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Re: Lifespan of electrolytic caps

Posted by Al Sekela (A) on January 7, 2002 at 14:57:29

In Reply to: Lifespan of electrolytic caps posted by Allan on January 6, 2002 at 20:13:50:

Allan,

The quoted lifespan for electrolytic capacitors relates to a specific stress test done by the manufacturer. This involves running samples of the capacitor at a high temperature, so is very conservative compared to our typical use.

Electrolytic capacitors have a wet electrolyte as part of their structure. This can dry out from high operating or storage temperatures, and increase the effective series resistance (esr). High esr causes the filter circuit to misbehave. Some capacitor manufacturers publish esr versus frequency curves on their web sites.

See http://www.execpc.com/~endlr/ESR/esr.html for a more detailed discussion. Note that high ripple current in a filter application causes the dielectric to generate heat and shorten the life, even in a well-ventilated application. The maximum ripple current is usually specified by the manufacturer as well.

The capacitors are specified to maintain a low esr for a certain period of time at a certain temperature. As long as the application and circuit design keep the dielectric of the capacitor at a significantly lower temperature, the life is indefinite.

There are thin film replacements for electrolytic capacitors used in Tomahawk cruise missiles and other critical applications. These do not dry out from storage or use, but I image they are very expensive. See:

http://www.eci-capacitors.com

There are special meters made to measure esr of electrolytic capacitors, since these are still the primary failure point for consumer electronics. There is one unit built by a gentleman in Australia and sold as a kit or assembled by an organization for service technicians. See

http://www.anatekcorp.com/testequipment/esrcompar.htm

for a comparison with other units. This one tickles me because I spent several years at Zilog supporting the manufacturing line for the chip it is based upon (Z86E08). If I had an income, I would buy one just to have around.

I'll be at CES until the weekend and out of touch with this site.

Best regards,

Al

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