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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.
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
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
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.
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