> 2. Aluminum and ceramic capacitors are less touchy and can be used at clo=
ser
> to the stated value but still should be derated (say, to 75%).

Wet aluminium electrolytic capacitors should be run nearer to rather
than further from their rated voltages. Running at low voltages wrt
rated will lower lifetime.

Lifetime ~=3D Hours_rated x 2 ^ ((Trated - Toperating)/10)          T's
in degrees C.

Operation at high temperature without applied voltage greatlyreduces
lifetime compared to power on operation at same temperature.

Solid or "dry" aluminium electrolytics have the advantages of tantalum
and the disadvantages of neither T or wet-A.

Consider wet-Al to have C of +100%/-50% unless otherwise stated.

Be aware of leakage currents - specs are provided by manufacturers.
Different options are available.

Ripple current will allow rated lifetime if rated spec not exceeded.
Excess ripple currents derate lifetime. Formulae available. Based
mainly on internal temperature rise. Don't push too far.

You can heatsink large electrolytics with some lifetime gains, withion reas=
on.

Positioning electrolytics in higher airflow areas and avoiding higher
temperature pockets or adjacent heat sources can greatly enhance
lifetimes (or rather, avoid greatly decreased lifetimes).

Very large wet Al electrolytics stored unused for extended period may
benefit from a low volatge "reforming" period. This is not commonly
done with most modern caps.

Brand matters. The very best are only somewhat better than the good,
but the very worst are very very bad indeed.



      Russell McMahon
      Applied technology
--=20
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