Lifetime of CD942C20P15K is ~75 hours

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Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx>

Hi All,

I finally did the great CD942C20P15K MMC capacitor life test.

I ran three of them at 1414 VAC (60Hz) which is 2000Vpeak.

http://hot-streamer.com/temp/GreatCapTest-002.jpg

http://hot-streamer.com/temp/GreatCapTest-004.jpg

http://hot-streamer.com/temp/GreatCapTest-001.gif

The goal was to see how long the dielectric would hold up to 
ionization at the edges of the plates which is the limiter.

The results are:

At 10.3 hours, capacitor #3 went out of circuit with blown fuses.  It 
was torn apart and only "one" blow hole was found.  The dielectric 
looked fine...  Hard to say what happened, but this capacitor seems 
to be fine.  The dual 100mA fuses are right on the edge so it is easy 
for something odd to blow them.  A data point to be thrown out...

At 103.2 hours the fuses on the other two caps went open at the same time.

Capacitor #1 was torn open and it had substantial dielectric damage 
with a few hundred self healing blow outs.  It probably could have 
gone longer, but it was definitely at or very near end of life.  The 
edges of the plates had burned and weakened the dielectric at the 
plate edges.  The dielectric turned a chalky white color and it 
easily tore along the damage lines.

Capacitor #2 was put back into the test, but it only went a few 
minutes more since it was making consistent popping sounds several 
times a minute.  Obviously, it was having serious problems and was 
arcing consistently, but not failing outright.

The data was put in the form of change of capacitance with time:

http://hot-streamer.com/temp/GreatCapTest-result.gif

At about the 75 hour mark, the capacitance started to drop at an 
obvious fast pace and ended up at about 1% lower than the 20 hour 
mark.  This is very likely due to internal arcing.  Both capacitors 
seem to degrade at about the same time and pace.  Capacitor #2 also 
had "something" happen at 45 hours but it did not seem to really hurt anything.

So it appears that the CD942C20P15K capacitors can go for 75 hours at 
2000Vpeak with little problem.  The next 25 hours will see 
significant internal arcing with total failure probably at the 100 
hour mark.   At 10 hours, practically no damage is observed.


So not too bad!  As expected, the high voltage does kill them after a 
while.  But 75 running hours is a pretty long time for most of 
us.  There was no high current pulsing in this test like in a real 
coil, but I don't think that will significantly change anything 
here.  Simple AC line "on-time" is the killer...

It is interesting to note that the DRSSTC only puts high voltage 
stress on the caps during the actual firing cycle.  Thus, the life in 
a DRSSTC would be about 20+++ times longer!

Sometime I will try to run three at 1000VAC too.  But I need the 
space back now for the OL-DRSSTC work.  Fortunately, this test 
finished right on time ;-)

Cheers,

	Terry