Old Capacitors Reading Too Good!

[Dan Green]

Hi all,

Is it advisable to change old electrolytics which are reading a
capacitance considerably higher than their stated value? I know they
vary a lot anyway, but if I have say a 480uF cap which is reading
640uF, should that be a concern? Does it indicate some underlying
issue with the cap?

[Peter W.]

When any capacitor starts to read "high", that is the first sign of leakage.
CAVEATS:
a) An unmarked (electrolytic) capacitor may be anywhere from -10% to + 100% of rating. Modern caps, (made since 1990 or so) tend to be -5% to + 20%.
b) Beware outliers - if you have a dozen caps of the same rating and voltage from the same manufacturer from (approximately) the same lot, they should all be within a few % of each other. One that tests more than 5% low or 15% high in that lot from the average will be suspect.
c) And why it is that I keep an ESR meter as an additional screening tool over just capacitance.

Peter Wieck
Melrose Park, PA

[Three Jeeps]

On Thursday, July 6, 2023 at 11:51:06 AM UTC-4, Dan Green wrote:

Yes, higher values indicate a leakage issue. IMHO, capacitor measurement reading should be taken with a grain of salt.
Where they occur in the ckt needs to be taken into consideration as well as the age and type of cap. The most often cited usage is in power supplies (both linear and SMPS). As a general rule of thumb, for me, if it is a linear PS, over 20 yo, and there are PS behaviors that suggest a capacitor issue, replace all the electrolytics with equivalent capacitance and slightly higher WVDC if possible. Not worth revisiting that repair later down the road. If one comes across a dead SMPS and it is a capacitor issue, replace them all. It is not worth saving 50 cents just to rebuild it later.
I believe a better measurement approach to diagnosing bad caps is a capacitor checker in conjunction with an ESR meter.
As I mentioned, these are my guidelines/approaches. I am sure other ppl have theirs and may disagree with me. I'll just say to each their own.
Good luck

[HW]

On Thu, 06 Jul 2023 16:51:01 +0100, Dan Green <dhg9...@hotmail.se>
wrote:

>Is it advisable to change old electrolytics which are reading a
>capacitance considerably higher than their stated value?

Yes, definitely.

If you measure the cap with a "simple" meter, like a multimeter, a
failed or failing capacitor can "trick" the instrument to read high.

[Dan Green]

No, I'm talking about bespoke ESR/capacitance meters here.

[Phil Allison]

Dan Green wrote:
---------------------------

** Usually, the first warning of electro cap failure is when the ESR reading is higher than normal.
This indicates most of the electrolyte has been lost by evaporation.
Significant loss of capacitance starts later.
Increasing capacitance indicates a different condition, where the max voltage goes down in reverse to the increased C value.
Usually this only accompanies many years of non use.


..... Phil

[Arie de Muijnck]

If you use a real C / ESR meter, it may indicate the isolation layer has thinned over time.
In that case, reforming with a small current to the proper voltage could repair the electrolytic.
However, it also indicates the age - and replacement is certainly better.

Arie