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Re: Safety gap issues
Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>
At 04:24 PM 11/25/2005, Tesla list wrote:
Original poster: "JT Bowles" <jasotb@xxxxxxxxxxx>
You said:
"The breakdown for air is approximately 30KV per cm. This is a
local field strength and if the field is uniform (constant) then you
can measure the distance to find the total breakdown. The 25KV per
inch you found probably assumes a geometry and applies to a short
range of distances. "
Well holy crap, that throws ALL my measurements with high voltage
off a lot. My sparkgap for example is set at 7.5 to 8 mm. That means
my sparkgap is set at 22.5kV? NO WAY; MY TRANSFORMER OUTPUTS 12KV
ONLY. SO, IF IT WERE SET AT 22.5 KV, IT WOULDNT FIRE WORTH CRAP.
THIS MEANS THE FORMULA: 1cm=30KV cannot be correct
Thanks a ton for the help, but I THINK you're wrong buddy
JT... all caps, as you know, implies yelling..
First.. the breakdown field for air is one of those things that is
very well known, and, sure enough, it's pretty close to 30 kV/cm in a
*uniform field* at standard temperature and pressure.
Second, there is a very strong dependence on breakdown voltage for a
particular gap on the electrode configuration. For instance, needle
gaps break down (for low voltages) at 1/3 to 1/4th the voltage of a
uniform field gap. Producing a gap with uniform field is a challenge,
however, there are some electrode designs that do this: Rogowski and
Bruce are examples. Unnecessary for tesla coiling, where you can
just adjust the gap distance to compensate for the field enhancement.
Third, for gap voltages above, say, 500kV, a lot of the "rules" don't
work very well, even if scaled for the dimensions (i.e. you make the
electrodes correspondingly bigger).
So... If your gap is firing with your 12 kV transformer (I assume
you're giving us peak voltage = 1.4 Vrms) and a gap length of 8mm, it
means that your gap is nowhere near a uniform field gap. This is no
surprise. A field enhancement of a factor of 2 is not too unusual
for arbitrary gaps.
A couple other things to be aware of: Are you measuring your
breakdown with just the gap hooked up to the transformer? Once you
get the primary capacitors into the circuit, the voltages can easily
rise well above the nameplate voltage.