NST protection

From:  Malcolm Watts [SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent:  Tuesday, April 14, 1998 5:13 PM
To:  Tesla List
Subject:  Re: NST protection

Hi Sulaiman, all,

> From:  Sulaiman Abdullah [SMTP:sulabd-at-hotmail-dot-com]
> Sent:  Monday, April 13, 1998 6:25 AM
> To:  tesla-at-pupman-dot-com
> Subject:  NST protection
> Hello all,  I had a 15 kV 30 mA NST to start my first Tesla Coil.
> I'd been "playing" with various capacitors and inductors,
> a simple ball-bearing spark-gap, Jacob's Ladder etc. ... no problems.
> Trying to be "smart" I made a 'protection' circuit/filter and used
> the ball bearings as safety gaps.
> For each side of the NST;
> Safety-spark-gap to ground ( 5 mm gap, 1" steel balls)
> 1.1 nF Ceramic to ground   (two parallel sets of 4 x 2.2 nF each)
> 7 kOhm 30 W wirewound resistor in series with
> 18.5 mH air-core inductor (0.4mm magnet wire, 109mm dia, 750 turns)
> Great (in theory).
> Surprisingly the safety-gaps fired before 100% on the variac,
> with no filter the gaps didn't fire.
> No problem, simulated resonant-rise on PC so decided to adjust gaps.
> During the adjustments (each adjustment involved 'firing' the gaps)
> one side of the NST died :-(
> After consideration I beleive that safety gaps SHOULD NOT be across
> the NST as they will cause 'nasty' voltages, I've always seen the
> safety gaps shown across the NST, and I've read many sad tales of
> NST's forming 'tracks' internally. I think that all gaps should be
> isolated from the NST by at least a resistor, preferably a filter
> such as above.

Safety gaps can't cause nasty voltages. My advice - throw the chokes 
away and get your transformer secondary leads as short as possible. 
If there is no wire/chokes etc between the main gap and transformer 
secondary, there is no way a safety gap set wider than the main gap 
can possibly fire. Again, research I've done in the past suggests 
that any parasitic energy storage component between the transformer 
secondary and main gap is responsible for the severe voltage hikes 
and these occur *when the main gap initially fires* as an energy 
source is then trapped between a virtual short circuit and the 
transformer secondary.


> Was it coincidence that the NST failed under the above conditions?
> or is a spark-gap (even a "safety" gap) directly across an NST a
> bad idea? Comments ? ............
> Bye ... Sulaiman