> Electrolytic caps have a limited lifetime that's worsened by heat.
> Good caps have an MTBF of 2,000 hours, but that cuts in half at the
> temperatures that exist in a PC case near the CPU.

Sort of.

Electrolytic capacitor life is at  arated temperature - typically either 85C 
or 105C.
Rated lifetimes are usually 1000 or 2000 hours.
Capacitor life approximately double er 10C drop in operating voltage so eg a 
1000 hour cap run at 45C average can be expected to have a lifetime of ABOUT

        ~~~   1000 x 2^(85-45)/10 = 16,000 hours or about 2 years 
continuous.

A 105C cap in the same environment SHOULD give an extra factor of 
2^(105-85)/10 = 4x or 8 years continuous operation.

The mechanism for this is mainly electrolyte dryout. This applied to "wet" 
aluminium electrolytic capacitors but not to eg solid Aluminium or solid 
tantalum (should anyone be daring enough to use the latter in a power 
circuit).

Interestingly, the above is for applications with rated voltage applied. Run 
them at a given temperature with no voltage applied and the life time is 
much shorter than with rated voltage applied.


   Russell McMahon

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