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Re: Low voltage sparkgaps for ignition coil driver...

Tesla List wrote:
> 
> Original Poster: Bryan Kaufman <bryan-at-apexrad-dot-com>
> 

> This is a wonderful list, I presume this is DC.
> Anybody have the data for HF broken down by frequency or know what the
> reference is. Do the CRC manuals have this? Which one? I don't seem to be
> able to find it in the Handbook of chemistry and physics.
> 
> <<<< These are for DC only.  AC gets strang.  Perhaps someone else knows
> about this?  Only the OLD CRC books have this in them.  The new ones droped
> the chart out. - Terry >>>>
> 
> Bryan Kaufman
mea culpa, I did have a typo (thanks, Terry for mentioning it). It is 30
kV/cm not mm, and in a uniform field, to boot. Needle (yes, the standard
is a pair of sewing needles) gaps are typically 1/4 the voltage of a
sphere, but it varies a lot, particularly for very short gaps (low
voltages).

However, to answer Bryan's questions, I have a reference on spark
discharge (Bazelyan and Raizer) which describes some data on breakdown
voltage variations with various rise and fall times for impulses. This
is for BIG gaps (several meters) and BIG voltages (around a Megavolt),
but, it looks like the breakdown voltage has a minimum at around 100
microsecond fall time, with the time getting longer as the gap gets
bigger.  There is also a noticeable increase in breakdown voltage for
very short fall times: from 3MV at 2 uSec pulse width to 2 MV at 100uSec
(8 meter(!) gap between 2.5 cm rod and a plane).

They postulate that it has to do with the leader formation, and the
speed with which the leader propagates across the gap. 

Somewhere, I have data on 60 Hz breakdown vs DC breakdown. Most of the
sphere gap tables (like the one on my web page) are for standard test
impulses (i.e. 2 uSec rise time, 50 usec fall time), because that is
what they are measuring. As I recall, the 60 Hz numbers are the same as
the DC numbers. I would expect the breakdown voltage for higher
frequencies (say in the 10's to 100's of kHz) would be higher than for
DC, but probably not noticeably for short gaps on the order of
centimeters.
-- 
Jim Lux                               Jet Propulsion Laboratory
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