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Re: Overcoupling?

>Original Poster: Terry Fritz <twf-at-verinet-dot-com>

>>I was tuning up my coil again a couple of days back (I now have my other
>>NST back from repair and have stretched by alu duct toroid to 36" dia - 2kW
>>in) and at full power (2kVA) it now tunes at around 8.3 turns (WinTesla
>>predicted 7.6). However when I hit this tune I think that I identified
>>symptoms of 'double-humping' - I saw a ring of heavy inter-turn corona and
>>mild sparking about 3/4 of the way up the secondary, and the occasional
>>spark all the way down the secondary coil.
>>
>>I raised the secondary up about 1/2" and this was reduced, although not
>>absent. The sec base is now about 1/4" above the level of the primary, and
>>the gap around the sec is about 1.5". My toroid's centre disc is temporary,
>>not covered completely with foil, so maybe this is causing probs too.
>>
>>Anyone else have any experience with coupling adjustments? If you have any
>>advice I'd be happy - I don't want to reduce the coupling too much...
>>
>>PS Spark length so far is 60", although since I am in a confined space, if
>>I try for beyond this, the coil seems to favour the roof beams and walls as
>>targets. I hope I can achieve more - do you think I can get that 6 feet??
>>
>>Alex Crow

>Hi Alex,
>
>   When in doubt, I would reduce the coupling.  Too little coupling and your
>sparks are short. Too much and your coil may burnup!!
>
>	The corona area you mention is probably a natural high field stress area
>that linear wound Tesla coils have about 2/3 up the secondary.  Raising or
>lowering the top terminal may help this if it looks like it is going to
>arc.  I would fill in the center disk covering as this may help the high
>field stress things you are seeing.  I think your coupling is really too
>high and the output sparks may easily grow longer if you reduce it.
>Overcoupling is really bad and can cause many problems.  It is always a bit
>odd to move the secondary away from the primary to get longer sparks but
>this is often the case...
>
>	Terry

If the main consequence to overcoupling is damage to one's secondary from
the sparks that race along the length of the secondary, could this be
helped by adding several disks of an insulator, say LDPE, along the
length of the secondary, disk ID tightly hot-melt glued to the secondary,
to effectively increase the top-bottom surface length of the secondary?
I seem to recall seeing this done on someone's wep page, no idea who that
was.  Same idea as on the ribbed HV insulators we see on pigs, NST
bushings, etc.  In clear acrylic, it had that kind of futuristic, Jetsons
look to it too!

Gary Lau
Waltham, MA USA