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Old 8th October 2013, 09:15 AM   #1
Kev06 is offline Kev06  England
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Default Reservoir capacitor lifetime?

I'm deliberating over the reservoir capacitors (for the supply to class AB power amps); my system tends to be powered-up continuously most evenings and weekends, so I want to be sure I'm going for suitable lifetime ratings.

I can do a rough estimate from manufacturer's data: say a 2000hr @ 85c capacitor is run at only 45c it looks like the lifetime would be 32000hrs, or around 10years if used for 8hrs per day (within the permitted ripple current). But I beleive thats the mean time to failure - so 'very' significant numbers will fail well before then, but i don't know the statistical distribution of failure times to know how many/when. Nor do I know 'actually' how warm the capacitor will be internally in my amp - just another 10c and I'm suddenly down to 5years 'even' if it reached MTBF. So maybe 3000-5000hrs @105c would be a safer choice; quite a lot more expensive but on average they last for considerably longer...

I realise there are many factors but just roughly as a ball-park sanity-check, would 3000-5000hrs @105c seem reasonable or is that excessive?

Many thanks,
Kev
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Old 8th October 2013, 09:25 AM   #2
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Is operation at 105 deg C ever practically possible ? Most amps will shut off ( if designed with temp protection) at 65 to 70 deg C!
As far as I know , no supply cap should be hot enough that you can't touch it continuously with your finger. That puts it below 50 deg C under worst conditions !

Some tubed amp do have capacitors reasonably close to the tubes and hence run at higher temperatures than solid state amps. But you will find many old tubed amps with dried out supply caps and need a change !
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Old 8th October 2013, 09:45 AM   #3
AndrewT is offline AndrewT  Scotland
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The ambient will be less than the rated. You have already accounted for this bit.

The ripple will be less than the ripple capacity. That will reduce the heat generated internally. Try to find or just guess that this will at least double the lifetime.

A ClassAB amplifier is generally playing music at average levels way below maximum and thus runs much cooler for much of the time. Could this be another doubling of lifetime? ClassA is very different.

Quiescent dissipation overnight, or while you are not listening, adds a bit more lifetime to the para above.

Don't worry about it.
Just allow plenty of ventilation and locate the caps on the cool (perforated?) floor of the chassis.
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Old 8th October 2013, 10:12 AM   #4
Kev06 is offline Kev06  England
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Thanks! As usual then, I may be worrying too much; thank you for the perspective

Cheers
kev
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Old 8th October 2013, 10:37 AM   #5
Kev06 is offline Kev06  England
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Quote:
Originally Posted by ashok View Post
Is operation at 105 deg C ever practically possible ? Most amps will shut off ( if designed with temp protection) at 65 to 70 deg C!
As far as I know , no supply cap should be hot enough that you can't touch it continuously with your finger. That puts it below 50 deg C under worst conditions !
Yes, thats true, the capacitor's temperature rating is indeed well above what it would be actually run at in a normal amp. It is relevant to me/us though because the lifetime quoted in the spec sheets is actually a ballance of time and temperature; a capacitor that can stand 2000hrs @105c is notably more robust than one which can only manage the same lifetime @85c, and so it will last longer even at typical operating temperatures. This is particularly so because the lifetime and temperature relationship is not linear - as you operate the capacitor further below its quoted temperature the lifetime extends greatly.

Cheers
kev
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