NST protection

From:  terryf-at-verinet-dot-com [SMTP:terryf-at-verinet-dot-com]
Sent:  Monday, April 13, 1998 7:23 AM
To:  Tesla List
Subject:  Re: NST protection

Here's my guess....

>From:  Sulaiman Abdullah [SMTP:sulabd-at-hotmail-dot-com]
>Sent:  Monday, April 13, 1998 6:25 AM
>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.

How old was the neon?  If it had been outside for years and years, the
sudden new stress may have been too much.

>For each side of the NST;
>Safety-spark-gap to ground ( 5 mm gap, 1" steel balls)

Each side should be protected to 15000 * SQRT(2) / 2 or 10600 volts.  That
implies a gap of ~3.2mm with 1 inch diameter electrodes.  A 5mm gap fires at
about 15000 volts!  That may have been the problem.

>1.1 nF Ceramic to ground   (two parallel sets of 4 x 2.2 nF each)

These caps must be on the transformer side of the resistors and chokes.

>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,

The 1.1uF caps were resonating with the output inductance of the neon.  This
causes the output voltage to go much higher than expected.  If the gaps
fired the neon was pushing about 22kV RMS at 40mA!  :-(  If you ran at 100%,
the stress may have been very high.

>with no filter the gaps didn't fire.

No resonant rise.

>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 :-(

If you widened the gap the voltage may have gone very high!

>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?

Gap too wide and resonant rise probably killed it from over voltage.  The
earlier ladder stuff may have caused damage that took awhile to show up.  A
track can start and cook for awhile before the output is lost.  

>or is a spark-gap (even a "safety" gap) directly across an NST a
>bad idea? Comments ? ............

If everything is designed well and the variac isn't turned too high, the
spark gaps would not be needed.

>Bye ... Sulaiman