Re: Why do my caps keep blowing?

David, see below:

> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Why do my caps keep blowing?
> Date: Tuesday, January 26, 1999 5:01 AM
> Original Poster: "David Hosking" <davidhosking-at-bigpond-dot-com> 
> Hi all,
> I am making my first tesla coil and am having trouble with caps blowing
> I am making them from 1mm thick poly (10layers of 100um) 
> They are vacuumed & filled with fresh transformer oil.
> When I place 12kv30ma a.c. across them (0.008uf)  with no load to test
> they puncture through the middle of the poly within 5 minutes.
> Perhaps I shouldn't test them like this? 
> I clean everything with alcohol and wearing rubber gloves during assembly
> then vacuum them for 4 days.


> What should I do?

You are way, way overstressing your 1 mil poly!!  I am amazed they last for
5 minutes!!  Typically, thicknesses are 60 mil and often 90 mil for 12KV! 
So, you have two choices:  1) use at least 600 layers of your 100 um poly,
or 2) put about 60 of your 10 layer caps in series.  Of course, each cap
will have to have about 60 times the plate area to end up with the same
total capacitance.  The latter is slightly less prone to failure because
each cap will be less voltage stress and thus will have less tendency to
have problems with corona and edge creepage.

> What is the life of one of these caps anyway?

5 minutes, as you found out!  But with 90 mil thickness, they should last

> When I have constructed my coil I want maximum life out of the caps so
> long should I set for each coil run?

If you use 60-90 mil caps, you can run your coil as long as your family and
neighbors can stand it!  Most runs are typically several minutes duration.

I recommend you use layers of 2 to 20 mil poly to save yourself the huge
job of preparing and aligning 600+ layers of .1 mil poly.

> Thanks in advance for the help.
> David Hosking.
> email <mailto:davidhosking-at-bigpond-dot-com>davidhosking-at-bigpond-dot-com