Re: [TCML] MMC Caps

[Paul Brodie <paulb.brodie@xxxxxxxxx>]

I suppose I am once again guilty of blindly following "data" posted on what
pass for top notch Tesla Coil web sites. I stupidly assumed that when
someone posted a list of "good" and "bad" capacitors for use in building
MMC's, that they had done just what Steve suggests and has done and not
simply listed certain caps as unsuitable because someone reported that they
had failed when used in this application. I mean, let's face it, if you try
hard enough, you can destroy any caps you use to build an MMC.

All I can say is thank goodness for the Steve Wards of the world, and I
would be remiss if I didn't mention Gary Lau in this, who take the time to
actually investigate these sorts of things. I don't mean to leave any else
out but both Gary and Steve have assisted me in this way on more than one
occasion. I know there are many more of you out there. Thanks guys for all
that you do to keep those like me in the straight and narrow.
Paul Brodie
Think Positive

On Mon, Aug 9, 2010 at 1:19 PM, 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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