell-Dubilier 942 series snubber capacitors to build a multiple-mini capacitor (MMC), or build yourself a homemade capacitor, or possible look into finding a surplus commercial high voltage pulse capacitor of the appropriate voltage and capacitance rating.

As for your spark gap, you are going to have to actually spend a bit of time and money to design a reliable and reasonably faultless gap. Google up "RQ spark gap" or "Sucker spark gap" to get some design ideas.

Finally, I would strongly recommend doing some further research into Tesla coil design. Google up Richie Burnett's Tesla coil page and sign up online for the Tesla Coil Mailing List (TCML) and learn more about Tesla coil building before proceeding further, as this is a rewarding, but potentially very dangerous hobby!


Besides all the safety concerns other people have mentioned, some MOTs also have unusual input and output wire configurations. A long time ago, I tried to build a MOT Tesla Coil (luckily I found an electronics guy in my town who helped me to not end up electrocuted) and it turned out that the metal casing of the MOT itself was one of the output leads. I don't know how universal that is, but it's prevalent enough that my mentor knew to look for it when he was helping me out.

  • \$\begingroup\$ Tuning of both input and output circuits is important to get right.The high Q of the system which is essential for voltage magnification means tuning is critical .Input must be aligned to output. \$\endgroup\$ – Autistic Nov 13 '15 at 8:48

I cannot stress strongly enough that you should not be using a MOT here. They are VERY dangerous, and will kill you if not treated with respect from a distance. Pick up an old neon sign transformer (one without a GFI) and for an inexpensive capacitor bank look up how to make a bucket capacitor or beer bottle capacitor.



One Q: On your capacitors pic, you have both terminals connected on the top cap. Were bypassing one of them?

Found the spark far too small and buckled down for a cheapish transformer ($150). I had ENDLESS trouble trying to do things the cheap way. Also trying to get it to fire right out the gate has endless^2 problems as well unless you are following a specific guide - winds, diameter etc.

To follow Tesla's work you will have to put yourself in Tesla's shoes. He didn't have a guide but he was lead there because of his instruments. People nowadays used oscilloscopes. Back in his day he did things by visually seeing the output. That's why he built a machine that sparks - because there was no meter that could measure how powerful it was.

So, in short - be careful. Sure, it's only electrical burns or death. But in the end you will only have a sparking machine capable of a 2" spark. It's a good place to start but not a good place to end.

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    \$\begingroup\$ This question is over four years old and already has numerous answers... \$\endgroup\$ – Hearth Dec 3 '18 at 13:54

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