neon trannie loading?

From:  Bert Hickman [SMTP:bert.hickman-at-aquila-dot-com]
Sent:  Saturday, June 20, 1998 9:20 PM
To:  Tesla List
Subject:  Re: neon trannie loading?

Tesla List wrote:
> ----------
> From:  Jeff Corr [SMTP:corr-at-enid-dot-com]
> Sent:  Saturday, June 20, 1998 1:33 PM
> To:  tesla list
> Subject:  neon trannie loading?
> I got a flyer here from Franceformer saying that
> neon transformers should be loaded to 75% to 85%
> of the designated max. current.  Ex : a 30ma neon
> should be loaded to 24ma or 25ma.  If this was true,
> would this not explain why tesla coils kill neons
> fast?  (charging up the cap would be like a temporary
> short, pulling much more than 25ma, right?)
> Could this also be why resisitors in series with the
> chokes help save neons?  Since adding resistors, I
> have not ever lost a neon, and was getting very good
> sparks.
> Any thoughts?
> Jeff Corr
> 2114 Monitor
> Enid, OK 73703
> http://www.harvestcomm-dot-net/personal/corr/


Good ideas! And paritially correct. 

When loaded to 75% to 80% of design current, the voltage output of a NST
will drop to less than half the open circuit voltage. Normally, shorting
the secondary will still only cuase the faceplate current to be drawn.
It's more the combination of 60 Hz high voltage and RF HV backfeeding
into the windings which tends to destroy these transformers. NST's tend
to fail shorted (either carbon tracking or shorted turn(s) which
eventually short out the secondary). The added low-pass filtering that
comes with RC or RLC protection networks helps block most of the high
voltage RF generated when the gap fires from getting to the delicate
transformer windings, prolonging its life. However, given enough running
time, your transformers are STILL most likely doomed. 

If you're running with resonant charging, the tank cap recharging
current can be 2-3X+ the faceplate rating - this can cause current
overstress and overheating of the interior of the windings during
extended runtimes. Overall, 60 MA transformers seem to be a bit more
robust than 30 MA ones for Tesla Coil service, especially in resonant
charging operation.

-- Bert --