At 20.09 2012.02.18, you wrote:
>> Hello,
>> I need a >=3D400V >=3D4.7uF pulse capacitor with a dV/dt rating >=3D20V/=
us.
>>=20
>> I can find them on Mouser, Digikey, etc.. e.g.:
>> http://www.kemet.com/kemet/web/homepage/kechome.nsf/vapubfiles/F3294_PH
>> E426.pdf/$file/F3294_PHE426.pdf
>>=20
>> But they are all too big/bulky!!
>>=20
>> What should I look for? Is there a different technology than
>> polypropylene, still suitable for >=3D20V/us pulse applications, and is
>> not as bulky as those caps?
>>=20
>> 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=20
>connection to 230V mains for use in mains filters or as motor run=20
>capacitors and the like, often referred to as X2 rated, or something=20
>like that. I don't think you will find anything else that will do what=20
>you want.=20
>
>Note also that you don't quote a dI/dt rating - but this will probably=20
>be the rating that will bite you in the backside. It is an easy rating=20
>to ignore, and will often result in shorted capacitors, a familiar=20
>form of failure for anyone that attempted to use the old Philips=20
>'lollipop' polycarbonate capacitors in items like capacitor discharge=20
>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 much
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 would
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/EC=
WF%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

--=20
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