What Is The Shelf Life Of Electrolytic Caps?

Thread Starter

Glenn Holland

Joined Dec 26, 2014
I have 12 home built power supplies (with outputs ranging from 12 to 24 VDC) using Mallory brand electrolytic capacitors. These caps range from 2100 to 4500 ufd and they were purchased in new condition.

The power supplies are over 30 years old but, they were never used. So I'm wondering if these caps can deteriorate by just their age or can they be safely energized as if they were new?

Defective can-enclosed caps can explode and I don't want to have a 4Th Of July on my work bench!!!


Joined Jan 8, 2017
I would suggest reforming then before applying a voltage directly to them. Reforming is just connecting them to a power supply near their rated working voltage via a high value resistor. Monitor the current and when it has reached a low value they should be OK to use. If you Google "reforming capacitors" you will find lots of information on the subject.



Joined Mar 10, 2018
I have Tek scopes back to the 50's still OK, and also have fixed
ones where caps dryed out and had to be replaced.

Fixed more modern equipment where they failed short.

As mentioned prior reforming is first, and measure ESR to see if they
are functioning properly.

Here is ESR table, an approximate guide -

Its a bit of a crap shoot.

Regards, Dana.


Joined Feb 19, 2010
I did not know about reforming.

I know about ESR.

Here is something you two did not say...
- For reforming, does OP have to remove capacitor from the circuit? If they desolder the cap from pcb, might as well replace it with new/"fresh" cap.
- For ESR measurement, does OP have to remove capacitor from the circuit? Because if they do have to remove it.......


Joined Mar 10, 2018
A variac is the age old standard coming out of the vacum tube era.
I still use it on old scopes that I repair.

Bad news is there was a period of IC design that did not tolerate supplies
out of spec for prolonged periods, eg. they may or may not malfunction in
unpredictable ways. IC specs call this out if properly written. Most modern
circuits today, in their design specs, fix this problem..

So proceed at your own risk. Carefully observe current consumption,
heat, can sometimes save you from cooking off your equipment under

Regards, Dana.
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