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Old 25th July 2003, 10:35 PM   #1
Doh is offline Doh
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Join Date: Oct 2002
Location: New York
Default Capacitor derating/lifetime question

Hi,
I've recently began wondering how long the power supply capacitors in my amplifier are going to last, based on a worst-case scenario, since I am not the original owner of the amp. Based on what I've read, and assuming a 2000 hour at 85 degree rating, and the "rule of thumb" where the lifetime doubles with every 10 degree decrease in temperature, this is what I've come up with.

2000 hours at 85 degrees
4000 hours at 75 degrees
8000 hours at 65 degrees
16000 hours at 55 degrees
32000 hours at 45 degrees
64000 hours at 35 degrees
128000 hours at 25 degrees

Assuming operation for 12 hours a day, this gives me operation for 29 years at room before the capacitor moves outside of its +/- 20% rated range. Even at 35 degrees, I'm looking at 14.5 years.

Since the capacitor is not likely to be operating at maximum ripple current, and is about 8 volts under its rated voltage, I'm guessing that the capacitors will be good for another 10 years before they fail. (It's already 10 years old) Does this sound right to anyone, or should I be more worried about replacing my power supply caps before something nasty happens to the amp?

Doh
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Old 27th July 2003, 01:37 PM   #2
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Join Date: Jan 2003
Location: Sweden
Hello

You have to make a difference between lifetime and MTBF, (Mean Time Between Failures) which are a bit different.

Electronic components do not have a predetermined lifetime instead they fail randomly at a certain rate.

When a capacitor manufacturer say that a capacitor will last 2000H at 85 deg it means that if you use many capacitors of the same type at 85 deg 50% of them will have failed after 2000 hour, but some of the failed ones will have failed maybe already after 100 hours and some others will fail after 4000 hours.

For each hour there is a risk of 1/2000 that a capcitor will fail.

Another thing to note is that failure of a capacitor is usually not catastrophic so it stops working but instead maybe cap value has been reduced or leakage current has increased so usually it works more or less as a capacitor for much longer time then the rated value.

Also note that capacitors degrade even when they are not used and most serious manufacturers do not guarantee function more then 10 years.

As there is a very small risk that the capacitors fail catastrophically I would not bother to change them if I didnt noticed any degradation like increased hum or so.

Regards Hans
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