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Re: [TCML] MMC Caps
bartb wrote:
Hi All,
Before assuming too much, maybe dig into the TCML archives to find out
the history of MMC's in TC use. The "guy" referred to below is none
Indeed, Bart's advice is good..
As I recall, the 940 and 942 have superficially similar ratings (942 is
a bit better) as did the WIMA MKP, I think, but the difference was in
the internal construction (Terry and others cut a whole raft of
commercial capacitors open after testing to do Destructive Parts Analysis)
The difference had to do with the partial failure..the 942s are
basically a whole bunch of smaller caps in parallel internally, each
individually fused to the external connection. The 942s would punch
through in one segment, the internal fuse to that segment would blow,
but the overall cap would remain good, just slightly lower capacitance.
The 940s didn't have this type of construction, so a overvoltage that
punched through killed the cap.
(I may be misremembering here...)
But overall, the idea was that the 942, in addition to having better
ratings, was MUCH more failure tolerant. This allowed running much
closer to the edge (if not well beyond it) on the ratings (e.g. running
a 1000WVDC cap on 1000VAC rms, which is actually 1400V peak).
There were also some thermal analyses done.. the 940 has higher ESR than
the 942, so requires more derating when run with high peak currents.
One can certainly design a TC using the databook values for the caps,
using suitable derating, etc. But most hobby TC builders are cost
sensitive, and want to push components as much as possible (e.g. spend
$30 on 10 942s at $3, at the ragged edge, rather than $60 on 30 940s at
$2 each, running at databook values).
The capacitor count adds up quickly when you start stringing more in
series to get voltage margin, and then you have to string more in
parallel to get the capacitance back up (and increase the current rating)...
I think that as of when the analysis was first done, the economics said
942s are a "better deal" than the 940s
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