Hi Mario The best discharge capacitors ever are silver mica plate capacitors. They are not cheap or small but they will fit the bill. I have used then as pulse dischage capacitors in ultrasonic avalanche pulse generators producing <5nsec 600v pulses. On Sat, Feb 18, 2012 at 4:44 PM, Electron wrote: > At 20.09 2012.02.18, you wrote: > >> Hello, > >> I need a >=3D400V >=3D4.7uF pulse capacitor with a dV/dt rating >=3D20= V/us. > >> > >> I can find them on Mouser, Digikey, etc.. e.g.: > >> http://www.kemet.com/kemet/web/homepage/kechome.nsf/vapubfiles/F3294_P= H > >> E426.pdf/$file/F3294_PHE426.pdf > >> > >> But they are all too big/bulky!! > >> > >> What should I look for? Is there a different technology than > >> polypropylene, still suitable for >=3D20V/us pulse applications, and i= s > >> not as bulky as those caps? > >> > >> Even half the size would already be a big win for me. > > > >What you are have found is a capacitor that is rated for continuous > >connection to 230V mains for use in mains filters or as motor run > >capacitors and the like, often referred to as X2 rated, or something > >like that. I don't think you will find anything else that will do what > >you want. > > > >Note also that you don't quote a dI/dt rating - but this will probably > >be the rating that will bite you in the backside. It is an easy rating > >to ignore, and will often result in shorted capacitors, a familiar > >form of failure for anyone that attempted to use the old Philips > >'lollipop' polycarbonate capacitors in items like capacitor discharge > >ignition systems. > > Yes, it's a CDI. :o I calculated a 50A peak current.. but ignored the > dI/dt. > While the dV/dt is the discharge rate of the capacitor (the charge is muc= h > slower, and the turn ON time of the SCR doesn't matter in the dV/dt, but > only the time it takes to discharge into the coil), the dI/dt instead wou= ld > be linked to the rise time of the SCR going into conduction? > > I would never use a polycarbonate capacitor in a CDI.. but I wonder if > polypropylene is my only practical choice? > > I found this alternative to the PHE426 that I've been using so far, it's > smaller (not by a huge amount, but smaller) and even cheaper: > > > http://media.digikey.com/PDF/Data%20Sheets/Panasonic%20Capacitors%20PDFs/= ECWF%28A%29%20Type.pdf > > But they don't quote a dV/dt nor a dI/dt.. I wonder if it's suitable for > my CDI? > > With kind regards, > Mario > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .