Re: SV: NST resonance, also NST Failures

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jon Tebbs by way of Terry Fritz <twftesla-at-uswest-dot-net>" <jgtebbs-at-eos-dot-net>

Hi Jan,

I've inserted some observations below:

Tesla list wrote:
> 
> Original poster: "Jan Ohlsson by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <jan.ohlsson-at-mbox319.swipnet.se>

snip

> On a more practical level I had great problems when starting my first coil
> with the safety gap taking over the main gap regularly after a brief moment
> every time I started the coil. The safety gap was set so that it never
> fired before the main gap without the primary connected. I am an audio guy,
> so it took me a while to conclude that the only reason why there could be a
> vastly different voltage at different points of short and very heavy
> conductors was that there were large amounts of RF of at least a couple of
> hundred MHz present to cause standing waves on the short connections.
 
A possible contributing factor to the safety gap _taking over_ may be due
to the
differences in electrode shape. If the safety gap electrodes are of a very
small
radius, or even pointed, they may end up breaking down undesirably due to the
voltage stress being concentrated in a significantly smaller area. A larger
radius of curvature may prove helpful.

> I then built a simple RC filter with non-inductive resistors in series with
> the primary and capacitors of  500 pF to earth across the NST. That should
> take care of the RF, I thought. The safety gaps still took over the main
> gaps. I did not understanding anything at first and was at the brink of
> giving up the whole coil project after having fried one NST after beeing
> stupid and having widened the safety gap.
> 
> Then I tried the same filter across the NST without the primary connected.
> The safety gaps still arced violently. It was then I realized that it was
> probably two mechanisms at work at the same time , RF feeding back from the
> primary and NST resonance with my filter caps. I tried series resistors
> between the NST and the filter, and voila, the arcing disappeared.
 
I suspect that you have the safety gap on the NST side and not the TC primary
side of the filter.
If you take a look at Terry's filter circuit, you will see that the safety
is on
the TC primary side. This has the same effect as the series resistors that you
introduced between the NST and the filter. You can find Terry's filter at:

http://hot-streamer-dot-com/TeslaCoils/Misc/NSTFilt.jpg

I successfully use this circuit in conjunction with both a TCBOR static gap &
SRSG with only occasional firing of the safety gap.

> That was a bit puzzling, so small capacitors should not cause resonance,
> according to what I had read  on this list and elsewhere. That is what
> prompted me to try to measure the resonances.

Any time you combine an inductor (NST primary or secondary, etc.) with a
capacitor, you create a resonant circuit. That resonance may not be what or
where you expect, but it is there...

snip
 
> Regards,
> Jan

On the topic of NST failure analysis:

>Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-uswest-dot-net>"
><evp-at-pacbell-dot-net>

>> At 400kHz, the high
>> frequency voltage is just not going to get very far in the output windings
>> and that high voltage my hit only a few layers and BLAMMO!  Since trying to
>> figure out how the high frequency voltage distributes in the output winding
>> of an NST is rather messy.  
>
snip  
>
>        I think careful post-mortem examinations of failed transformers should
>yield information on exactly where the insulation failures occurred and
>thus to a much better understanding of failure modes.  To perform the
>exams one first needs failed transformers along with the circumstances
>of their failures, and then dig into the bloody tar (very carefully) and
>see what happened.  In this regard, the frequent reports of transformer
>resurrection by melting the tar or "de-potting" makes me suspect failure
>unrelated to inter-turn or inter-layer shorts.
>
snip
>
>> Reports like yours of NSTs taking this high frequency voltage are
>> encouraging.  However, I suspect that is an exceptional case.  
>
>        I'm not so sure, but it would sure be interesting to know!
snip
>
> 
>> Cheers,
>> 
>>         Terry
>
>Ed

On the topic of NST failures related to HV RF, I have performed a post
mortem on
two such about ten years ago. In both cases I found the failure mode to be
inter
layer arcing along the outside edge of one secondary coil. Both arcs were from
one end of the outer layer to a point about one third in towards the bobbin,
burning the edges of the paper insulation between these points and the
insulation off the wire. The wire was also found to be burned open at the inner
most point of the failure site. At the time of failure, these had been in use
without safety gaps or other filtration.

For the record...
-- 
Jon G. Tebbs
<jgtebbs-at-eos-dot-net>