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Re: Safety gaps destroyed my Neon ?



Hi Vivian,

> Original Poster: "Vivian Watts" <V.C.Watts-at-btinternet-dot-com>
>
> It has occurred to me that my safety gaps may have destroyed my Neon
> because they DID fire.  I had two 5KV Neons in anti-phase with a common
> ground (I couldn't remove the ground).  I had two safety gaps, one from each
> HV leg to ground.  Also a further safety gap across my rotary - which did
> not fire.  My thoughts are, that say at a peak the Neons where producing +/-
> 5KV i.e. 10KV to the tank circuit.  With a bit of rise the voltage became
> say 15KV thats 7.5KV with respect to ground.  Now if ONE of the safety gaps
> fires to ground thus shorting one Neon, the tank capacitor at 15KV presents
> its full voltage across the second Neon (until that safety gap fires) and so
> may have destroyed it.  The solution I think, should have been, not to have
> centre grounded safety gaps with a centre grounded Neon set-up.

You donīt seem to have a lot of luck with neons. Sorry to hear that. Are these
5kV NSTs center tapped? If I understand you correctly, you are seriesing the
outputs of the two for 10kV, right? IF this is what you are doing AND IF the
NSTs have center taps, I think I know what your problem is: The insulation
of your NSTs is designed for only 2500V above ground. When you series these,
you are practiclly overstressing the insulation by a factor of two. This might
be the max. it can take. Any increase (due to whatever) will fry them, pretty
much, instantly. Seriesing two non center tapped NSTs shouldnīt be a problem
tho. The connection between xformer #1 and xformer #2 should be grounded
(becoming effectively a center tap). That way you wonīt overstress the
insulation. This will, however, ONLY work for NON-center tapped xformers.
For center tapped units you would need to run each neon through a 1:1
isolation transformer on the 220V side. Depending on the VA of each neon
this might pose some problems finding such an isolation transformer.

Iīm not sure if your safety gaps killed the xformer. If one gap fires AND the
voltage is STILL (after the gap fires) higher than the allowed 5kv, the second
gap would conduct and relieve the neon of itīs burden. While this might take
several ĩsecs (for the gap to fully clamp the excessive voltage), I donīt think
the insulation would die that quickly. Properly set, the voltage across the
safety gaps will never be higher than 5kV. I think it is more the problem
discussed above, as the 5kV is already stressing your neonīs insulation.

5kV is a little on the low side (hard to make a good quenching gap for this
voltage), but canīt you find a NST with say 7-8kV? This is the "norm"
voltage for german NSTs. While a lot of coilers say high voltage is better,
my "low" voltage coil seems to thrive on 7.5kV NSTs (50" with 900VA and
a not yet maxed out 57-63" with 1257VA). My NSTs (75mA  rated) weigh
about 18kg (40 U.S. lbs) each. I have sucessfully modified a 75mA unit
for 350mA. This does limit the run time to about 30min per hour, (copper
losses are increased by a factor of 25-30!!), but modifing them for 170-
200mA poses absolutely NO thermal problems. They hardly change
temperature within a 30min run at this current rating.

More luck to you with new neons!!


Coiler greets from germany,
Reinhard

(P.S: if I were you, I would still bring the coil to the Teslathon. Surely some
coiler would let you use his xformer).