electrolytic capacitor leakage current behavior

[lilmantis]

                    Hello everyone, I have a problem understanding the behavior of the leakage current of electrolytic caps.  It seems that after a few minutes (2-5 min)the leakage gets to a low value close to the minimum according to the datasheets, but what happens if the voltage is changing? Does the leakage current increase with applied voltage or current, or does it stay to the low value the datasheets specify?

[TheElectricChicken]

You'd need to hold the voltage steady to measure it because the capacitor will be filling up. It may also take time to fill up every nook and cranny, so you have to give it a minute. (or more) I'd think it may be working fine.

[lilmantis]

     Suppose you charge a cap for 2 min. and then you discharge it, but before you discharge it you measure the leakage current. After repeating this procedure for a hour will you have the same leakage current as the datasheet specifies or the minimal leakage current possible? I would suppose the leakage current would have finished by now the process of oxidizing the imperfections between the cap armatures.

[T3sl4co1l]

You have two components at work:

One is due to the true DC leakage current of the dielectric itself.  The dielectric is present as the result of an electrochemical reaction, one which is still ongoing.  The voltage rating of the dielectric corresponds to the voltage it was "formed" at; over time, this gradually dissolves, reducing the "formed" voltage.  In normal operation, just enough leakage current is drawn to oppose this inverse reaction, and operation remains nice and stable.

If equipment is left unpowered for a very long time (decades), the "formed" voltage decays, and leakage can be so high that capacitors heat up, produce hydrogen gas (which is held internally, increasing pressure), and in very bad cases, breaking the seal or relief, or causing an explosion.  (Of note, old style metal cans were sealed without relief cuts, so they tended to go sky-high in such cases.  This is not an exaggeration.)

If you have a very slowly changing voltage (like, days to years), more leakage will be drawn during the higher peaks, and less during the lower valleys, but ultimately being about the same average.

The second part is dynamic, an AC phenomenon that can manifest as leakage.  This is dielectric absorption.  This acts like a smaller capacitor (some percent of nominal), connected in parallel with the nominal capacitor through a very large resistance.  The time constant is on the order of minutes to days, so the equivalent resistance might be gigaohms for typical parts.

For this reason, you must make sure to "soak" the capacitor at exactly the nominal voltage, for an extended period of time, and monitor the leakage after that.

A notable case is the LT appnote about low noise: AN-83 http://cds.linear.com/docs/en/application-note/an83f.pdf
They used wet-slug tantalum capacitors (these use something like gelled sulfuric acid for electrolyte -- good to much higher temperatures than the gunk they stuff in aluminum electrolytics!), which cost something like 50 bucks a pop, and exhibited a week-ish "soak" period before they could finally be selected for least "true DC" leakage.

Tim

[Deathwish]

having read what Tim said i googled soak and got this.

http://www.electrocube.com/electrocube/assets/File/documentation-files/capacitor-documents/electrocube-tech-bulletin-10-caps-dielectric-absorption.pdf

[apis]

You have two components at work:

One is due to the true DC leakage current of the dielectric itself.

Shouldn't the leakage due to the dielectric be resistive, i.e. proportional to the voltage? Or is it the chemical reaction complicating things?

[lilmantis]

       I found this document from Tadiran that seems to explain quite well the leakage behavior of electrolytics.

Leakage current properties of modern electrolytic capacitors
www.tadiranbatteries.de/pdf/applications/leakage-current-properties-of-modern-electrolytic-capacitors.pdf

[T3sl4co1l]

You have two components at work:

One is due to the true DC leakage current of the dielectric itself.

Shouldn't the leakage due to the dielectric be resistive, i.e. proportional to the voltage? Or is it the chemical reaction complicating things?

Yes.  It's a barrier effect.  I'm not clear, myself, on what it is about anodization that causes the large voltage drop (well in excess of any in-solution or surface/interface potential), but the fact that it exists, and that the thickness of the layer is proportional (give or take?) to voltage, is the important part at work here.

The above article is an excellent, detailed read.  Good ref.

Tim

[3roomlab]

nice thread about leakage currents

am i right to assume that by subjecting new caps to a working voltage for a period of time, say few hours will help condition the e-cap ? (as in force oxide layer build up?)

[Cliff Matthews]

nice thread about leakage currents

am i right to assume that by subjecting new caps to a working voltage for a period of time, say few hours will help condition the e-cap ? (as in force oxide layer build up?)

Agreed, I just checked out residual charge on a lot of 6" tall 250v and 450v electrolytic caps I charged up and posted a question about in May. All the 250v units had between 0.3 and 1.0 volts charge on them, while the 450v units had between 6 and 14volts remaining. So yike's, thanks for this posting! And the video was good too (if it saves somebody or keeps them alert, it will have been worth it)
https://www.eevblog.com/forum/beginners/19-yr-old-250v-cornell-dublier-electrolytic-caps-still-trustable/msg676027/#msg676027

[macboy]

nice thread about leakage currents

am i right to assume that by subjecting new caps to a working voltage for a period of time, say few hours will help condition the e-cap ? (as in force oxide layer build up?)

Yes, this is referred to as "reforming" the capacitor.
That's another thing for you to Google and read up on