What they all said ..

and

even ecaps involve a degree of dark magic!

1. Too LOW a voltage also kills caps quicker.
For wet aluminum caps, 15V on a 35V cap is too low!

2. Putting another comment into a formula
Rule of thumb to be used with care but useful.

                     Lifetime ~~~= Hours_rated x 2^(Trated-Toperating)/10

so REAL 105C caps are well worthwhile.

(2^(105-85)/10 ~~= 4 times longer lifetime.

3000 hour versus 2000 hr caps = +50% longer lifetime.

All very approximate.


3. If it doesn't say Panasonic on it then there's a reasonable chance
that it hasn't got Panasonic in it. Farnell are reputable but bad
things happen. I'd check that. Really.

4. Ripple current was mentioned by several people. This matters much.
Look it up in the spec sheet and see how it relates to application.

5. Failure as seen is not unusual. As noted, cap dries out, then
overheats and boils or shorts and vents.
Full explosion with guts thrown out messily is not unknown.

6. Cooling can help. Even heatsinking caps can help !!! BUT just
maximising air flow helps.

7 Storage at temperature WITHOUT voltage applied is FAR worse for
lifetime than operation at rated voltage !!!
 Forming voltages are not present and dry-out is accelerated.

8  Size as noted is some indication of capability although high
quality caps MAY be smaller.

9 As noted - for demanding applications DEMAND name brand with real
specs and traceability. Farnell are real enough that they should care.

10 7 years sounds OK in that sort of application day to day.

11. Not applicable here, but using several smaller caps to make a
given capacitance often can give better net ripple current rating.


  Russell McMahon
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist