NST protection

From:  D.C. Cox [SMTP:DR.RESONANCE-at-next-wave-dot-net]
Sent:  Monday, April 13, 1998 10:25 PM
To:  Tesla List
Subject:  Re: NST protection

to: Sulaiman

If you have the protection caps and sparkgap in place it can produce RF due
to the inductance of the xmfr leads which generated VHF frequencies which
blew your NST.  Set the protect gaps only with 60 Hz from the xmfr and no
caps in the circuit.  Also, be sure the series resistor is directly off the
xmfr bushing and BEFORE the protect to ground caps.


> 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.
> 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