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Re: [TCML] Spark gap
Hi Vinnie,
Excellent question. In days past, coilers would build a series of pipes
spaced apart as based on advise from others or their own experience.
Then use trial and error taking the gap and transformer to the bench.
Start by using 2 of the pipes, then 3, then 4, etc.. until the
transformer would finally not fire across the gap. Then go back 1 or 2
pipes and check for consistent firing. This sets the gap for the
transformer. This is still a recommended method to ensure you don't open
the gap up too wide (for NST and Cap health). BTW, good to let the gap
cool down between tests.
However on the front end of design, it is nice to have a good ball park
for building (via advise or calculations). I use Javatc's Static Gap
procedure. Input asks for the number of pipes, pipe diameter, and the
total gap width. Javatc runs a somewhat extensive number of equations.
It will determine your cap voltage, field strength, breakdown voltage,
distance each electrode should be spaced, etc.. All these equations
could be performed on paper, spreadsheet, or whatever as well. The
breakdown voltage should be set just below the charging voltage (which
is the peak voltage at the cap). Change the total gap input until arc
voltage is just below the charging voltage. This gives a good ballpark
to build from with some design data at the front end. But also you need
to look at bps and percent cap is charged. These are also important and
based on cap size and transformer power.
It is still important to take the static gap and transformer to the
bench and test it once built.
An example: My RQ static gap is 6 electrodes of 1.25" diameter, total
gap spacing is 0.25".
Here's an output of Javatc for this particular case:
----------------------------------------------------
Static Spark Gap Outputs:
----------------------------------------------------
0.05 [inch] = Gap Spacing Between Each Electrode
19799 [peak volts] = Charging Voltage
19414 [peak volts] = Arc Voltage
34822 [volts] = Voltage Gradient at Electrode
77654 [volts/inch] = Arc Voltage per unit
98.1 [%] = Percent Cp Charged When Gap Fires
9.265 [ms] = Time To Arc Voltage
108 [BPS] = Breaks Per Second
3.54 [joules] = Effective Cap Energy
382 [power] = Energy Across Gap
47.8 [inch] = Static Gap Spark Length
----------------------------------------------
For reference:
**
**The arc voltage is the voltage potential at which the air gap between
electrodes becomes conductive.
The Arc Voltage is equated by first identifying the field strength at
breakdown, then incorporating equations where the electrode shape and
size are considered (William North). The field strength is taken from
the website of Jim Lux and supported by various papers on the subject:
Field Strength = p * ( B / ( C + ln ( p * d)))
where
p = pressure in Torr (mm Hg). For air, this value is 760
B = 365 Vcm-1 Torr-1
C = ln( A / ln ( 1 + 1 / gamma))
d = gap width
where
gamma = 0.095 (secondary ionization coefficient)
A = 14.6 cm-1 Torr -1
Electrode geometry equations used for the arc voltage are from the
L.A.N.L. report "High-Power Microwave-Tube Transmitters, chapter 7, by
William North".
As I said, there's a lot that Javatc does in this area and I can't list
it all here. I use Javatc to get decent data before I build. It's just
an option for coilers.
Take care,
Bart
Vinnie wrote:
Hello People
While everyone is on this subject.
How does one determine the number of gaps and spacing between each
segment in a cylindrical R.Q.S.G?
It seems like from what I read that if you don't have enough gaps
that's bad and if you have too many that's also bad.
Is there anyway to determine what is actually needed? Is it determined
by your input voltage or you cap or a combination
of both?
Thanks for your time.
Vinnie
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