Re: Why does running an NST on an async gap kill it?

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

Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>

Hi Gerry and Mark,

The filter at:

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

works like this.

MOVs - The MOVs provide absolute "fool proof" protection against over 
voltage right at the NST.  They are 1800V +-10%.  Unlike a safety gap, 
there is nothing that can go wrong.  As I was working on this filter, 
reports of miss-adjusted safety gaps were very common.  So I added the MOVs 
since there was no adjustment.  Today, folks seem better at adjusting gaps 
so it is not as much of a problem, but the MOVs simply can't allow the NST 
to go far past spec no matter what happens.  Ideally, if the system and 
real safety gap are adjusted right, the MOVs will never do anything.  I 
have had reports of MOVs burning up, but practically no reports of failed 
NSTs :-)))  If run too hard, the MOVs melt internally and hard short out so 
even under the worst conditions they will still save the NST.  Also, if the 
MOVs "get warm", that is an early indication of system problems.

RC Filter - The filter is set at about 290kHz.  It is really not designed 
to filter the Fo frequency but rather the 30MHz type harmonics and giant 
initial firing noise.  If the frequency were much lower, the loss in the R 
or the large C would start messing with the coil's operation.  The way NSTs 
are wound, the inductance, resistance, and capacitance of the HV windings 
is "sort of" uniform across the winding.  "Few hundred" kHz stuff does not 
do much harm.  But >1MHz bursts can hit the outer layers of the winding 
pretty hard and cause local breakdown.  So the RC filter really is to 
stop >1MHz type signals rather than the fundamental.  The filter also 
provides drain resistors to the entire system (they are hiding in there 
;-)) but now with MMCs that is not a big deal.  There is inductance in the 
resistors I use but ringing affects are swamped by the resistance and the 
small inductance really has no affect other than perhaps lowering the 
cutoff frequency a little.

BTW - Normally, power resistors like these run at 350 degrees C at rated 
power!!  That is fine for the resistors, but early filters with small 
resistors got all kinds of complaints about the resistors getting super 
hot!!  So I made them way oversized just to lower their operating temperature.

Safety gap - This safety gap really should take the brunt of over voltage 
events.  It is designed to take many hits and make a lot of noise 
indicating a problem.  Properly set, the MOVs will never see high voltage.

Probably the biggest preventer of Fo signal getting across the NST is the 
main gap being placed across the filter's output.  When the main gap fires, 
it effectively shorts the output of the NST shunting the RF voltages.  This 
paper explains the details or ideas that went into the design of the filter:

http://hot-streamer-dot-com/TeslaCoils/MyPapers/rcfilter/rcfilter.html

I should also say the fusing the input of the NST is a very good 
idea.  There is a resonant mode that can saturate the internal shunts of 
the NST causing very high currents to be drawn.  This effect is very 
possible with large primary cap sizes.  A simple fast blow input fuse can 
prevent damage in this case.

With the filter and fuse,  The NST should never fail unless it is just too 
old and damaged to begin with.  Resonant voltage rise probably accounts for 
90% of NST failures.  The safety gap, MOVs, and even fuse will stop 
that.  A 15kV NST in uncontrolled resonance can hit 80kV!!  Failure is 
certain.  That simply cannot happen with the filter in place.  I think the 
RC portion taking out the >1MHz noise will take care of the other 10% of 
problems.

Admittedly, the filter is over designed and sort of "overkill" but I did 
not want to take any chances.  I figured it was better to have a rock solid 
protection circuit that was simple and easy rather than having a lesser 
circuit we would have trouble with.  Unfortunately, it is sort of pricey, 
but may of the parts are not critical and you can substitute caps and 
resistors of sort of close value.  I have never had any complaints about he 
cost and "thank goodness" none about the filter working.

Since it has been a few years now. I am curious if there "have" been any 
failures of NSTs with the filter?  A NST may fail due to old age or 
previous damage, but failures of good NSTs with the filter should be almost 
none??  We can talk of all the theory behind the filter all day long, but 
the proof is in how well it has actually done out there!!  If the filter 
has let anyone down, I would be happy to hear of the details and see if 
there are any further improvements we could make.  I did change the number 
of MOVs for various NSTs and allowed a little extra room for 140VAC 
operation since folks seemed to like to run coils at "11" ;-))



The original poster here asked why ASYNC gaps are bad with NSTs...  A NST 
with a resonant sized cap "must" be drained with a spark gap.  Without a 
spark gap firing, the voltage at each peak will go like 20kV, 40kV, 60kV, 
70kV, 80kV....  So in about 1/50th of a second, the NST will be 
destroyed!!!  ASYNC gaps fire whenever "they" want to rather than when they 
should...  In a resonant system there is no room for error.  If the gap 
does not fire, the NST will die, and instantly!!  A safety gap would 
prevent damage but it "must" work!!  There is no room for any wiring 
mistakes or miss settings.  "Resonant" size cap systems used to be very 
popular since they could produce long arcs until the NST blew up.  But that 
was due to running the system at far high voltage that they should have 
been run.  Usually a "WOW! look at those great streamers... POOF!!!..." 
situation...  Turning systems up until they failed was common.  But now, 
NSTs are not available in giant piles behind the local sign shop for free 
like they were 20 years ago...  NSTs are no longer just fuses...  Now, an 
NST failure is a pretty big problem!!!

Now days, we like to use LTR size caps that are free of the resonant rise 
problem and far far more forgiving.  Sync LTR system voltage actually drops 
way down if the gap fails to fire!  SYNC LTR system can run at very nice 
power levels surpassing even the highly overloaded resonant systems.  That 
is the only type system I would recommend.  There are also static gap LTR 
systems for those that do not want to mess with a rotary or triggered 
gap.  They don't have the power of the SYNC systems but they run very 
comfortably.  MMCs now allow us to easily make caps of just the right 
size.  We don't have to take what ever we can find or make a big oil rolled 
cap and just use the value it turns out as.

Modern coilers may not appreciate how tough it was to make an oil rolled 
cap or blow 10 NSTs a night trying to get 1 foot streamers ;-))  That is 
they way it was ten years ago...  But LTR sized caps, MMCs, SYNC gaps, 
filters, computer programs,... have all come together in the last several 
years to make coiling a far more reliable hobby.  Even I am amazed by how 
"fast" things really did change...  I thought MMCs would take about ten 
years to become main stream :o)))  It turned out to be ten weeks!!  Even 
the non-Internet connected coiler community caught on super fast!!  I don't 
think one MMC post ever made it to TCBA...  We were all so busy, and in the 
three months between issues, it was over ;-))

We really did not write much down either ;-)  But now that the storm of the 
last few years has slowed, we have more time to write.  I am planning on 
finally writing a bunch of stuff up and I hear there are some actual books 
in the works too!!  Hopefully, with some much more solid references 
detailing the last several years, things will be much more clear...  It 
looks like solid state is going to be "BIG" so we have to get the old work 
wrapped up...

Cheers,

         Terry Fritz
         terrellf-at-qwest-dot-net


At 11:08 AM 8/8/2003 -0600, you wrote:

>If you look at the RC portion of Terry's filter (1000 ohm series R and .0006
>uf effective shunt capacitance to ground,  it is a one pole low pass filter
>with a cut off frequency of 265 KHz.  Any frequency below this will
>essentually be unattenuated by the filter, especially 60 Hz resonance
>effects.  I believe the RC portion of the filter is intended to only filter
>the high frequency harmonics of the spark gap firing from getting back to
>the NST.  It looks like the resonance frequency of the tank (assuming
>resonance < 265 KHz) will not be attenuated much.  However, the conduction
>resistance (~2 ohms) of the main spark gap will attenuate much of the tank
>voltage.  I have scoped the voltage across a static gap and found only RF
>hash during conduction (measurement is a little tricky to keep RF fields out
>of the measurement loop). The magnitude of this hash was maybe a couple
>hundred volts.
>
>I believe, resonant charging effects is the main culprit on killing the NST
>when the spark gap of an ARSG fails to fire at the appropriate time
>(especially if the ARSG is not turned on before the NST is turned on).  The
>MOV's  and safety spark gap will help clamp this overvoltage resonant
>charging effect.
>
>I'm wondering, if the safety spark gap was moved to be directly across the
>NST bushings and ground, if the NST would be better protected.  Since the
>charging current is coming from the NST,  the sooner this can be shunted the
>better. .  I have to think what this would do to the 1000 ohm power
>resisters since any Cp energy would then be dissapated in the resisters.
>Maybe this isn't such a good idea.  Comments??
>
>Gerry R
>Ft Collins, CO
> >
> > I can personally attest that the MOVs will fail if you don't set (forget)
> > to properly set the safety gap....
> >
> > The wirewound resistors in Terry's filter design do have a fairly large
> > inductance.  I remember trying to measure it once (the 1kohm, 100W
> > Ohmites), but the RLC meter I was using had issues with the large
> > resistance, and wouldn't give me reliable results.
> >
> > Another aspect of async gaps killing NSTs is that when used in conjunction
> > with near resonant sized capacitors, resonant rise can overvolt things if
> > the gap fails to fire due to falling on a voltage null.  I would expect a
> > good "Terry" style filter and LTR capacitors to significantly reduce NST
> > deaths from use with async gaps.
> >
> > Cheers,
> >
> > Mark Broker
> > Chief Engineer, The Geek Group
> >
> >
> >
> >
> >
> >