I'm building a Tesla coil and the secondary coil (about 800 turns of 0.85 mm diameter enameled copper wire) is wrapped around a cardboard tube (about 8 cm diameter and about 1 m long).

I was planning to wrap the primary coil (about 10 turns of >= 1 mm diameter insulated copper wire) around a smaller cardboard tube (about 4.5 cm diameter and about 40 cm long) that will fit inside the larger tube with the secondary windings, however, most Tesla coils I have seen have the primary coil on the outside of the secondary coil and I could not find any information regarding this.

Rough diagram (not to scale) of a Tesla coil with primary windings on the inside (Not to scale) I hope this rough diagram helps with understanding.

  • Will the Tesla coil work with the secondary windings (red) on the outside of the primary windings (yellow)?
  • If not, what is the reason for this, but if so, will I have to reverse the polarity or direction of the windings since the magnetic field is going in the opposite direction on the outside of the coil to the inside?
  • And finally, would the inner tube's diameter be large enough for the primary coil to work?
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    • \$\begingroup\$ seems like the primary is too far away from the secondary ... try using a 7.5cm tube for the primary \$\endgroup\$
      – jsotola
      Commented Mar 12, 2018 at 6:35
    • \$\begingroup\$ How many volts to you want from the top end of the secondary? How many volts do you expect half way up the secondary? haven't you got the primary a little close to the secondary? paper isn't going to stop an electric arc, neither is the lacquer on the wire. \$\endgroup\$ Commented Mar 12, 2018 at 7:42
    • \$\begingroup\$ How did your resonance/Tesla coil work out with the primary inside the secondary? I am thinking of doing the same thing but I will size the primary diameter to just slip into the ID of the secondary tube and only use 3-5 turns on a 1k-1.5k turn secondary. But I would be very interested to know about your experience. Thank you for taking the time. \$\endgroup\$
      – oddjobbob
      Commented May 6, 2021 at 1:12

    2 Answers 2


    There is a lot of 'technology' associated with Tesla coils. I use technology in its true sense of 'knowing how to make things work'.

    You can make an air-cored transformer with the setup you show. It will work as an air-cored step-up transformer. However, it won't work as a good Tesla coil.

    A Tesla coil will produce some sparks. A good Tesla coil will produce sparks longer than the height of the secondary, and hopefully as long as the John Freau equation of 1.7*sqrt(watts) inches.

    To this end, most successful Tesla coils use a flat primary at the bottom of the secondary, to keep it away from the high voltage end. Your primary is very close to high voltage parts of the secondary. Don't think the core material will do much to avoid breakdown between them. At the voltages these things can run at, the core material might as well not be there.

    When it breaks down through the core, it will destroy your secondary. This is another reason primaries go outside, nothing to do with performance per se, but to reduce the hassle factor of the inevitable flashover.

    Go to a specialist Tesla coil forum. Read, and see what works. It's great to experiment, but there's a line between trying things out, and failing to learn from everybody else's hard-won experience. For instance, 1m x 8cm is far too long and skinny to be a good coil. 800 turns is sensible however.


    Pretty late to respond to this, but I was doing the same thing and tried to look up info, but the thing is, it works, I've tried.! The whole black part is secondary, leaving 2cm from both sides, roughly900 turns, gives resonant frequency of about 800-900KHz. I drove it with a single mot pulsed DC supply, so max of 6kv input, and it gave max 3 inch sparks from hv to ground, the blue coil is feedback and black coil(black wire) is primary.primary and secondary


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