It is typically recommended to use a big neon-sign transformer (NST) for a physically big spark gap tesla coil.

Assuming the primary and secondary of a small tesla coil are tuned to have resonance on the same frequency, are there any problems associated with feeding a physically small tesla coil with a big NST (like 15kV 60mA)?


The original secondary has a diameter of 10 cm and a height of 50 cm (1:5 size ratio). The alternative secondary for the smaller tesla coil has a diameter of 5 cm and a height of only 25 cm. The wire diameter is also put to half the size so the same number of turns can be used for the same height. This should give roughly half the inductance of the first coil (10 mH vs 20 mH). Since the resonance frequency will be higher, this must of course be compensated for on the primary side. Let us say this is compensated for in the primary coil only and the capacitor bank remains the same.

Will this blow up? Will the streamer arcs be shorter, the same, or longer than with the bigger coil? (assuming things are optimally tuned).

  • 2
    \$\begingroup\$ You need to provide details like turns ratios and inductances for the transformers, otherwise this question is a shot in the dark \$\endgroup\$
    – Voltage Spike
    Feb 18, 2021 at 0:13
  • 1
    \$\begingroup\$ Assuming you're talking about a spark gap Tesla coil no most likely there's no problem at all... Higher input voltage will result in higher output voltage... If this thing is really small and you have a really high quality coil then you might have some issues with too good of performance that you have high voltage Arc over to your primary or places you don't want it that damage the coating and insulation on your coil etc... \$\endgroup\$
    – MadHatter
    Feb 18, 2021 at 2:14
  • \$\begingroup\$ The biggest concern is clearance and creepage. If you use an NST which is "too large" for your coil, I would expect unwanted arcing between the primary and secondary, or the topload and the primary, or racing arcs up and down the secondary. \$\endgroup\$
    – DerStrom8
    Feb 18, 2021 at 12:52
  • \$\begingroup\$ A streamer that visits a transformer primary or secondary terminal and then visits you is going to be as good as a solid wire carrying 110v, 220v, or the transformer secondary voltage, and I believe "coilers" have been killed that way. \$\endgroup\$ Feb 18, 2021 at 15:28

1 Answer 1


The key to understanding is: think about constant-voltage power supplies, where the load determines the current, which can be anything from zero to I(max.) A very large AC line-transformer will happily power both the kilowatt bulbs and the milliwatt bulbs.

If your TC system is designed under the assumption that the HV supply is constant voltage 15 kVAC, then the physical size of the main TC coil is irrelevant ( ...well, as long as your air-gaps are wide enough, and will still prevent arcing during the peak volts: 1.414*rms.) In that case a smaller Tesla Coil would just draw a lower current from your NST, compared to a larger Tesla Coil. As with any transformer, extra iron and copper are required in your NST to avoid excess heating during maximum wattage, but this isn't a problem when your load draws less than the transformer's maximum watts/mA.

On the other hand, I remember old exotic designs where the NST and the main primary capacitor were critical for attaining max TC output (treating the NST secondary as a 60Hz or 120Hz resonant circuit, or, as a series load-inductance, where Vout is intentionally pulled down to less than 15 KVrms.) In that case, the NST isn't just a constant-voltage power supply, so simply reducing the size of the TC coil might give unexpectedly poor results. (But none of this matters for a standard, non-exotic TC circuit.)

Heh, if you have a huge NST, then think about ...christmas-tree lights! Do something similar: try to power many little bitty Tesla coils, rather than one large coil. I once made a Tesla-coil array with 80 TCs, each driving one fluorescent tube, as a "Plasma sculpture" at Seattle Folklife fest.

Also, years ago people were trying to achieve tiny TCs with yard-long lightning, by building them into the bottoms of tall plastic pipes full of oil! Just how small can a TC-secondary be, while still putting out one entire megavolt?

  • \$\begingroup\$ Christmas teslas are a great idea! I was thinking that the TC system was usually designed to draw near max current from the NST - in this case up to 60 mA - and thus use the NST as a current limited source. Can you please clarify what you mean with "prevent arcing during peak voltage"? Nothing will happen if the main spark gap does not ignite below the transformer voltage the way I intuitively understand it, so I guess you are talking about a constant arc that never dies? Thanks \$\endgroup\$
    – rubund
    Feb 21, 2021 at 8:38
  • \$\begingroup\$ @Ruben I meant, prevent unwanted arcing between secondary and primary, because entire device had been shrunk down in size. ( Need insulated primary turns or something.) Also, NST secondary must act as series inductor whenever the main TC spark gap is wired parallel to NST output, w/primary capacitor then in series (not placed across NST output.) That was a N. Tesla trick to prevent RF damage to the secondary of the AC supply xformer \$\endgroup\$
    – wbeaty
    Feb 21, 2021 at 15:21

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