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Re: MMC Testing vs equations (plus MMC bulk update)



Hi,

At 11:27 PM 8/29/00 -0500, you wrote: 
>Greetings,
>Earlier today, I went through Terry's MMC power handling calculations. I 
>made a few mistakes at first, but I think I got it right... finally. The 
>capacitors are the ones in the latest bulk buy. They are rated 2000vdc 
>(700vac) .068uF with a lead spacing of 27.5mm... I plan on using 13 in each 
>string,
>Do these results look typical of a well made mmc?
>Ecap=.08872 Joules Per Bang
>Rcap=.02029 Ohms Internal Resistance
>Wo=.07152 Watts Per Capacitor
>To=2.86085 degrees celcius
>Life Expectancy of ~555 Hours
>Does that all add up?

I don't know all the numbers, but it sounds perfectly reasonable.

> 
>Wouldn't the bleeder resistors play a role in any of these equations? 

No.  Their added loss is tiny.

>How do you know how long the cap will last? I'm sure I can't time it down to 
>the hour. I usually keep runtime down to 1-2 mins at most. I don't know how 
>anyone could kill a cap by using it for 555 hours. BTW- Is that time 
>continuous, or intermittent operation?

No one really knows life numbers to anything better than an average order
of magnitude.  The equations just help to insure it isn't super bad (like 4
micro seconds ;-))  MMC caps seems to last just fine.

> 
>Also, I tested a cap to destruction earlier today. It took about 1 minute of 
>run time at 4kV firing voltage to kill it.  Before that, I ran it at 2kV for 
>3-5 minutes with no heating....Using Terry's equations, shouldn't it last 
>over 40 hours at 2kVac to kill it?  

The fancy equations were for "normal" operation.  I give the life equations
to the best known values but that is really not saying very much.  If the
caps do not get hot and you are not running them significantly over the
rated DC voltage, they should last "forever" in Tesla coils use.

>Upon autopsey, I found that every layer had been punctured. There were two 
>dissimilar burned holes all the way through the cap on opposite sides... 
>Ouch.  When it failed, all I heard was a quiet little snap. No cracks in the 
>potting...

The cap was internally arcing and taking thousands of shorts and clearing
them.  I have seen poly caps that look like Swiss cheese inside still run
perfectly for years...  They are darn tough.  Be sure to note the thickness
of the poly and compare that to the volts/mil our old rolled poly caps used
;-))  I think modern technology has put the rolled caps into the museums
next to the glass plates ones...

>Don't failed MMC caps usually just explode?

Exploding caps is no longer allowed in "world class", UL, VDE, SA, m.i.c,
k.e.y, m.o.u.s.e... electronic equipment...  They can burn up however but
only current does that... 

> 
>For those of you still waiting by your mailboxes for these caps...
>I called the distributer yesterday, he told me the caps would be in the mail 
>today. Aparently www.illcap-dot-com  has quite a good quality control team. They 
>have to check each of the 500 caps I ordered. : O 

I am not sure of the exact caps your getting but I see they list RMS
current at 100kHz :-)))  Very nice!!!

>The digi-key stuff is coming tomorrow. So, if all goes according to plan, 
>I'll have your caps in the mail by monday. 
> 

Sounds like you are having fun with your bulk buy!  Been there, done that ;-))

Cheers,

	Terry