Re: OT: the capacitor curse


Robert Simpson
 

Fred,
Thanks for the notes and your website. Ecaps drive me crazy sometimes,
this may help restore some sanity.
Bob

--- In TekScopes@yahoogroups.com, Pa4tim wrote:


No, it was something I had experienced and then when I was reading
Analog SEEKrets DC to daylight (
http://www.logbook.freeserve.co.uk/seekrets/ ) to my supprise it told me
the same.

My experience is for this mainly on older equipement ( were 16V is not
the most used value) in modern gear working on max 12V 16 and 10V are
the most used caps and because modern Chinese junk dies after 2-5 years
because of caps failure this could imply that < 16V is also the most
used cap today and so also seems to have the highest failure in numbers
( but that is not the same as % failure within a class) i exchanged lots
of caps in modern conumer suff and every cap I replace gets the full
examination on all parameters. The nicest failures I keep, solder them
on pcb strips for testing LCR gear and experiments.

I think the cause is the low voltage rating. To get a high voltage
rating you need more "paper" between the foils, and so have a bigger
sparesupply of electrolyte that will be longer able to keep the
isolating oxidelayer in good shape. The dielectric is not the
electrolyte/paper but the oxidelayer. For higher voltage this must be
thicker.

A cap starts its life as two etched strips of alu foil. This etching
is to increase the surface and so capacitance. The manufacturer then
forms the oxide layer by applying (sometimes upto twice) the working
voltage on the foil. Then two layers of foil are rolled together between
the paper soaked in electrolyte. The anode and kathode strips are first
welded to the foil.
The low voltage caps have probably a thinner oxidelayer, thinner
aluminum, thinner paperlayers between the foils.

An other failure mechanisme ( and I think cause for increased ESR) are
the junctions foil to strip and strip to pin. The oxide layer seems to
do its best to separate those junctions or ear away the strip ( that
probably is not pre-formed and also is not in the electrolyte. I have
seen severals caps that allmost lost capacitance due to partly corroded
( or totally eaten away) connecting strips.
http://www.pa4tim.nl/?p=1385 here you see some pictures from the guts
of some Tek used caps.
( this capacitor porn is not for the faint hearted, non of the caps
was harmed during the process ;-) )

Fred PA4TIM

Op 22 apr. 2013 om 09:10 heeft Dave C davec2468@... het volgende
geschreven:

On Apr 21, 2013, at 11:30 PM, Pa4tim wrote:

The worst caps are < 16V versions
Interesting. Can you offer a reason why this is the case?

Thanks,
Dave

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