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Re: MOTs Rule!
Hi Greg, all,
Comments interspersed.
> Original Poster: "Gregory R. Hunter" <ghunter-at-accucomm-dot-net>
> I've just tested my twin MOT supply with dual voltage doublers on my
>4" x 24" coil. The supply is based on two MOTs salvaged from
>discarded 900 Watt ovens. I borrowed the idea for it from Finnish
>Coiler Marco Denicoli. I normally run the coil with two 9KV/60ma
>neons. I use a single static vacuum gap made of 1" copper pipe
>fittings (yes, beefy) and a .023uF MMC. Toroid is 3.5" x 14". I had
>to close the gap a little to get reliable firing from the MOTs, so I'm
>guessing voltage is in the 8KVAC range. I used no ballast--the MOTs
>go as hard as they want to. I ran the vacuum cleaner motor on my
>gap at full throttle in hopes of quenching the higher current.
Ahhh. Another low voltage coiler ;o)). I donīt like MOTs (see my mail
from last week as to why), but you arenīt overvolting your dual MOT
setup, as long as you ground the "center tap" between the two MOTs.
However, I doubt your gap will quench properly at this power level.
It doesnīt matter how hard your "blower" is running. Itīs just too much
power (1800VA as of the spec plate). Your setup is probably giving
you MUCH more power as the MOT shunts donīt work in a true short.
As you are only using 2 MOTs, a normal 3 piece safety gap will provide
your MOTs with adequate protection.
> Results were pretty satisfying. The streamers aren't any longer than
>with the NST supply, but they are much thicker, brighter, more
>intense, and more chaotic. They look as hot as lightning bolts. There
>also seem to be more of them. The spark gap sounds different too
>--much lower in pitch--almost a mechanical grinding sound.
You made up for a MUCH stiffer PSU and didnīt change the primary
cap, so what you are seeing, is the effect of what increased BPS
does to your TC output. If you switch over to a SRSG, your spark
length will increase (in my opinion) dramatically. If you then increase
your primary cap (donīt go LTR, tho, it will become ridiculously large),
you would be set for large sparks. Guessing, I would say it is time to
build a larger secondary. That 4" isnīt gonna survive that kind of
power for long ;o))
>It gives me the creeps. Best spark length is about 30" (to my strike
>rail), and rarely a single streamer may reach out 36" or so. This
>thing really wants a bigger tank cap & larger toroid. I plan to remedy
>both very soon.
Yes, do so. Youīll love the results. With a SRSG and a bigger cap,
(and more secondary), I donīt think 80+" are unobtainable. However
keep a close watch on those MOTs. They may not last for long (I
bet they get pretty warm already). DONīT go for mains resonance
during cap design. Rather, increase your breakrate. Iīd shoot for
240 bps (200 if your a 50Hz coiler). Also remember, due to your low
voltage AND your big cap, you will be running a large (TC) primary
current, which calls for beefy components.
> I may even immerse the MOTs in oil to enhance cooling.
I WOULD do this, but not only because of cooling, but rather for
insulation reasons. MOTS ARE just as fragile to overvoltage as
NSTs are, if not even more so.
> Low budget coilers unite! MOTs rule!
Remember, the (E)MMC has itīs price based upon the Joules it
can store. It wonīt make a (noticeable) difference in price if you
build a 5 Joule MMC cap using high voltage and low capacity or
if you build that same 5J (example) cap for low voltage and high
capacitance. However, reconfiguring an already built (E)MMC
is as near as your soldering iron ;o)).
Coiler greets from Germany,
Reinhard