Re: My extended-foil cap failed

• To: tesla-at-pupman-dot-com
• Subject: Re: My extended-foil cap failed
• From: Mark Finnis <mefinnis-at-medicine.adelaide.edu.au> (by way of Terry Fritz <terryf-at-verinet-dot-com>)
• Date: Sun, 11 Oct 1998 06:09:36 -0600
• Approved: terryf-at-verinet-dot-com

At 11:25 10/10/98 -0600, Gary Lau wrote:

... Snip ...

>Last night I ran the coil for 1 minute, checked for any signs of cap
>heating, ran for 2 minutes and checked.  The ends were just slightly
>warm.  I ran for 3 minutes and checked.  The ends were again just
>slightly warm.  I was about 2 minutes into the next run when the roar
>diminished, then POOOMMM!  One of the PVC end caps burst, leaving about a
>pint of mineral oil on my garage floor and egg on my face.

Sorry to hear this.  PVC is a poor heat conductor & internal temp can be
quite high before it becomes obvious ........ as you seem to have clearly
demonstrated :-(

... Snip...

>Unclear is how long this cap would have survived
>had I limited run-time to a more conventional 30 seconds at a time.  Is
>running 7-8 near-continuous minutes something that is foolish even with
>the most robust capacitors?

I run my rolled caps for up to 5 minutes or more.  The filter resistors on
the NST protection circuit get hot, which I why I have them on heat sink.

The caps are barely warm ..... BUT, if you feel the terminal bolts, they
are quite warm and probably a better guage of internal temp.

( oh ..... and yes I only feel one at a time & discharge the cap first !! )

Mark

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