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Re: [TCML] Advice for my first SGTC
Ok, pay no mind to the Tesla map's "optimal capacitance." The reason
I say that is it doesn't really apply to high power systems. I'll explain
this as it makes sense to me, picking a "resonate" or "larger than
resonate" tank cap is a trick to suck more power out of current regulated
supplies (like NST's.) You see, if you match the impedance of the tank
capacitor to that of the power supply at 60 Hz, it will transfer current
much faster leading to higher power, this is great for low power NST's. If
you are NOT using a current regulated supply there really is no need to do
larger than resonate or resonate. In the case of MOTs, they are fairly
hardy when used this way, but I suspect if you used a resonate cap, they
would go up in smoke pretty quickly. In the case of larger coils like pole
transformers, using a resonate cap just means you draw more current through
your carefully sized ballast than it normally would. With that said, MOTs
are technically current regulated yes, but they wont really draw more than
the Tesla tank circuit will allow, so the regulation does next to nothing
(think of it like using a small NST that can give 30 mA, and only drawing
15 from it, you can remove the current limiters but the system will still
only draw 15 mA.) With that said a capacitor of around 80 nF should only
want to draw about say 20-25 amps from the wall outlet at a break rate of
about 700, you can ballast it back more but on its own it won't go any
higher if you omit a ballast (unless you use a bigger cap or a much higher
break rate.) What kills MOTS in this setup is POOR GROUNDING. I don't
kill mots using this setup, unless my ground connections are bad, and then
ground strikes cause bright arcing in the mots until eventually they die.
For one reason or another, bad grounding has been a real system gremlin for
me, so I keep extra MOTs on hand, and if one blows I plug in another and
go. I guess bad grounds are to be expected when you take your coil places
and have to use what is available... At any rate drawing arcs will always
pull more current than running a TC, so yeah they will get hot, but in TC
service they won't really get that warm except on super long runs.
As for an MMC, yes in theory you can use just an 8 kV cap, but it will
probably fail... to calculate a good ceiling for your capacitor (and other
may chime in here) take you supply voltage 4kV/.707=peak volts 5.6kV*2.5
=14kV. This seems much higher than it needs to be, and it is, but it will
make for a much longer lasting cap, and is still significantly less that
would be required were you using a doubler (rerunning the math 8kV with
doubler/.707=11kV*2.5=28kV...) assuming you use the .15uf 2kV caps you
should be able to get away with 4 strings of 7 caps, which is very
reasonable (at $6 a cap you come out to 160 some dollars.) I bought caps
in the 940C series for my MMC, substantially cheaper BUT they do not have
nearly the pulse and RMS current ratings of their 942C counterparts, and I
don't think they self heal. But then again I used way more caps than
should be necessary, so I should be fine once I get around to actually
using them... for a cap this small I recommend the tried and true, $160 is
not bad for something that will last longer than you do.)
Finally, my website is down, and has been for quite some time. I'm
not sure why, it is a shame actually, I put a lot of work into and I'm
looking to move it elsewhere, just haven't found time...
On Tue, Nov 20, 2012 at 9:26 PM, Atomic <atomicrox@xxxxxxxxx> wrote:
> Scott, that's a great video! Can't access your site, though:
> I'll probably drop the doubler.
> How do you calculate the optimal capacitance for dual mots? Tesla map gives
> me 315.8nF which is definitely not gonna happen :(
> Also how do you keep the MOTs from frying without a ballast? Mine gets
> *very* hot if I draw arcs for about 20s.
> TBH I don't think the doubler will cost that much - after all it's just 4
> microwave caps, 2 ceramic caps, a lot of cheap diodes and two high power
> On the other hand I could probably design the MMC for 8kV without the
> doubler, right? that's 4 times less caps for the same Farads!
> Jon, thanks for finding that. Better not risk getting the Russian caps.
> Matt, unfortunately almost all beer brands here use brown bottles. I might
> see if I can find wine bottles but most of those seem to be dark green and
> I definitely won't be able to get many with the same bottle type.
> OK guys, if I decide to go with the 942C MMC what are the odds that they'll
> blow up? I really don't wanna spend 200 dollars on this just to blow it up
> on the first light.
> Thanks for all the tips!
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