Mini Tesla Coil design

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I was daydreaming about building a miniature Tesla coil at some point as  
a weekend project. Like an 8-10" secondary with 500-700 turns of 28  
gauge magnet wire, maybe.

I thought it also might be fun to make it a hybrid design, using lil  
power triodes or tetrodes for primary drive, driven from an xtal  
oscillator or SPI-controllable oscillator like the DS1077.

I'd imagine having a bifilar-wound center-tapped primary with B+ on the  
tap and each side going to an LC-tuned plate circuit tank would work,  
yeah? Could also drive the grids through a center-tapped transformer to  
have negative bias on the grids for class C, and match impedances from  
the generator to the grid circuit.

My question is that I don't know what kind of load the secondary would  
present to the plate circuits in that arrangement - would you have to do  
something different with the secondary as well to achieve balance?

Tuning could be difficult; I guess you could match the primary resonance  
to the secondary resonance by dynamically adjusting the clock frequency.  
Or a VCO/PLL to ensure that the primary voltage and current is always in  
phase. Bleh this is already getting complicated.

Re: Mini Tesla Coil design
bitrex wrote:
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Most small vacuum tube Tesla Coils (VTTC) work quite well with a simple  
single-tube or push-pull Armstrong oscillator. Either circuit uses a  
tickler winding for feedback. You really don't want to use a  
crystal-controlled or other fixed-frequency source since the optimal  
self-resonant (and optimal) frequency of the secondary changes  
significantly with loading and proximity to nearby stuff (you included).

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Sounds similar to push-pull Armstrong power oscillator circuit with a  
center-tapped tickler winding:
http://capturedlightning.com/Papers/Push-Pull_Armstrong.jpg

You may want to make a small toroid-shaped topload on the secondary to  
protect the top of the winding and coil form from corona damage. A foil  
wrapped polystyrene foam toroid (craft store, for flower arrangements)  
should work.

Using an unfiltered FW rectified B+ supply will give you longer,  
sword-like sparks. Using filtered DC will give you hotter, hissing,  
flame-like corona/sparks.

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Tuning the primary LC circuit to be "close" is usually good enough.  The  
high Q secondary and moderate coupling (k typically between 0.2 - 0.35  
for a vacuum tube-driven coil) tends to pull the primary LC circuit  
along for the ride.

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Tune the primary to the same F frequency as the secondary. You want to  
operate the coil at the lowest resonant frequency for the secondary and  
topload. The secondary frequency can be measured by using signal  
generator to drive the base of the secondary, and  looking for a current  
peak. This should be done with the secondary in situ.

The primary circuit should be independently tuned by first removing the  
secondary and moving it far away before tuning the primary LC circuit.  
Trying to tune the primary when coupled to the high Q secondary will be  
an exercise in futility.

Using a simple feedback winding eliminates virtually all frequency  
control complexity versus loading...




Re: Mini Tesla Coil design
On 07/05/2017 11:19 AM, Bert Hickman wrote:
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Great information, thanks. The topology you posted looks like a winner.

I have a box of 6N7G/GT power triodes; I wonder if they'd work for a  
small coil. The coke-bottle shaped ones look suitably mad-scientist.

<https://frank.pocnet.net/sheets/127/6/6N7.pdf


Re: Mini Tesla Coil design
On Wednesday, 5 July 2017 14:17:46 UTC+1, bitrex  wrote:

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Tesla coil plus capacitor is self resonant. I'm not seeing any advantage in trying to externally control that.

They're trivial to make. All you need do to drive it is throw it a low frequency square wave. Chopped mains (or battery) will do, and was the basis for all violet rays from the 1890s to 1950s.


NT

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