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From: "Bill Sloman"
Subject: Re: Capacitors for high current high voltage pulse
Date: Thu, 31 Oct 2002 00:27:42 +0100
Organization: Planet Internet
NNTP-Posting-Date: 30 Oct 2002 23:27:45 GMT
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
"Darrell Harmon" wrote in message
> I need a 1500V 0.3uS pulse to power a flashlamp. It is an EG&G FX-280
> in one of those trigger sockets. I see CDE DME series caps (Metallized
> Polyester) in the mouser catalog that look good, but it says the
> maximum pulse dv/dt is 70V/uS. I will have about 2500V/uS assuming I
> use two 1KV .22uf caps in series. Thats 550 Amps. I will only be
> firing about 1 pulse per second so will these caps be harmed? Another
> cap option are some 800V .56uf mylar caps that I found surplus. I
> have no info on them. Would they be better? If you have any other
> suggestions that would be great, but it needs to stay cheap since I am
> a college student with a limited budget. Yes, I know this is high
> voltage. Power will only be applies when it is in its plastic box
> with the lid on for safety.
Metallised polypropylene would seem to be the standard dielectric for the
job. The 1999 Farnell catalog I've got at home lists a bunch of BHCAerovox
RBPS capacitors which seem to be designed for this sort of service in IGBT
modules, probably at much high repetition rates than you need. They are not
cheap or small - a typical price for a one-off would be about $10.
I don't know what the failure mechanism is when you over-rate capacitors
with excessive voltage slew rates and thus pulsecurrents, but I'd be worried
about "channelling" where the high currents tend to concentrated into narrow
filaments - even at low duty cycles you could end up with sufficient
localised heating in the dielectric for the metallisation to be dragged
through a osft patch of dielectric to short out the whole capacitor.
I've got no idea if this is a real problem, but that kind of behaviour is
often possible and can make life unexpectedly complicated.
Bill Sloman, Nijmegen
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