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creepyCrawler
Joined: 26 Sep 2004
Posts: 3
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 26 Sep 2004 7:21 Questions On Capacitor Lifespan and Pairing |
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my first post, so, a big hellooo to everyone here :sm9:
I'm new to electronics and I just found out that capacitors have limited lifespans, for example:
- electrolytics dry out
- ceramics go back to some "original state" or something
My questions:
1) Do tantalum caps age similarly too?
2) Will using an overrated cap help combat ageing in some apps? (eg. such as using a 470uF instead of a 220uF for a power supply input filter, or using a 25V cap where its only necessary to use a 16V)
3) What really happens when a cap ages? Does it affect capacitance/working voltage or both?
4) Finally, on pairing, are there any drawbacks to using an equivalent pair of capacitors instead of using a single cap to make up the necessary capacitance?
TIA for any answers posted.
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djalli
Joined: 10 Nov 2001
Posts: 887
Helped: 15
Location: 1600 Pennsylvania Avenue, Washington DC 20500
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| 26 Sep 2004 7:43 Questions On Capacitor Lifespan and Pairing |
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1) yes!
2) it is stupid to use a 470uF instead of 220uF only for aging purposes.
Frankly the filter characteristics will change entirely and second you plan to use the device until next millenium comes?
3) I do not have a great answer for that but it is safe to say it will be affected greatly
4) If you connect them in parallel they add and make your single cap you are talking.
Depending on conditions, frequency, amplitude there are drawbacks. If frequency gets up discrepancies will become visible.
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creepyCrawler
Joined: 26 Sep 2004
Posts: 3
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| 26 Sep 2004 7:57 Re: Questions On Capacitor Lifespan and Pairing |
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Thanks for the reply djalli, but you kinda raised even more questions along the way (the q's below are in the same order as my original post):
2) Why would it be stupid to use a 470uF instead of a 220uF to counter aging? Aside from cost and size, of course. Lets say we just want to use the cap as input-side "buffer" for a switching power supply and actual filter characteristics and efficiency weren't really a big issue.
4) Regarding pairing, I was actually wondering if using caps in parallel may have some bad influence on (or be badly influenced by) ripple.
I am embarking on a simple switching power supply build for a 12V LED nitelight that I want to run for, well, basically forever, without maintenance (its in a REALLY hard to reach place, hence the maintenance-free priority). As far as I can see, seems like the ageing of caps are the biggest thing stopping this project from working in reality.
Once again, any suggestions are appreciated! 
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djalli
Joined: 10 Nov 2001
Posts: 887
Helped: 15
Location: 1600 Pennsylvania Avenue, Washington DC 20500
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| 26 Sep 2004 9:34 Questions On Capacitor Lifespan and Pairing |
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hi creepyCrawler
2) Well one thing for sure 220uF or 470uF would act as DC blocking capacitor. Did you consider this? Use a balanced buffer which is done using with opamp for a perfect buffering.
4) As frequency goes up things get worse. Two caps near each other let say at 2MHz is asking for trouble. Even less frequency depends on other circuitry.
I guarantee you by the time caps are aged your house was over-aged a long time ago.
djalli
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FoxyRick
Joined: 09 Jan 2004
Posts: 598
Helped: 35
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| 26 Sep 2004 10:09 Re: Questions On Capacitor Lifespan and Pairing |
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Hi creepyCrawler,
One of the biggest effects on electrolytic capacitor aging is temperature. The hotter they get, the shorter the life. Read some manufactures' datasheets - they will give an expected life at certain temperatures. It really makes a huge difference.
If you use quality electrolytics and don't let them get much over room temperature too often, they could last 10 to 20 years!
Edit: Forgot to say that capacitors can both lose capacitance with aging, and increase their ESR (equivalent series resistance). These two factors are checked when testing a capacitor for 'goodness'.
Cheers,
FoxyRick.
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duguzhang
Joined: 28 May 2004
Posts: 21
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| 26 Sep 2004 21:47 Questions On Capacitor Lifespan and Pairing |
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There are various capacitors made of different materials. Can anyone give a full comparison between them?
Thanks.
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creepyCrawler
Joined: 26 Sep 2004
Posts: 3
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| 30 Sep 2004 5:47 Re: Questions On Capacitor Lifespan and Pairing |
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Thanks to DJ and foxy for all the replies. :please:
I'll lash something together with this info and will post my results if they're any worth writing about!
cheers!
- creeps
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