Kenneth Lumia wrote

>>
>>
>> Spehro Pefhany wrote
>>
>> .>The rated life is usually only 2-3K hours for the
>>> cheaper caps, which is only a few months of 24/7
>>> operation-- not
>>> acceptable in most applications, so derating is
>>> virtually always required.
>>
>> Yes I saw that while I was looking for replacements
>> and worked out at 8
>> hours a day they should only last about a year, so to
>> get 7 years was
>> probably pretty good.
>>
>
> As you may be aware, this rating is based on maximum
> temperature and ripple current.
>
> Two rules used for derating are:
> 1.  Life expectancy roughly doubles for each 10C drop
> in temperature from max temp.
> 2.  You can calculate lifetime it a little more
> accurately if you know the temperatures
>      and the ripple currents using the Arrhenius
> equation.  I think someone already posted
>      the temperature portion of the equation.  For a
> good summary of equations see:
>      http://www.elna.co.jp/en/capacitor/alumi/catalog/pdf/tecnote_e.pdf
>
> Ken

Thanks Ken, that looks like a good place for me to start. I shall read it a 
few more times and see what sinks in.

Probably my biggest problem will be getting an accurate substitute for the 
load to run the tests. These things run up to about 80 pulsators or so.

Regards,  Roger

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