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Re: [TCML] LTR/STR and spark length
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 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.
Neal Namowicz wrote:
Hi, and thank you to everyone for your responses-
Bart, it's very interesting that our coils are so very similar.....
except for that whole, underperforming thing on my part. Let me give
you a little more info on my specs before we go on.
My first topload was a dryer-duct, covered with foil tape, type, 19
3/4" X 3"
I had arcs off of it, but from all over the place, even with a
screw/breakout point taped to it.
So for the moment, at least for consistency purposes, I'm sticking
with a 10" ss sphere with a 12 5/16" steel rod inserted into it. All
the arcs are headed relatively straight up now.
My gap is single static, quenched with a blower/motor from a
microwave. The electrodes are round, "almost flat" surfaces, 1"
across, made out of brass (old drawer pulls). I'd like to stick with a
ssg for now, but, if I feel ambitious, I do have a couple little
motors around that could probably give me at least an asrg.
Since my last email I tweaked a couple things, and I'm getting easy
24+" arcs now. I opened up the gap a little bit, but still backed off
if I got any arc on the safety gap. I also ground a mild point on the
rod sticking out of the sphere. Racing arcs were mentioned earlier,
but the only time I experienced them was very briefly while adjusting
the primary tap.
*Could you explain how one goes about determining the resonance for a
given system, so that they could know (within the proverbial
ballpark's range) as to whether they are str/ltr/R? Thanks everyone,
Just thought I'd share my little coil with you since it's similar (4"
x 20" these days) and uses a supply voltage about 10.6kV at 200mA
(modified 12/60 NST), so similar to your power supply. I'm also
running a static gap on this coil. Early on and in the pics below I
have had some primary strikes but hardly get those any longer. In the
pics (at the link below), the coil ran with a rather tight coupling.
Some months later after these pics, I ended up catching the bottom of
the secondary on fire (bottom turns shorted). When I realized the
glowing red spots, I decided to just continue and let disaster happen
(which did). I ended up removing about 1/2" of winding which also
lowered coupling. The coil these days is painted red, but nothing
else has changed other than a slightly looser coupling. No more
problems with the sec bottom turns. This coil has also ran very long
runs (which I did mainly to verify coupling was good for it's
duration and I was curious as to how everything would do over a long
duration). I was happily surprised.
Well, I just wanted to show why I think your coil could probably do
better than it is now. BTW, the MMC is 18.8nF, so I also am running
STR with this setup. Other than maybe the gap and toroid, our coils
are probably not that different. Also, on this coil, I typically run
about 90 to 100 volt input via a variac to keep sparks out of harms way.
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