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Re: [TCML] LTR/STR and spark length
The 25nF your using is fine. Yes, it's STR, but well within an
acceptable range. If you were to increase cap size, I would recommend
going past Cres onto LTR to get to the other side of resonance which are
the values mentioned previously. For some of us who have run simulations
to view resonance analysis (when the voltage begins to rise, it's peak,
it's waveform, it's decent, back to norm), it's rather easy for us to
visualize. For those who haven't, well it may be more difficult and
simply a step of faith. Where you are now is perfectly acceptable. It's
great Gary suggested upping your cap value from your original value.
This really helps with bang size. Now you just need to work on reducing
Quenching is not really the issue with your gap but rather just keeping
the gap at a low temp so the arc voltage is as high as possible (ideally
your transformers peak output voltage). A higher CFM airflow will help
"tremendously", but even with that, the electrode type and shape makes a
big difference. Tubes are naturally ideal for this and is why they are
used by so many of us. I prefer something around 1" in diameter as
opposed to small 1/2" tube stock. Air cooling can then flow over the
outer and inner surface which greatly enhances the thermal dissipation
of heat. As you probably saw with my gap, it's designed around the
classic Richard Quick style. My particular airflow sucks in air rather
than blows it out, although I doubt it would makes a difference. The
bottom line is a cool gap at high power. This will keep arc voltage
higher, bang energy higher, spark lengths longer, and lower bps.
I see no need to adjust airflow with a dimmer. "If" you had such massive
airflow that it did cause problems, then a simple mechanical baffle (air
valve) will deal with this just fine. Start full force and build a gap
that will not limit your power supply and you'll be on your way to
greater spark lengths.
Some have theorized that gap losses are not as big as has been mentioned
on TCML. I and many others know differently simply from our experiences.
They are in a matter of words "power robbing losses". I've seen the
difference in my own coils and I am a firm believer that power is lost
in the gap when the gap is not appropriately designed for the energy
"banged" across the gap.
Neal Namowicz wrote:
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 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.
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