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Author Topic: Tesla Coil Alternative Primary - New Cuisine  (Read 4393 times)

Offline MileHigh

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Tesla Coil Alternative Primary - New Cuisine
« on: February 24, 2015, 06:13:57 AM »

For your Tesla coil builds, perhaps spreading the primary out along the length of the cylinder would be interesting.  See attached pic.  You are putting much more magnetic energy in the coil, so it might really be something.  I wold not recommend this for AC's kid's projects.

You could also try a long spiral, but the "multi-ring" design is sexier.


Offline TinselKoala

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Re: Tesla Coil Alternative Primary - New Cuisine
« Reply #1 on: February 24, 2015, 08:03:09 AM »
Well, it's an idea. But... There are a couple of reasons why it might not work.

One is that for the secondary of a true TC to ring properly, the primary must be relatively loosely coupled to the secondary, with coupling coefficient k well under unity, like between 0.1 and 0.2. With high k you just get transformer action rather than VRSWR (voltage rise by standing wave resonance) , the "magic" of a ringing 1/4 wave resonator.

Another is the voltage distribution along a true TC secondary that _is_ ringing properly.

Here's the voltage/length distribution for the "sample coil" from the Javascript TC designer page at

The voltage in ringing secondary increases so fast with distance up the coil that you very quickly get to the point where arcing to the primary is a major consideration. This is why you see primarys that are shaped like inverted cones or completely flat spirals: One tries to arrange for the proper loose but not too-loose coupling, with the outer turns of the primary far enough away from the secondary that arcing between primary and secondary cannot happen.

For a solid state TC, one often sees tighter coupling between primary and secondary, because the secondary can be stimulated cycle-for-cycle at its resonant frequency by the primary driver circuit so it doesn't need to ring freely between "strikes" like a spark-gap coil must do. Still, the primary must be kept low on the secondary because of the voltage buildup. Low-aspect-ratio secondaries (short fat), with more tightly coupled primaries of only a few turns, are something that I've been experimenting with, with good results, in SSTCs.

If you play around with the Javascript designer on the link above, you can enter various parameters, then let the calculator "auto-tune", or you can "adjust coupling", and see how the "picture" changes the height of the primary on the secondary. In the real coil, it pays to include some means of varying this "height" difference. For recent coils I've made I do it by mounting the primary rigidly to the baseboard, then including a spacer system on the secondary mounting arrangement so that the secondary can be raised or lowered inside the primary to adjust the coupling. For my earlier low-aspect-ratio SSTCs "TinselKoil" and "SassyClassE"  I varied the height of the closely-coupled primary itself. Just a few mm can make a big difference in performance of the coil.

Offline MileHigh

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Re: Tesla Coil Alternative Primary - New Cuisine
« Reply #2 on: February 27, 2015, 07:06:20 PM »
Thanks TK, I was not thinking and didn't consider the issues you raised.  I now know about stuff that I don't know!