Re: The death of a classic - First look

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Terry,
           Thanks for doing an analysis of the cap. First of all, I 
am relieved to hear that the probability of the resistors being a 
cause has now diminshed. That was of particular concern to me since I 
also use (and have recommended using) those particular resistors. My 
face hase gone from crimson to a somewhat paler shade.


On 15 Aug 2002, at 21:09, Tesla list wrote:

> Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
> 
> Hi Finn and All,
> 
> Today I got the failed strings from Finn's cap.  It sure does smell!  I
> love the smell of burning electronics :o)))
> 
> I tested a number of the resistors and they were all perfectly in spec.
> They showed no signs of stress.  The resistor I was concerned with before
> was in the middle of a pretty bad fire.  The fire damage to the boards is
> very profound.  Seeing the boards first hand and testing the resistors, I
> can find no evidence the resistors caused the failure.  They seemed to be
> perfectly good and adequate.
> 
> I disassembled a number of caps that were not involved in the fire or were
> in the "good" string.  The is no evidence of end plate or current heating
> damage.  They appeared to run cool without any thermal stress.
> 
> However.........  All of the cap's internal plates and dielectrics are
> "machine gunned".
> 
> http://hot-streamer-dot-com/temp/finn-001.jpg
> 
> There are literally thousands of holes blown in the dielectric layers that
> have self healed.  Some areas are very bad.  Considering the large number
> of caps in Finn's MMC, catastrophic failure was almost certain.  One cap
> had a section blown out of the case but did not burn up.  It was a disaster
> that never happened..., but it tells a story of serious problems...
> 
> I will have to get to a high power scope tomorrow, but there are three
> possibilities:
> 
> 1.  Over voltage on the cap caused millions of dielectric breakdowns.  A
> "few" did not self heal and ignited the caps
> 
> 2.  Ion damage to the dielectrics caused weak spots that eventually failed.
> 
> 3.  High currents in the internal metalized plate degraded and failed the
> thin metal center internal layer.  

I was struck by the fact that the worst sign of damage in the board 
was directly below the cap. Now had an open resistor caused it to 
pop, that would alos have been reasonable. However, the impedance of 
the resistors is so high compared with the cap while running AC 
voltages that the probability of it being the cause was low to begin 
with. I sort of sat down and did a bit of Sherlock Holmes on it last 
night. I think an open resistor would cause its capacitor to 
accumulate a charge over time if the unit was repeatedly powered up 
and then switched off with an applied voltage of the same polarity 
each time - not itself a highly probable series of events. The 
resistors really just ensure that each cap starts out on an even 
footing at switch-on.
      The *best* way of ensuring that the voltage across each cap is 
equal or nearly so at all times is to closely match each cap in a 
string, something I did right at the outset.
  
> Finn stated a very possible clue:
> 
> "Gap voltage is 32 kV (most of the time, but I am sure I`ve hit it with up
> to 45kV for quite some time as well)"
> 
> 45kV is 40% over the caps rating.  "I" would only go to 25% on EMMCs.  If a
> few caps in a string breakdown and go to zero volts, it can put even more
> voltage stress on the others in the string causing further breakdowns.  I
> note that some areas have slight corrosion suggesting that the damage is
> older than a few days.
> 
> When I can see the areas under high magnification tomorrow, I can determine
> which it was.  But it sure "looks" like the first possibility.
> 
> BTW - No doubt about MMC's ability to self heal now!!!  Even though it
> "eventually" failed.  Finn's MMC easily has millions of self healing events!
> 
> Unfortunately, the rest of Finn's cap is certainly heavily damaged too.
> Time to take it out of service and put it in a trophy case so It can be
> admired for generations to come :-))  Maybe if you explained to the museum
> that it was one of the first of it's kind, they would accept it as a
> display item.  100 years from now, it will be a big piece of Tesla coiling
> history!
> 
> Cheers,
> 
> 	Terry

My statement about conservatism in design is perhaps emphasized by 
this event. Thanks again for examining and reporting. 

Regards,
malcolm