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Registered Member #1262
Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451
I was wondering how small you can make a spark gap, and how low of a voltage, before it refuses to fire. I know about the 1kV/mm approximation, but it doesn't look like it holds up for very small gaps, as 300V can't seem to arc over 300 microns.
So basically I was wondering about the operation and theory of very small spark gaps.
The dielectric strength of clean air is 30kV/cm, but the thing is that many parameters affect the final "seen" strength of a given gap, and that is humidity, air pressure, surface defects, surface contaminants, geometry of electrodes etc.
Small gaps are more likely to break under much the same voltage, but the localized geometry and surface condition (field enhancement factor) plays an important role on the final strength... Longer gaps are more affected by the medium condition and might seem macroscopically more unstable.
I guess with small spark gaps, you have to make them mechanically steady, clean, with small rounded tips, and out of a hard metal so you can get repeatability. And of course you have to somehow calculate the electric field distribution over the electrodes, either with some software, or with approximations on typical geometries (sphere-sphere, sphere-plane, pin-plane gaps), if you want to get some numbers on the needed gap length...
Apparently, there is a minimun in the pressure curve (Pashen Curve), that a gap arcs under very low voltages, and that is the glow discharge area, around 1 torr or sthing.
Registered Member #1739
Joined: Fri Oct 03 2008, 10:05AM
Location: Moscow, Russia
Posts: 261
PanosB_GR wrote ...
The dielectric strength of clean air is 30kV/cm
...based on the linear approximation for the higher voltage ranges ;)
watch this! ps sorry for the russian pic, the air curve is the one that ends higher then the other ones, the X axis is pd [cm*Torr], Y - voltage in volts.
Registered Member #350
Joined: Mon Mar 27 2006, 05:14PM
Location:
Posts: 106
Dr. Kilovolt wrote ...
"For example, at 1 Torr... If 500 Volts were applied, it would not be sufficient to arc at the 2.85 mm distance, but would arc at a 5.7 mm distance."
Now that's totally wierd
That is due to the way a spark forms in air. It has to do with free electrons. The process is called avalanche breakdown. At very small distances electrons don't have enough space to accelerate to the required speeds.
Registered Member #1143
Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721
i know men, who make SGTC without HV transformer, he use rotary spark-gap, like 1mm distance, direct from 220V ac line, whit large choke, spark is 1-2m long
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Minimum spark gap voltage
