Capacitor info (was Re: tesla-d Digest V99 #153)

To: tesla-at-pupman-dot-com
Subject: Capacitor info (was Re: tesla-d Digest V99 #153)
From: "Marco Denicolai" <Marco.Denicolai-at-tellabs.fi> (by way of Terry Fritz <twf-at-verinet-dot-com>)
Date: Wed, 07 Apr 1999 18:15:57 -0600
Approved: twf-at-verinet-dot-com

Get a good Philips capacitor handbook.

There you can find all derating curves for their capacitors + a lot of useful
infos.

A couple of useful things:

1. Capacitor rated voltage decreases dramatically as frequency increases. A 2
kV
DC rated cap. at 10 kHz can be good for no more than 400V! Each capacitor has
got its own derating diagrams.

2.You can easily calculate what current peak a capacitor will withstand from
its
dV/dt rating (usually V/us). It goes like:

    I = dV/dt * C.

e.g. a 1000 pF capacitor rated for 10000V/us will withstand 10A surges (10000 *
1E6 * 1000 * 1E-12 = 10).

Good luck!






Tesla List <tesla-at-pupman-dot-com> on 06.04.99 21:45:49

To:   tesla-at-pupman-dot-com
cc:    (bcc: Marco Denicolai/MARTIS)
Subject:  Re: tesla-d Digest V99 #153




Original Poster: Phil Chalk <phil-at-eagles-dot-com.au>

>
>
> Down in Australia these are about $2 each, and we are now talking the same
> money as a Maxwell pulse cap !!
>
> For cost-effective use the million dollar question is how far "over-rated"
> commercial DC caps are and how far we can push this for AC Tesla use.
>

Hi Mark, et al.

Just looking thru RS & Farnell catalogues, and any way you look at it, you're
up
for close to 200 bucks (Pacific pesos).  BUT, for ME, if it'll work I'd much
rather
do this than mess around with poly & oil (low tolerance for tedium !)  I spoke
to a
local importer today (looking for single, very high voltage units - no joy, but
they can probably import one for me.) & will go to see them soon.  I'll see
what I
can do on prices for these litle PP caps.

Two things worry me a little - possibly already well covered on the list
previously.  The quoted ac / dc ratings of these caps seems to me a little
arbitrary, but it seems, in general, that the ratio of the ac to the dc rating
reduces as the rated voltage increases. for example a (Arcotronics) 250V dc cap
is
rated at 125V ac (50%), but the 2000V dc cap is rated at 500V ac (25%).
Secondly,  I have a derating curve here for a 1500V dc / 450V ac PP cap showing
full rating (450V) up to 10kHz, dropping to just under 100V at 100kHz.
For the record, the blab for this capacitor goes :-
"A very low loss dielectric suitable for continuous use at high ac voltages.
It
will withstand high voltage, fast rise-time pulses and has an excellent high
frequency performance"

However, there's one very good way to find out how they
go.......................

Cheers,

Phil C.