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

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

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Chris,

At 10:49 PM 11/25/2002 -0800, you wrote:

>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.

At 2000 volts AC, a plywood may be conductive.  Best to just leave an air gap.

>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.

It probably arced right through the center of the wood.

>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.

Replace the cap and all should be well.


>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?

2000 volts of RF will conduct through wood.  Next time, just leave an air 
gap between the resistor and cap and it will be fine.


>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.

At 9000 VAC, the peak voltage is 12728 volts.  Six caps are rated at 12000 
volts.  So you are a little over.  However, I would not worry about it at 
all.  Six caps per string should be fine.  If you are using 10M resistors, 
they are at their power rating (0.45 watts) so no problem there 
either.  However, the resistors may get a little warm but spacing them 
should keep them from heating the caps.  If you wanted to be real safe, you 
could use two resistors in series per cap.

Cheers,

         Terry




>-Chris