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Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
If R>>d the formula tends to 30000d, as expected. If d>>R it tends to 60000R, meaning that both spheres produce corona when the surface electric field on the spheres exceeds 30 kV/cm of radius. This formula works well if the polarities in both spheres are opposite. If one is grounded it predicts too high voltage for large gaps, because the single sphere connected to high voltage starts to produce corona at just 30 kV/cm of radius if far from the other. The value of 30000 is approximate. Any irregularity on the spheres can reduce it. The value is approximately proportional to the air pressure in atmospheres.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
At sgtc kinds of current the spark gap appears to me to be more like a zener diode than a resistor, the conducting gap seems to drop abut 300-400 V irrespective of the current - just an observation from memory, not a fact.
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
Mattski wrote ...
Looks like a useful reference.
I tried to make some experiments with small gaps, and noticed that the breakdown electric field really exceeds 30000 V/cm in a clearly visible way at a few mm of spacing. I get 4.2 kV for 1 mm spacing very consistently, not 3 kV. This reference has a good formula for the breakdown voltage in this case, that appears in several books too: x = 293 * p * d / (760 * T) Vbreakdown = 24.22 * x + 6.08 * SQRT(x) p = pressure in Torr (mm Hg), d = distance in cm, T = Temperature in Kelvins Vbreakdown in kV The formula for sphere gaps can be used for small gaps if the 30000 is replaced by the value obtained in this way divided by the distance d, but the formula above is enough for R>>d.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Antonio wrote ...
I tried to make some experiments with small gaps, and noticed that the breakdown electric field really exceeds 30000 V/cm in a clearly visible way at a few mm of spacing. I get 4.2 kV for 1 mm spacing very consistently, not 3 kV. This reference has a good formula for the breakdown voltage in this case, that appears in several books too: x = 293 * p * d / (760 * T) Vbreakdown = 24.22 * x + 6.08 * SQRT(x) p = pressure in Torr (mm Hg), d = distance in cm, T = Temperature in Kelvins Vbreakdown in kV The formula for sphere gaps can be used for small gaps if the 30000 is replaced by the value obtained in this way divided by the distance d, but the formula above is enough for R>>d.
yes, i saw this on Jim Lux's site and others, i was wondering if the 30,000 number could be changed to increase accuracy. Jim lux and others suggest accuracy and/or precision of +/- 8-10%.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
OK ive been keeping this device in storage for use as a HV variable cap, however without the paint i think it would be good as a parallel-plate spark gap.
10M ohm load, 10 Watts, 20kV, with thermistor, all encapsulated in cell-cast polyester.
Registered Member #343
Joined: Fri Mar 24 2006, 02:45PM
Location: Los Alamos NM
Posts: 10
After recently working on a 'Can-Crusher' (simplified explination of it, it is really 8 MASSIVE caps arranged as Marx banks, when errected is 4 banks, 4.8 MILLION amps, 10 us discharge, MEGA-JOULE range...just a little 'toy'), I learned that for good low inductance spark gaps to use a large surface for the face of the gap.
For my home project, I used 2 copper rods, about 5" in length. Each rod is supported with a bracket on opposite ends. The gap distance is spaced further than the breakdown voltage. To trigger the gap, a razor blade is inserted vertically closer to one of the gaps, in my case the positive side. To fire the gap I trigger it. The trigger arc goes from the razor to the neg gap. This in sence makes the spark gap really short so the entire charge fires.
Look up ATLAS and sparkgap for more info. (I will try to find a link to the info also)
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
(If I look for Atlas and sparkgap I find the post above...) From the discussion above I found several ways to measure DC high voltage, that I started to describe in the page that I posted. The last method, that works surprisingly well, is the old pair of pith balls. I even found a good formula relating voltage and separation of the balls. It's a common basic Physics exercise, but strangely recent texts don't try to find that relation.
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a question about spark-gaps...
