Re: UK polyprop cap source

To: tesla-at-pupman-dot-com
Subject: Re: UK polyprop cap source
From: "Harri Suomalainen" <harri.suomalainen-at-pp.inet.fi> (by way of Terry Fritz <twf-at-verinet-dot-com>)
Date: Tue, 13 Apr 1999 12:18:20 -0600
Approved: twf-at-verinet-dot-com

>I've just found another UK catalogue listing for HV poly caps
...
>Wima FKP1 polyprop film caps - says they're designed for pulse & High
>current use

Sorry to dissapoint you but...

FKP1 series is designed for pulse loads. However, it does not have too high
voltage rise-time limit (dV/dt). This on the other hand does limit available
current. (The more current the faster the voltage changes)

Using those would require many units in series and many chains in parallel.

An example 10 in series would be rated for 16kV peak (1.6kV each). However,
due to high-frequency high-current pulse (dV/dt rating) that would probably be
usable for 8kV peak. For some marigin you would perhaps use that as a 6kV
peak or 4kVac. So, for a 20kVac rating you would use perhaps 50 (!!) of those
in series. Even if I had somewhere an error making 25 or so enough that is
a lot.

So, even with 25 100nF caps in series  you would get a 4nF cap. You probably
need something like 10-20nF. Having 5 chains in parallel would also be usefull
in reducing series resistance. The chain of 25 caps would have resistance
equal to 25 times the single cap resistance. So, with 5 chains in parallel
you would
only have 5 times the resistance of single cap. This would require 125 caps.

Unfortunately the situation just is such that it would be too expensive.
The legs of
single caps would also have nasty sharp edges requiring special good care. For
that price tag you could purchase a commercial pulse cap.

I'd say just forget it unless you run into a huge stack of surplus caps.

Harri