Re: Dead MMC. was: AAAA! My coil...

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Mddeming-at-aol-dot-com>

In a message dated 11/26/02 11:06:18 AM Eastern Standard Time, 
tesla-at-pupman-dot-com writes:



>After reading the list posts and seeing that most everybody agreed on
>raising the resistors away from the perfboard, I raised the resistors off
>the perfboard by about 1/2 of an inch. I then put a small square of plywood
>underneath the one resistor that is in the spot that keeps getting blown,
>to better insulate it. I then fired it up. Instantaneously, there was a
>spark in that area, and I shut it down again. However, I noticed that the
>carbon mark was only on the top of the wood, not on the bottom or sides
>indicating an arc. I didn't pay much attention to it at first, and thinking
>it was solely the resistors fault, I took out the blasted one and didn't
>put in a new one. I figured the charge would just bleed into the other
>adjacent resistors. When the coil was fired up again, it worked okay for
>about 20 seconds, then a loud 'pop', and I recognized that special type of
>orange glow by the MMC. I rushed over, blew out the small fire, and took
>the coil! l in for the night. Rrrgh... there is a beautiful (sarcasm) black
>crater about 1/4 of an inch diameter in the top of the cap that the failed
>resistors were right underneath. So my coil is now declared dead until we
>can get another cap.
>
>However, since there was no evidence of an arc, and the cap failed seconds
>after I removed the resistor, and since every other cap and resistor are
>doing fine, could the problem be the cap? It might have been that the cap
>was shorting out, dumping the charge across the resistor, and when the
>resistor was removed it simply carried the current itself and blew up. Any
>ideas?
>
>Also, (sorry about so many questions) someone told me that with 2 strings
>of 6 geek caps, the mmc is not suited to handle the voltages at the peak
>voltage. (9000 VAC r.m.s. voltage, much higher peak voltage) however, the
>geek group recommended the current setup, so am I okay as is or do I need
>to redo my mmc completely? (I thought I was doing fine, now I'm confused
>again). Thanks.
>
>
>-Chris
>


Hi Chris,
        Just a couple of thoughts on your problems, past and future.
1) At NST output voltages, the residual moisture in plywood makes it 
somewhat conductive unless it is well-sealed.
2) With 6 caps and 9 kV rms, the peak voltage across each cap is 2,121 
Volts, a marginal arrangement at best.
3) Having one cap in a string with no resistor, does place a higher current 
stress on that cap. Any time the symmetry is broken there is a rebalancing 
of stresses. Said rebalancing always occurs in the most detrimental direction.
4) Always buy more caps than you need, as manufacturing QC is never 
perfect, and "compost occurs". ;-)
Matt D.