[sdiy] How do old electrolytics go bad?

[harrybissell at wowway.com]

ALUMINUM Electrolytics have (you guessed it) aluminum for
the plates and (nonconductor) aluminum oxide for the dielectric.

The "electrolyte" does nothing more that make sure that the aluminum
oxide coating does not degrade with time / heat / use. Eventually it dries up
and the capacitance goes up, or the aluminum oxide breaks down with voltage
and it goes shorted.

This is why you will hear of "reforming" electrolytic caps that have not
been used for a long time, by applying voltage s-l-o-w-l-y and waiting for
the dielectric to reform.

I'm not certain but probably the capacitance goes up as there is no
electrolyte to maintain the oxide... but I have seen some capacitors that go
up because the connections to the leads (or terminals) slowly fail with time.
This can be seen as an increase in ESR (equivalent series resistance).

H^) harry



On Fri, 06 Feb 2009 15:40:34 -0700, Bob Weigel wrote
> I typically see old electro's drift higher and higher..even to 10X 
> the original value and then they'll plummet often to dead or they'll 
> even short sometimes.  This in observing Fender and other amps of 
> various ages largely.
> 
> The *source* of this phenomena, I assume, is that the electrolyte 
> gradually degrades and in doing so some functional distances may be 
> altered..and of course closer plates mean higher capacitance.  
> Shrinkage of the dielectric material.   And then as it gets too 
> close it punctures and leaks sometimes.  Or something totally 
> disconnects in other cases.  Not sure but rambling possibilities 
> anyway :-)
> 
> Anyway that phenomena is common whatever the case, let me assure 
> you.  -Bob
> 
> Ingo Debus wrote:
> 
> > Hi,
> >
> > we all know that electrolytic capacitors often go bad when they are  
> > old. In the past I had replaced some that had a significantly lower  
> > than rated capacitance. But now I got two that show a higher than  
> > rated value on my DMM: one is rated 100uF/3V and shows 270uF on the  
> > meter, the other is rated 16uF/16V and shows 23uF. Of course I  
> > desoldered them from the circuit before measuring.
> > Is this normal? Or did they have the higher values from the  
> > beginning? Or are they leaking and thus fooling my meter?
> >
> > I estimate these caps are about 40 years old.
> >
> > Ingo
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Harry Bissell & Nora Abdullah 4eva