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I am trying to build a Tesla coil. I have a neon sign transformer and a couple of microwave oven transformers. I am trying to understand why using the coils of an additional transformer to act as the primary and secondary won't work. I am doubtful though because I haven't seen anyone else trying it. Any Help?

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    \$\begingroup\$ Welcome to EE.SE! Please draw a schematic of your transformers and how you have, or intend to, connect them. Click on edit and the schematic symbol and a fairly competent schematic editor will open up. \$\endgroup\$
    – winny
    Jun 9, 2020 at 8:54
  • \$\begingroup\$ Realize how a transformer works, the primary coil induces a changing magnetic field inside the transformer. As a result of that the secondary coil develops a voltage. The secondary coil of a different transformer will not develop a voltage because there's no magnetic flux inside it. \$\endgroup\$ Jun 9, 2020 at 9:14

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From this website entitled Stefan's Tesla coil pages: -

Typical coupling coefficient for standard Tesla coils are in the range between 0.05 and 0.2. The looser the coupling, the shorter the sparks. But if the coupling is too tight, there will be racing sparks along the secondary and breakdown between the primary and the secondary.

See also pretty much the result of any search on the subject: -

enter image description here

So, if you are unsure about what I'm saying go google some stuff. A regular transformer (be it for a neon sign or for use in a microwave oven) will have a coupling factor of nearly 1.

You need a loose coupling factor so that the individual coils (primary and secondary) can be resonant tuned. It's the resonant tuning that creates a monstrous voltage magnification and not necessarily the turns ratio although more turns on the secondary will produce more voltage.

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One of the biggest issue is matching the resonant frequencies. A Tesla coil is a resonant transformer, meaning the voltage rise at the secondary is due to resonance between the primary and the secondary coils. Matching the primary and secondary resonant frequencies of a conventional transformer is not easy.

Another issue, as Andy mentioned, is the coupling. Coupling of conventional transformers is generally much higher than what is desired for Tesla coils. This will likely lead to internal arcing and permanent damage of the transformer.

Yet another issue is voltage. The resonant tank circuit of a Tesla coil (the resonant capacitor and the primary coil) can often see voltages of several thousand volts in solid-state systems, or several tens of thousands of volts in traditional spark gap coils. Conventional transformers cannot handle such voltages, as the insulation between the windings and the core will most likely break down and short out, permanently damaging the transformer.

I'm sure there are more reasons but these are the main three that popped into my head.

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Congratulations on scoring a neon sign transformer (NST), that's usually the difficult bit these days. As long as it's an old style 'big iron' transformer without ground fault protection, as the latter spoils the operation for Tesla coils (TCs). There are modern high frequency ferrite neon sign supplies (called neon sign transformers so as not to confuse the installers) that are no good for the purpose.

A couple of microwave oven transformers (MOTs) can provide wire for the primary and secondary of a rudimentary TC. Saw the cores apart, and unwind the secondaries carefully onto a PVC pipe former to make the TC secondary. Use the thicker primary wire, in a bundle of several strands, to make the TC primary.

Neither your primary nor secondary will be ideal when made from reclaimed wire like this. The secondary needs to be very smooth and even, which can only really be done with new wire. Your primary would benefit from being made of bare tube (plumbing micro-bore), to provide tapping points for tuning.

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This is not an answer, but rather a description of the Tesla coil I built for a 8th grade science fair project.

I used a neon sign transformer (they were easier to find back in those days). Across the secondary I put a home made capacitor, made from a piece of glass in a wood frame (~12"x12") with a piece of (I think) aluminum foil or aluminum/tin sheet on each side for the plates.

The secondary outputs were then fed into the main coil. The primary of that main coil was 20-50 turns of high voltage wire (thick) insulation, wound on a wood core with a 6" or so air gap in the middle. The secondary was a 4" diameter piece of PVC, 4 feet or so long, that was wound with a single layer of 30 gauge shellac (?) insulated magnetic wire. This ended up being several hundred, maybe a thousand or more turns.

I could draw easily draw a 12" or 18" arc from the top terminal of the secondary coil. Made quite a racket, mainly from the plates of the capacitor vibrating.

Put on quite a show. I cannot envision being allowed to do something like that today!

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