Re: What happens when electrolytic capacitors 'dry out'?
- From: Tim Wescott <tim@xxxxxxxxxxxxxxxx>
- Date: Fri, 09 Nov 2007 00:44:02 -0600
On Fri, 09 Nov 2007 05:52:44 +0000, /dev/phaeton wrote:
I hear about this a lot when it comes to old amplifier equipment. "The
caps were dried out in my 1971 Doumaflauuchi Quadraphonic HiFi Set so I
had to replace them all".
Does this mean that the dielectric has met the destructive end of a slow
chemical reaction? Does it mean the capacitor package sprung a leak and
released whatever 'moisture' it had in it, thus changing its properties?
How long does it take for electrolytics to 'dry out'?
Are there any preventative measures, such as powering up the device every
few months to re-energize the plates?
Once the caps are all dried up, what are the options?
Replace them all with new?
Remove them, bake them in an oven, reinstall them?
Power up the circuit and leave it on in an 'idle state' for an extended
period of time?
Thanks, and sorry for the wordiness!
-ph
The mechanism by which an electrolytic capacitor "dries out" is that the
water in the electrolyte evaporates. This is just like the dregs in a
bottle of beer drying out, only with quote marks (and without the mold).
This "drying out" happens because all common* electrolytic capacitors
aren't hermetically sealed, and the electrolyte _does_ have to stay wet.
Electrolytic capacitors that don't dry out can lose their ability to
withstand voltage, because the oxide layer on the aluminum gradually goes
away**. When powering up equipment with high voltage electrolytics that
have been unused for a long time the capacitors must be "reformed" -- do a
web search for the procedure.
* I know wet-slug Tantalums, as produced for aerospace applications, are
sealed -- I know of no aluminum electrolytics that are so constructed.
--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com
Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
.
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