Capacitors: Who is Right? What ages them faster?

I have seen data sheets that give operational life as much as 300,000 hours (34 years), with a shelf life as high as 10 to 15 years "...without deterioration of quality". What will affect the life span is the ambient temperature (if too high, the caps will start leaking current), the ripple current frequency and the initial temperature rise (on/off operation). If you leave the amps off, then you increase the life span by virture of zero frequency but lose it when you turn on the amps (high initial temp rise). If you leave the amps on, then the reverse is true - so, to me, it's a wash whether you keep the amp on or off.

But 8 years to a ps filter cap? Doesn't sound right to me, nor have I heard that Krell caps have such a problem; however 34 plus years for a cap is, IMO, a bit on the optimistic side. I would guess that 15 to 20 yrs would be in the ballpark. Then again, my ML 20.6 pc boards were replaced twice in 15 years. Because of bad caps.

What does this mean with regard to those amps that use few large value caps as opposed to those amps that use more smaller value caps?

One of my collegues does research on capacitor reliability (for aircraft systems) and he agrees that thermal cycling is the main problem. I will ask him again now that he has had time to look more into the issue and let you know. In my experience with using caps in designs is that 10 to 15 years is typical in 55C ambient conditions (for electrolytics).

As for the on/off bit, I have thought about it at length since I am a thermal guy by education and have decided that off once a week and when I leave town is the best balance.

For large value caps, the thermal lag time is longer so perhaps they normally last longer than smaller (physically and value) ones. I will ask my buddy tomorrow about this too if I see him. Arthur

Arthur,
I spoke to Nelson Pass today. He said that, over time, the fluid in the cap slowly dissipates from the cap, until it eventually dries out. He believes that if the amp is left on, the higher ambient temperature is accelerating the process, but by how much, he didn't seem sure.

Your colleague is probably more knowledgeable than Pass is on this issue, since this is his profession. I'll be very interested in his answers......

Kevziek: Most of Nelson Pass' amps are of a high bias design and run hot. Leaving them on exposes the caps to a greater amount of heat, so they will end up "cooking" themselves and drying out. Then again, if one turns the amp off and on as needed, the caps are exposed to big temperature swings from the amp being stone cold and then coming up to operating temperature. Doing this repeatedly stresses the caps more than leaving them turned on all the time. That is, so long as the amp has proper ventilation both internally and externally to begin with.

This is not to mention that leaving them on reduces the amount of exposure to high levels of in-rush current. It is in-rush current that damages equipment the most due to the huge surges involved and the rapid internal temperature changes that take place when this occurs. This is why light bulbs typically blow when turning them on, not once they are already on.

Other than that, Seandtaylor did a nice job. GS's comments are right in the ball park too, but one has to realize that not all caps are made the same or have the same spec's and reliability. For that matter, caps from the same manufacturer and of the same product line may differ due to variances in the dielectrics used from batch to batch. Some batch's of products are much better / reliable than others.

As a side note, just a while back i pulled some caps from an amp that Nelson designed and released in appr 1990. While this amp is 13 years old and idles at about 130* at the heatsink, it has been on 24/7 for over three years. With all of that in mind, the electrolytics in the power supply actually tested above their rated values by almost 20%. Needless to say, i didn't replace them as there was no need. Sean
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