At 10:27 AM 14/07/2010, you wrote:


>The originals were also only rated at 85 degrees, and I've fitted 105 degree
>replacements, but while excessive temperature can dry them out, I wouldn't
>have thought it would cause this problem.

When the cap starts to go the ESR will increase, which quickly
increases the internal I^2*R heating, and the cap can then vent.

The "X" embossed into the top of the cap deliberately weakens the
aluminum can so it will release pressure relatively safely and in a
relatively controlled fashion when the end comes. Smaller caps
typically vent out the bottom by blowing the rubber bung out.

For long life you want to keep the caps as cool as possible..
considering ambient temperature, external heat sources and internally
generated heat. As a rule of thumb, every 10 K increase in temperature
halves the life. 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.

And there's no free lunch, if you can find caps rated at 5,000 hours or
more they will usually be a lot bigger as well as maybe 10-15dB more
expensive.


>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



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