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Re: Help required - ignition coil Tesla

Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx>

Hi James,
The coil is really small and you have a lot of oddities (hv supply, primary, gap, etc..) and all that adds up to difficulty (for anyone).
The primary is just like 18 awg magnet wire? Might work, but you will probably want to "not" make this closewound as you would a secondary. The small wire size has a much smaller radius of curvature and the breakdown voltage (turn to turn) would be greatly reduced. The small wire can also only handle a small current. When the gap conducts, heavy currents will try to make their way through that primary. Small wire will limit the amount of power (thermal losses).
The cap also is super small. Your only 0.39nF (way down there). I know you likely sized it for resonance, but cap energy is another qualification to consider for spark lengths. I think your primary is close to resonance, there is just either too many losses or just now enough "umph" to do larger spark lengths.
I suspect your basically seeing a brushy pink 1" corona looking spark off the end. If you "really" want sparks, find an NST about 12kV at 30mA. Install a fitting topload and build the primary and cap to suit the resonant frequency.
I feel for you. Large coils are much easier to get working decently than those really small coils. 

Take care,
Bart


Tesla list wrote:


Original poster: "James Howells" <james@xxxxxxxxxxxxxxxxxxxxxxx>
I have spent every day since Jan1 working on my tesla and can not get more than a 1" spark no matter what I do 

To save my sanity - some help is requested - I WANT TO GET BIGGER SPARKS!
Power supply:- Twin Auto Ignition coils - 555 interrupter through two power transistors. A separate oscillator supply to main 12v battery-ball point pen case chokes - works well
Spark gap :- 6 mm copper tube fixed end to end but gap adjustable - best setting seems to be 0.1" Primary : 4 series 390 pF 15kv caps , and 3 in series 390pf 15kv caps ( should be 4 of each, but leg fell off one cap and no replacement yet) the three strings are in parallel 
{4 x 390}
{4 x390}
{3x 390}
Primary coil wound on 4.3" plastic soil pipe ( sewer pipe) with .048" enamelled wire Turns 18 at present but was 30 and reduced it and tested one turn at a time down to 18 turns
Secondary: wound on 2.64" dia white ( abs?) plumbing waste pipe approx 450 turns of .019" enamelled wire to a height of 10.25 " 

Various toroid top loads have been tried ... and non , which works best!
The present spark at the Tesla is about 1" which is about 20% of what I expected 
I have not been able to improve on this - PLEASE HELP!
I built a Tesla Coil Tester to match the resonant frequency of the coils ( it's a 555 and two LED job, and I think it works like the grid dip oscillator Thank one needed to have here for compliance with our Ham licence). 
The coils do show oscillation at the same point on this home made instrument
Originally the tester had a range too high for my coils so, I had to double the oscillator capacitor inside the tester - and as a consequence lost even the crude calibration that was there to give me a quantative reading. This said, I think both coils resonate at about 800 kHz, and this is about the area my calculation show also.
How close do I have to be with the frequency match ? Is it exact or nothing or is some mismatch possible but with reduced spark length? 


Thanks in anticipation of your replies

James Howells

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