Re: [TCML] MMC Caps

[Joe Mastroianni <joe@xxxxxxxxxxxxxx>]

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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