Re: HV Arc Resistance
Subject: Re: HV Arc Resistance
From: Terry Fritz <twftesla-at-uswest-dot-net>
Date: Sat, 10 Jul 1999 14:36:01 -0600
I have made a few gaps recently :-) so I'll give my two cents on this.
At 11:26 AM 7/10/99 +0200, you wrote:
>I am still trying to figure out the hows and whys of the spark gap
>for best design. My question pretains to the resistance of the
>spark gap. In which way does the resistance of the gap increase
>or decrease when compared to the distance? I.e: if my gap is
>set twice as wide, does the gap resistance increase by a factor
>of 2, 4, or ?? What (in comparison) effects would one see if I
>built a (for starters lets say a static) spark gap with:
I "think" if you double the distance, the resistance basically doubles. I
think a good measure of this is when one draws an arc such as off a Jacob's
ladder. If you double the distance, the arc just seems to get longer
without the intensity of the arc changing greatly. So if it is twice as
long, the arc is twice as big and the resistance would seem to be proportional.
>1.) Two electrodes with a distance X.
>2.) A 10 electrode gap with a TOTAL distance X and
>a gap to gap distance of x/10.
I think many electrode gaps probably have the same resistance when they
are conducting but have much higher losses at the zero current crossing
where the arc goes out. A single gap will stay hot and it is easy for the
arc to restart. However, a multi gap cools quickly and the arc has a much
more difficult time restarting. This appears effectively as higher losses
but more effective quenching. A gap with say a hundred small gaps would
have super good quenching but the losses would be extreme. A single gap
has very low losses but practically no quenching.
>3.) Same as 2, except 50% of the gaps are x/20 and
>the other half is x/5 (so total X is the same for all 3 cases).
I seems that the more gaps sections one has, the resistance rises but the
quenching gets better. I think it is possible for a low resistance poor
quenching gap and a high resistance good quenching gap to both do well but
it appears to depend on the system they are used on. In other words, some
systems seem to benefit from better quenching while others need lower
primary resistance to make the best sparks.
>(Right now) I am NOT (yet) interested in quenching or
>other gap properties, but soley on the I^2R losses
>involved in the gap setup.
>For example, I am using a 10 electrode setup w/o any
>quench help and get pretty good results. Awhile ago
>(this was not with my 8" coil, so I donīt have a direct
>comparison), I tried the Gary Lau method of vacuum
>(well, mine was really pressure) quenching a two
>electrode gap. My resulting sparks were a LOT less
>than desirable. Yet, Gary is getting good results. My
>setup was more of the makeshift kind, so it might
>just be design failure on my part, but I am wondering
>if my many gap design might have less (or more)
>resistance, which in turn might be leading to better
>results. Just FYI, I am running a 7500Vrms neon, a
>67nF cap, around 600A peak primary current an NO
>protection equipment except for a 3 piece safety gap
>(set pretty wide) directly across the neon. Would
>extra gap resistance be of help in harnessing the
>inductive kicking effects, described on the List some
I have 10K in series with the transformer but the voltage is very high and
the current is low. So even this high resistance is not a significant
factor in the "kicking effect". If you add resistance to the primary
circuit, your systems output sparks will fall dramatically!! It is very
important to keep the primary system resistance low but there is a point
where other factors start to take over. In other words, if your system
runs well now, gold plating your primary coil will probably have
practically no effect on performance. But if ones primary system is hooked
together with long lengths of 22 gauge wire, replacing that with wide
copper strap can make a giant difference.
>Is it good enough to just look at the lowest
>resistance of the gap or must we view the resistance
>per time period (much more complex) to fully
>understand the gap?
The best gaps I have used to date are high power sync rotaries with two
gaps with very close spacing. Forgetting all the sync effects, I have been
going for as few gaps as possible with very close spacing and not worrying
much about quenching. My rotary arcs about 1/4 inch before the gaps align
so the quenching should be very poor. Yet my systems does not seem to mind
that as long as the primary resistance is lowered.
This is a very complex and poorly understood area. I got side tracked
away from it but it sounds like you are picking it up again. I never
really came to any great understanding of gaps other than seeing that they
are affected by many things. Quenching, resistance, timing all have
different effects on different systems and it seems like what a particular
system lacks the most is what will give the most benefit by fixing. It
seems like everything is important. I keep the series resistances as low
as possible by using few gaps with heavy interconnects, I use sync gaps to
gain efficiency and get all the nice "kicking effects", and the spinning
electrodes seem to keep things cool (now days :-)) and give some quenching.
It seems like I really haven't been able to do much more to improve the
gap electrically and most of my recent effort have been making the thing
robust (I hope to have some pictures and such on my site soon of this...).
The best way I have found to study these effects is to use a primary
system WITHOUT the secondary system. A scope can be used to measure the
ring down times of different gap systems. The longer the ringdown the
lower the resistance and losses. Also, don't be surprised to see
significant effects of different interconnect wiring. The parasitic in the
primary wiring do all sorts of fun things from causing RFI, heat, losses...
I think the zero crossing effects also can play many games with all this...
I am not sure I said anything of real use here. However, I hope you are
able to REALLY understanding gap behavior better than I have been able to.
All I can really say, is that my gaps have reached a point where I am
really happy with them. However, I have reached this point mostly through
trial and error despite my attempts at being real scientific in their
design. I think I have been pretty good at "thinking and calculating" my
way through most Tesla coil design subjects but gap design at this level is
>Pondering in Germany,