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

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

Original poster: "Barton B. Anderson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <tesla123-at-pacbell-dot-net>

Hi Chris,

 From your description, it sounds like the cap was arcing internally and 
overvolting the resistors. Once the last resistor was removed, the cap 
itself went into a runaway condition and failed from the same internal 
arcing and all the fun dynamics that goes along with that. My guess is the 
cap was originally damaged due to being overvolted by the transformer. The 
NST at 9kV(rms) has a Vp of 12.7 kV. The cap bank was rated at 12kVp, so 
you were definitely on the edge or slightly over.

The rms current looks good and is the area where you will see reliability 
ratings (it is also the typical area of concern), but the cap voltage must 
also be considered. Maybe more emphasis on derating the cap should be 
rolled into cap reliability. Often it's not and just assumed the user 
should know this. Derating at 2.5 x Vp is a good rule of thumb.

Take care,
Bart

Tesla list wrote:

>Original poster: "Chris Roberts by way of Terry Fritz 
><twftesla-at-qwest-dot-net>" <quezacotl_14000000000000-at-yahoo-dot-com>
>
>
>Well, crud.
>
>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 adajacent 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 coi! 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 untill 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. (9000VAC 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 completly? (I thought I was doing fine, now I'm confused 
>again). Thanks.
>
>
>-Chris
>
>