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Re: So Maybe MOTs Don't Rule
Tesla List wrote:
> Original Poster: "Gregory R. Hunter" <ghunter-at-accucomm-dot-net>
>
> Fellow Coilers,
>
> My earlier enthusiasm for the twin MOT supply with two half-wave voltage
> doublers has faded somewhat under the harsh light of experience. After
> packaging the MOTs in a metal chasis (US Govt. .30 cal. ammo can, perfect
> fit!), I went through several pairs of blown doubler diodes. The culprit
> turned out to be the safety gaps across each MOT HV terminal. Each time
> the safety gaps fired, the doubler caps discharged thru the diodes and blew
> them with overcurrent. I tried discarding the safety gaps, but now the MOT
> terminals simply arced over the the metal chasis due to inadequate
> separation. This had not been an issue before, because the safety gaps
> were set more narrow than the chasis/MOT separation, so all transients
> simply jumped the safety gaps. Now the MOTs made their own safety gaps.
> More blown diodes.
>
> I added additional insulation, which cured the arc over problem, but now my
> filter resistors began to fail. I used the flat, wire-wound type which is
> wound on an aluminum strip with a hole punched thru each end--great for
> easy breadboarding but lousy for HV RF. Transients simply arced to the
> strip, bypassed the resistor body, and arced back onto the other end of the
> resistor. This HV RF defeated even my improved MOT insulation. More blown
> diodes (ARRRRGGGHHHH!). So I modified the resistors by removing the metal
> strip and suspended them from stiff wire leads. I also moved the safety
> gaps to the final output terminal of the supply, giving the transients some
> place to go besides into my MOTs. Ahhh, success. No more blown diodes.
> This foolishness used up almost all my junk box rectifiers. Oh well...
>
> Next problem: way too much current and not enough voltage. The MOTs got
> scorching hot even with runs of less than a minute, and the filter
> resistors were fairly smoking! The culprit was my .91uF doubler caps,
> which presented essentially a short circuit to the MOTs. I cured this by
> adding two more .91uF caps (from my junk box) in series with the first two,
> effectively halving the series capacitance. The doubler caps can now
> charge to the peak output available from the MOTs, giving my voltage a
> small boost (I can leave the static gap set the same as for my 9KV NST
> supply). This also reduced the current, so my MOTs run warm, but not hot,
> and my power resistors run hot, but not glowing. In other words, the
> doubler caps do double-duty as capacitive current limiters. Of course now
> my MOT supply can develop "only" 1700VA, but it sure is much better
> behaved. I plan to idle my valuable NSTs and just run with the MOT supply
> for my unstructured experimenting.
>
> And the little 4" x 24" coil? It crackles with an intensity that must be
> seen & heard to be appreciated--it likes 1700VA from the MOTs much better
> than 1080VA from the NSTs. The 60Hz pulsing not only sounds cool, it also
> affects the appearance and behavior of the streamers. They are hotter,
> jerkier, and more frantic than before. Maybe MOTs don't rule, but they can
> still make a very serviceable power supply if you have the patience to
> tinker with them.
>
> Next step? I bigger MMC to better utilize my stiff MOT supply.
>
> Best Regards from Middle Georgia,
>
> Greg Hunter
I plan to build a power supply like yours but for current limiting I'll
increase the value of the filter resistors : it's limit the current when you
start to charge the tank cap and when your cap are near full there resistance
are high and you have full voltage.If you play whit filter resistance value
,capacitive value of the tank and break rate I'm sure you could have a really
performing setup.
Luc Benard(Montreal)