Re: [TCML] MMC Caps

[eric goodchild <ericgoodchild@xxxxxxxxx>]

I personally don't see the big deal with 942C caps Ive used both 940 and 942
and both work about the same, I prefer the 940s because they are cheaper and
generally you can get them in higher voltage ratings.

Eric

On Mon, Aug 9, 2010 at 10:19 AM, Joe Mastroianni <joe@xxxxxxxxxxxxxx> wrote:

>
> Mea culpa. And it's easier to tell someone to watch out for something than
> to say - go ahead all is well within  design parameters.
>
> I've got both datasheets in front of me. The 942 series clearly has better
> pulse handling rating- dv/dt is an order of magnitude greater for 2kv .15uf
> models. Current at 100khz is about 50% greater in the 942 than 940.
>
> I personally have no idea how to translate that into a design suggestion
> for the man, because the on line resources are all over the map. Some guys
> say "specs be damned" and other guys ( me) are very conservative.
>
> I've used 942s in 5x18 parallel mmc boards and subjected them to all sorts
> of abuse as I can as an amateur, with no problems.
>
> So I guess the best I can do is recommend 942s and stay silent on anything
> else. Everyone should be allowed his own experiences I guess.
>
> Sorry if I offended. I'll go back to asking questions now.
>
> Cheers
> Joe
>
> Sent from Joe's mobile phone.
>
> On Aug 9, 2010, at 8:48 AM, Steve Ward <steve.ward@xxxxxxxxx> wrote:
>
> > Guys,
> >
> > There is nothing "wrong" with the 940C caps, look at the data sheet,
> > understand the specs.  You most certainly can use the 940C caps (i do),
> but
> > they cant handle as much current as the 942C's, and most of the MMC
> "design"
> > guides i see are honestly stressing the caps well beyond the data sheet
> > specs in terms of voltage, and possibly current, since i dont think many
> > people bother to even calculate that.  So while not exactly
> interchangeable
> > with the 942's, the 940's do work fine for TCs, you may just need to use
> > more of them in parallel to handle the peak current.  Sometimes (if you
> work
> > out the design details) the 940C caps are more economical.  For most
> popular
> > single-NST systems, the single string of 942C caps will be *just* enough
> to
> > survive the whole hour of run-time that most are after, while a single
> > string of 940C caps of similar V and C ratings will not, hence they got
> on
> > the bad list.
> >
> > I just get particularly annoyed by the lack of scientific reasoning on
> the
> > list sometimes.  Just because someone had their caps fail, doesnt mean
> the
> > capacitors were at fault/unusable, check their design and i almost
> guarantee
> > it was stressing the capacitors well beyond their ratings... its not some
> > mystery.
> >
> > Steve
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