Re: Partial Discharges.....A good reason for cap failure?

Hello Jim, Gary, Fr. Tom, all,

Jim wrote:
>>It wouldn't be too hard to set up a test setup with a NST and a variac
>>to actually look for partial discharge activity. This might give you a
>>way to evaluate a home built (or used) cap. Typically, HV components
>>show an increase in partial discharge activity as they reach the end
>>their life, shortly before catastrophic failure.
>>All you need is an oscilloscope and a suitable HV coupling cap (a few
>>is all you need, so it would be cheap). Hook the NST across the cap
>>hook the scope up with a series cap to one of the terminals of the
>>As the voltage increases, you get bursts of HF noise (from the corona
>>and partial discharges). Since the voltage is varying sinusoidally,
>>bursts will tend to be synchronised with the line frequency, so by
>>syncing the scope to line freq, you should see them.
>>One diagram I have shows a series C of 3300pF and a load impedance
>>(scope input) of 50 ohms.  I would think that something like 33 pF and
>>load of 5K would work just as well. The 33 pF cap could be as simple
>>a few feet of RG-8 coax

Could you tell me which file you are referring to? I downloaded
your site a while ago and did an offline search on the files I have
(maybe you added something to your website?), but couldnīt
find what you mentioned. I think I have the idea, I just want to be
sure of it. Thanks.

Fr. Thomas McGahee wrote:
>>Excellent post. However, I disagree with your statement:
>>While it does not *totally* prevent corona, it greatly reduces
>>it. Without the oil there would be air between the layers
>>of poly, and that definitely would encourage corona. I
>>believe what you meant to say was that the use of oil
>>does not reduce the corona completely, and so eventually
>>there will be degradation of the poly.
>>I am just concerned that a new coiler might read that part of
>>the post and then believe that there is no reason to use oil.
>>Oil is still one of our best ways to reduce corona.
>>BTW, the place where this degradation takes place the most
>>is at the ends of the capacitor where the "tabs" or
>>final connections are brought out. So the length of the
>>'margin' of the poly here should be at least an additional
>>half inch over what the 'margin' is on the sides."

Fr. Tom,
Thank you.
Yes, your point of a new coiler maybe believing oil
isnīt necessary, IS a good point. I didnīt mean to
say this, of course. I just wanted to make sure that
coilers should realize, that even though they are
pouring "gallons" of oil into their caps, this will not
totally eliminate the corona production (this is a
hard to believe fact, but after seeing this effect a
few times I DO believe it). Your last comment is an
important consideration to take, when building your
own pulse caps. It might be worthwhile to wrap the
connection tab with a few sheets of interleaved poly
and paper, all the way from the bolt hole to the point
where the connection strap turns into the actual
capacitor plate. Slightly off-topic:
But how did you get interested in Tesla coils? Thanks!

Gary wrote:
>>The phenomenon you describe is exactly what took out my last rolled
>poly cap. Scott Hanson did some microscopic analysis of a sample of my
>>failed cap's poly dielectric.  He sent me a photo of the poly just
>beyond the
>>foil edge towards the margin, and all along that foil edge was this
>>characteristic branching etching pattern.
>>I've appended this photo to the capacitor page on my web site:

Hi Gary.
Yes, shortly after I posted the original mail, Scott sent me
the picture of your cap sample. This is a very beautiful picture
and exactly shows the effect, I was talking about. Please DO
post it on your website. This effect is definately worth saying
a few words about. Esp. since the damage it does, does not
lead to instant failure. I am sure a lot of coilers out there do
not realize what is wiping out their caps (and hard work).

Coiler greets from germany,