Re: Blown Cap

David and Wesley, I had the same type of failure.

> David Trimmell <dwt-at-efn-dot-org> writes: 
> > I'm still working on the design) and then it died! Today I dismantaled the 
> > cap (a messy job) and located a hole the size of my pinky, it was allong 
> > the edge of the flashing threw the plastic. I was very carefull in sanding 


> David; 
>   Your comment that the point of failure was along the edge of the metal  
> brings up a possibility that I hadn't heard of before. I know you said 
> that you sanded the edge carefully, but a tiny missed burr, or even a  


I recently had a home made cap fail. It used 90 mil poly and was 
connected to 15KV neons..
It failed right at an edge of the plates. 

At least two of us have had failures at the same point.

I wonder if it might have been caused by an air bubble? The edge of 
the plates would be a natural place for a bubble to get stuck.

Since I didn't want to buy more materials I decided to repair the 
cap. I don't know if i'll do it again since it was a messy job!

It is not possible to repair the dielectric so I cut the plates away 
from the damaged area and then re-assembled it. I now have a slightly 
smaller value cap.

The reason I suspect a bubble is because of what happened when I was 
filing it with oil the second time.

I have two valves on the cap. One was connected to the vacuum pump 
and the other to the bucket of mineral oil. After it filled with oil 
I shut the valve to the oil and left the pump running. I decided to 
leave the pump running for a while since a little extra pumping 
couldn't hurt anything. 

I then picked up the cap to move it out of the way. When I set it 
back down I noticed quite a few bubbles coming from the top of the 

It seems that the jolt released some bubbles so I gently bounced it 
on the floor.

I kept getting more bubbles!

I kept the pump running and bounced it until the bubbling stopped.
I did this about every ten minutes for about 4 hours until I didn't 
get any more bubbles when I bounced it.

I then released the vacuum and broke in the cap like it was a new 
one. I let it sit for a week or so and then started out with low power.

So far it's been working but I have not yet pushed it as hard as I 
did before.

When I make the next cap I'm going to try running it at low power 
with the vacuum pump still connected. I think that might get rid of 
more air.

Luke Mester
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