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Capacitors What to pick?

MadMechanic

Aug 28, 2012
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Hello All,

I am selecting a capacitor for smoothing and buffering rectified DC voltage fluctuation, and looking at different types of capacitors I have questions. Digikey.com has many to choose from and I was mainly looking at aluminum electrolytic capacitors. I noticed that the electrolytic ones have a lifetime rating where other types do not list a lifetime. Most electrolytic capacitors show around 2000 hours @ given temp. although they do go higher.

The question being: is there any advantage to using another type of capacitor than electrolytic for what I need it for? Do these other materials have a longer life? some of them do not list a (lifetime) rating.

I know what values I need, but Digikey has ceramic, mica, tantalum, film, silicon, etc..
Too many options :eek::confused:
 

BobK

Jan 5, 2010
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Most of those types do not come in 1000s of uF, which is what you are going to need. Typically, electrolytics are used.

Bob
 

MadMechanic

Aug 28, 2012
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That makes sense. I started looking at some of the others and some are surface mount etc... They even have an Electric Double Layer, Supercap that is 5000F! I'm sure it costs thousands, but only rated at 2.5V. What would that be used for?
 
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davenn

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That makes sense. I started looking at some of the others and some are surface mount etc... They even have an Electric Double Layer, Supercap that is 5000F! I'm sure it costs thousands, but only rated at 2.5V. What would that be used for?

amongst other things, supercaps are often used as backup for memory etc do the same thing as a rechargable or a lithium button battery but last much longer

just keep in mind for your electrolytic cap for smoothing...
to have at least 1000uF per amp of current the PSU will be able to supply
so for a say 5 Amp supply a 6800uF or 10,000uF cap wouldnt be out of place

and a voltage rating of preferably twice that of the supply rail ....12VDC use a 25VDc rated cap


Dave
 

MadMechanic

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Interesting, I had never seen that much capacitance. I am looking at producing 12V 1.5A max I think, so I am going to use something like a 100,000uF 25V cap, I found one in that range with a lifetime of 7000 hours. Not bad! -Thanks again for the info
 
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rob_croxford

Aug 3, 2010
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Just as a side note... the temperature rating states that it will have a life time of X hours at X temperature. For every 10 degree drop in temperature the lifespan doubles.
 

shrtrnd

Jan 15, 2010
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I don't have specs to back me up, just experience repairing equipment.
Electrolytic caps are usually used for power supply smoothing because they're relatively
electrcially forgiving about power fluctuations (within their ratings). The reason they have
life-ratings is because they have a wet-electrolyte. Over time it dries out, and it dries
out faster if it's under high heat. If you read the specs, some are more expensive, that
have longer life-cycle ratings.
The mica's, tantalum, ceramic, and poly types are pretty-much solid material and have
much greater life expectancies (they DO have life-cycle ratings). (There IS a wet-tantalum
type available which is very expensive now, but most tantalums are solid)
I see a lot of non-electrolytic caps in low-voltage power supply boards, but I don't
remember ever seeing a high voltage power supply that didn't use electrolytics to
smooth the output ripple. I'm supposing there is a very valid reason for that.
I could Google the why's of electrolytics in power supplies, but so can you. I'm just
giving you a general explaination of what I've seen in use.
 

davenn

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Interesting, I had never seen that much capacitance. I am looking at producing 12V 1.5A max I think, so I am going to use something like a 100,000uF 25V cap, I found one in that range with a lifetime of 7000 hours. Not bad! -Thanks again for the info

a 100,000uF is so torally an overkill by many many magnitudes
and the physical size of a 100,000uF is HUGE!!

use a 4700uF or 6800uF is all you need


Dave
 

MadMechanic

Aug 28, 2012
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a 100,000uF is so torally an overkill by many many magnitudes
and the physical size of a 100,000uF is HUGE!!

use a 4700uF or 6800uF is all you need


Dave

True, but this whole project is overkill and overbuilt :D. I found one about 2 inches long which is not bad, should fit fine. I thought about using a smaller cap and a ni-cad battery, but I may just one huge capacitor instead, unless I can find a battery I like.
 
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