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Re: [TCML] LTR/STR and spark length

Thank you very much for inputting the numbers for me. (Math, not one of my strong points) So at 25nf I'm still well in the STR range, but you think it's good enough for bigger sparks? I did notice the brass warming up fairly quickly, and yeah, they are solid. I can redo my gap, maybe go to a copper fitting type. As far as quenching ability, I also have a fan from an old air hockey table which I'm sure moves a lot more air, so I can put that into my next gap. Do you think it's worth putting a dimmer on it to vary the speed, or just go full force? Thank you again for your help, and I'll let you know how things improve with a different gap.


Hi Neal,

Let me answer your last question first. An equation to find the capacitance which becomes resonant with the transformer is: Cres = 1/(2*pi*Z*Hz) where Z is the transformer impedance and Hz is the line frequency.

A good approximation for transformer impedance is Vout/Iout. I say approximation because that equation does not include reactive components. So, in your case you have a 9kV 180mA supply. Cres = 1/[6.28*(9000/.18)*60] = 53nF. Now, as you go above this value the tank capacitance becomes LTR (larger) and if you go below then STR(smaller). Resonance causes the voltage to rise, and this is typically what we try to stay away from with NST's due to their voltage sensitivity. As a matter of fact, we try to go LTR at about 1.5 x Cres or in that neighborhood. Check out Richie Burnett's page on resonant charging for a more detailed explanation.

I think much of your losses is mainly in the gap. Your using brass and I assume it's solid. Also, the fan your using is not enough to keep the gap cool. As I've mentioned in other emails, I like large copper tubing. This is only my opinion. My reasons are the large surface area and how airflow over (and within) the large surface area maximizes cooling. Solid stock takes a little longer to heat up, but it doesn't take long (few seconds) until the gap is running at a very high temperature. High gap temperature lowers the arc voltage decreasing the discharge energy available when the gap conducts (and shorter spark lengths are a direct result). Of the static gaps I've built, solid brass was one of the worst.

You mentioned you opened the gap up. This is not recommended and is how transformers end up in the grave yard. Yes, if you open the gap, the cap will have to charge to a higher voltage to arc across the gap, but then you begin to risk both the tank cap and transformer secondary winding failure due to over voltage. My recommendation is to adjust the gap with the transformer to arc consistently when there are no other components connected and leave it there. Then simply work on keeping it cool.

You have plenty of power for 50" spark lengths, but I think your gap is robbing you of the bang energy needed.

Take care,

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