Dan, this is a good thread. I had to revisit the subject when my old
Mag Innovision 17" monitor died a couple of years ago. It turned out that
electrolytic caps spec'd at 85C died taking out several components from the
H.O.T. to the flyback transformer... Talking with Mag confirmed the problem
and I got the replacement parts with 105C caps...

   The "rehealing" feature with electrolytics relates to the working
electrolyte in that any faults in the oxide layer will be repaired by
further anodization. This is a factor is the design of the cap.

   "Self-Healing" refers to metallized caps where a partial discharge
results in a localized failure of the dielectric which burns away the
metallized electrode effectively isolating the fault.

   Someone mentioned "reforming" electrolytics. I had not considered this
to be a problem but, after `blowing the dust off' ITT's "Reference Data for
Radio Engineers", I ran across the following:

   "Conventional aluminum electrolytic capacitors which have gone 6 months
or more without voltage applied may have to be reformed." A DC source at the
rated voltage with a 1.5K internal resistance for caps with a rated voltage
exceeding 100V or 150 ohms for <=100V must be applied for one hour after the
cap reaches it's rated voltage +/- 3%. The cap is then discharged through a
resistor of 1ohm/V.

   I would first check with the cap vendor to see what the shelf-life is in
this `day and age', and what, if any, `reforming' procedure is required.
Still, it's something to think about if you have a bunch of `ancient'
electrolytics laying around and you are experimenting for home use.

   - Tom

At 07:12 PM 3/8/00 -0700, Dan Michaels wrote:
>Interesting thread. I recall once reading that aluminum
>electrolytic caps have, to a certain extent, the ability
>to resurrect themselves after they have been hit with
>large "overvoltages", like ESD - eg, 5KV. Supposedly,
>the aluminum oxide layer can rejuvenate, even after some
>damage. Apparently doesn't work for tantalums, which don't
>have the oxide layer.
>
>Any additional experience/enlightenment along this line?
>Like how much damage can they take, etc?
>
>- Dan Michaels
>Oricom Technologies
>http://www.sni.net/~oricom


------------------------------------------------------------------------
Tom Handley
New Age Communications
Since '75 before "New Age" and no one around here is waiting for UFOs ;-)