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Re: So Maybe MOTs Don't Rule
Greg
What value resistors are you using? Are they between the doublers and the
gap/cap connection? This is the only thing limiting charging current, right?
Alex
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