RE: Dead MMC

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Gary.Lau-at-compaq-dot-com>

Hi Terry:

Yes, I see my error.  While the dv/dt ratings do in fact diminish with
increasing C, I failed to note that the product of C * dv/dt, or maximum
current, actually increased with C.  Thanks for catching this.

Regards, Gary


Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Gary,

Actually, the dV/dT rating is not the one we want.  The dI/dT rating is the
key.  It takes more current to change the voltage on big caps.  You will
find it increases pleasantly as the value goes up :-))

http://www.cornell-dubilier-dot-com/film/9422000.htm

Not that these currents in the above or for 60Hz not 200kHz.  Geek caps are
good to 8 amps RMS at 350kHz not 13.5.  Better point that stuff out before
somebody goes and...

Also not that the ESR drops with big caps that helps to keep them cool but
the real ESR is higher.  At 350kHz it is 9.5mOhm not 5mOhm as in the chart.

One may also note the measly 500 volt AC rating :-))

Cheers,

	Terry


At 10:43 AM 1/27/2002 -0500, you wrote:
>Hi Winston:
>
>Thanks for sharing this.  I think we learn more by failures than by 
>successes, I'm sorry it had to be yours.
>
>In addition to the unknown nature of the film metalization noted by Terry, 
>caps also have a maximum dv/dt rating that basically determines the maximum 
>AC current that the cap can withstand.  If you look at a manufacturer's data 
>sheet, this dv/dt rating goes down drastically as the uF-rating of the cap 
>increases.  The 1.0 uF rating of the caps you used is at least 10 times what 
>is typically used in MMC's.  (I'm a little leery of the Geek's .15 uF caps 
>for this reason, but I guess you can't argue with success.)  I suspect that 
>your caps just couldn't support the high current demanded by this application.
>
>Regards, Gary Lau
>MA, USA