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.
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