Gary, I had been reading your notes on filtering and resistor inductance
here: http://www.laushaus.com/rcl_filter_simulation.htm
I had to check with the manufacturer, but the self-inductance of my 500 ohm
wire-wound capacitors is 220 uH. My resistor's actual measurement is 470
ohm. An RCL simulator with 470 ohm, 220 uH and 1 nF (the setup I was using)
shows a small signal gain in the frequency range my coil resonates. It's
not much - less than 2 dB - but it's enough to make me wonder if this
contributed to the damage to my capacitors.
I will be choosing a capacitance value for the rebuild of the filter which
doesn't have any gain in my coil resonant range. As well as significantly
more capacitors to limit the stress per capacitor.
-Joshua Thomas
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.
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> > >
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> >
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--
Joshua Thomas
My new email address is: joshuafthomas@xxxxxxxxx
Please update your information if you have not already done so.