Hi Gary,
I was using 3kV Panasonic film capacitors; hereâ??s a link for the actual
product:
https://www.mouser.com/ProductDetail/Panasonic/ECW-HC3F822J?qs=5PZ87NsE%252BJ%2FduaOvzk%252BWIA%3D%3D
I chose these caps (total 1nF per-rail) because I get an RC filter with
cutoff of ~318khz with the 500ohm resistors. My coil has a resonant
frequency of around this value, so my intuitive thought is that I need to
bypass frequencies at or above the coil resonant frequency. The total is
2nF which *is* a significant fraction of my 10nF MMC. Is some assumption
here in error?
These were used with 3 between each HV rail and RF ground. The caps caught
on fire after short use. You can see video of that event here:
https://youtube.com/shorts/Sx_lHwjR_bs?feature=share
(I should likely have shut this down sooner).
The main spark gap is working well as you can see from the video and was
set to fire closely under the unloaded NST voltage. The safety gap was set
such that it fired at the NST unloaded voltage, and then incrementally
increased until this no longer occurred. I found that in actual use that
once the safety gap fired, it would continue to arc continuously. I
adjusted out the safety gap just until stopped happening. The fire you see
occurred after that.
I suspect I need more caps - perhaps twice as many to be safe - but Iâ??m
also unsure if the safety gap is working as I need it. Having it not stop
arcing once it fires makes it useless.
Thanks,
Joshua
On Sun, Nov 21, 2021 at 4:05 PM Gary Lau <glau1024@xxxxxxxxx> wrote:
> Hi Joshua,
>
> I've never heard of blowing the caps in a protection network. What kind
> exactly were they? Was the main spark gap and safety gap set properly? If
> not, voltages can easily FAR exceed the NST faceplate rating. If that's
> what happened, consider yourself lucky that you lost the protection caps
> and not your NST.
>
> I strongly advise against the use of any inductors in NST protection
> networks. They just make matters worse. I have studied and simulated the
> various protection network topologies extensively and documented the
> results on this web page - http://www.laushaus.com/tesla/protection.htm.
> PLEASE heed the warning to set spark gaps properly. No protection network
> will help if your gaps are too wide.
>
> The usual resistors (500-1000 Ohms) used in protection networks constitute
> an acceptable loss of power. Less obvious, if your protection CAPS are too
> large, that will constitute a significant loss. Remember, with each
> "bang", you are charging and discharging your main as well as protection
> caps, so your protection caps want to be much, much smaller than your main
> cap. The energy in your main cap goes towards sparks, but the energy in
> your protection caps is just burned off in the resistors. The inductance
> of wirewound resistors is not significant.
>
> Regards, Gary Lau
> MA, USA
>
> On Sun, Nov 21, 2021 at 8:00 AM <pupman.com@xxxxxxxxxxxxxxxxxxx> wrote:
>
> > Hi,
> >
> > I just use large air cored chokes and a spark gap. It's simple, and
> > impossible to burn out. There's no need for some rube goldberg setup. HV
> > capacitors are expensive and like fuses can only be abused once. The last
> > set I made was probably on 2" PVC with 6" of #28 windings or something
> > similar to that. Protection spark gaps should be pointy and just on the
> > verge of firing. If they arc at full line voltage and no load, even
> > better. You don't get full nameplate voltage on a NST at full load
> > anyways, so don't expect that with a coil running. It's a not that
> > different from how you should never run a microwave oven empty. They must
> > have a load to prevent arcing.
> >
> > The solid state ignition transformer sounds fun. I ran my coils off DC,
> > rectified from NSTs or even unshunted transformers.
> >
> > The real key to not burning out your NST is proper tuning. I could tell
> > from the video you posted a week or so ago that the coil was still not
> > tuned correctly, or other adjustments are still needed. The sound of the
> > spark gap and how the arcs looked was the key. You should be able to get
> > streamers that grow in length and do not not look like DC arcs from a
> > power pack or electrostatic generator. Even for a coil the size of yours
> > the tuning will change due to the proximity of objects nearby, even your
> > arm or a ground lead. For maximum fun, tune the coil to you and the
> ground
> > wand you draw arcs off.
> >
> > If you can look at the waveform for the coil with an oscilloscope, that
> > helps too. I always meter off the ground connection of secondary with a
> > shunt resistor. It's the safest place to obtain a low voltage with
> respect
> > to ground for taking measurements or attaching instrumentation.
> >
> > Congrats of the first light!
> >
> >
> > On Sat, 20 Nov 2021, Joshua Thomas wrote:
> >
> > > Hello all,
> > >
> > > I've been struggling to build a lowpass ("Terry") filter to protect my
> > NST
> > > from RF feedback. The first one suffered arc-overs because
> > > the capacitor leads were too close together, and the second one had the
> > > capacitors fail - apparently only 9kV per rail wasn't enough. It's a
> 12kV
> > > NST so I assumed 18kV > 12kVx(sqrt 2) and therefore would be
> sufficient.
> > > Apparently not.
> > >
> > > What suggestions are available for those who have made a NST filter
> that
> > > was successful? Particular capacitor types/brands would be welcome, as
> > well
> > > as how the physical layout was accomplished. I use two 500ohm/100W
> > > wire-wound resistors for the R half of the RC filter.
> > >
> > > On a different topic I got a used OBIT for $35, but it appears to be a
> > > solid-state one running pulsed DC. The model is Allanson 2275-628G.
> > 17.5kV
> > > "peak", 45mA, at 20khz. I mainly bought this for curiosity to see how
> it
> > > might perform on a coil. I'm a little suspicious of the voltage and
> > > amperage, as the total VA rating is only 87.5VA - which is a fraction
> of
> > > the 560VA that would be expected from 12.5kV RMS (17.5kV peak) at a
> full
> > > 45mA!
> > >
> > > What experiences has anyone had with one of these?
> > >
> > > Cheers,
> > > Joshua Thomas
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > > --
> > > Joshua Thomas
> > >
> > > My new email address is: joshuafthomas@xxxxxxxxx
> > > Please update your information if you have not already done so.
> > > _______________________________________________
> > > Tesla mailing list -- tcml@xxxxxxxxxxxxxxx
> > > To unsubscribe send an email to tcml-leave@xxxxxxxxxxxxxxx
> > >
> > _______________________________________________
> > Tesla mailing list -- tcml@xxxxxxxxxxxxxxx
> > To unsubscribe send an email to tcml-leave@xxxxxxxxxxxxxxx
> >
> _______________________________________________
> Tesla mailing list -- tcml@xxxxxxxxxxxxxxx
> To unsubscribe send an email to tcml-leave@xxxxxxxxxxxxxxx
>
--
Joshua Thomas
My new email address is: joshuafthomas@xxxxxxxxx
Please update your information if you have not already done so.