----- Original Message ----- 
From: "Roger Weichert" <rweicher@bigpond.net.au>
To: "Microcontroller discussion list - Public." 
<piclist@mit.edu>
Sent: Wednesday, July 14, 2010 7:52 PM
Subject: Re: [EE] Bulging Electrolytics


>
>
> 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 

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