Electro caps derating - Minimum Voltage - Page 1
Forum section: electronics / projects

author: jumbodas
author_rank: Newbie
timestamp: June 18, 2018, 12:53:41 am
content: This is the question about aluminum electrolytic capacitors. A document I saw somewhere written in 1993 says "operation below 20% of rated voltage causes the aluminum oxide dielectric to thin and is not recommended". In the document there was no other strong evidence / reference to support this statement.So my question is -a) Is it just a hearsay or even if has some theory to support it. have caps given problem when used at <20% of rated voltage? And how much the problem will be if I use at 10% of rated voltage?b) In 2018 that is 25yrs later when parts technology has progressed. Is it still valid? I was of opinion in 2018 one should be able to use cap even at 5% of rated voltage with no noticeable change in cap long term performance.Please share your opinions but facts supported with creditable references are even more appreciated. Thanks

author: helius
author_rank: Super Contributor
timestamp: June 18, 2018, 02:39:50 am
content: Aluminum electrolytics just sitting on a shelf have thinning dielectrics. If they are installed in equipment that is powered off, the same process takes place. If the working voltage is very low in relation to the rating, it may take place as well, albeit more slowly than at 0V. This effect is normally accounted for in the life rating; applying the full WV to a capacitor will just survive longer than the published figure.Nippon Chemi-Con:Quote

author: CatalinaWOW
author_rank: Super Contributor
timestamp: June 18, 2018, 03:17:04 am
content: I would assume that if the dielectrics are thinning that the capacitance would go up, and that going out of tolerance might be the life limiting factor, rather than dielectric breakdown. Only the OP can answer whether increase of capacitance is a defect in his application.

author: floobydust
author_rank: Super Contributor
timestamp: June 18, 2018, 05:06:40 am
content: I think that's bogus, 20% of rated voltage is can be quite high. 16V rated part=3.2V, 450V rated part = 90V!Electrolytic capacitors have a definite half-cell potential between the aluminium oxide and electrolyte.If a circuit has resistances such as bleeder resistors that discharges the cap to 0V then the oxide is for sure depleted in time, usually over year and reforming is necessary.Sitting on the shelf, I've had some in storage for years and a voltmeter gives 0.2-1V and I believe the oxide remains intact. But that's my theory.The original oxide layer is created applying 140%-200% of rated voltage to a factory new capacitor, so adding oxide seems verses reforming the original oxide, I don't fully understand the difference.

author: jumbodas
author_rank: Newbie
timestamp: June 18, 2018, 07:42:11 am
content: Quote from: helius on June 18, 2018, 02:39:50 amYes I read this document and it sayQuote"For a screw terminal type capacitor with the rated voltage of 350Vdc or higherLx=Lr×2 × 2 ×Kv ………………………（19）Kv ：Derating voltage factor"But I couldn't find value of Kv in the document. Nor I could find it in the datasheets for screw terminal type capacitor with the rated voltage of 350Vdc or higher

author: helius
author_rank: Super Contributor
timestamp: June 18, 2018, 07:46:15 am
content: Derating voltage factor is the ratio between the peak voltage applied in the circuit and the WV rating. So if you apply a maximum of 175V to a 350V capacitor it is 0.5

author: BravoV
author_rank: Super Contributor
timestamp: June 18, 2018, 08:06:14 am
content: Quote from: floobydust on June 18, 2018, 05:06:40 amWonder if ultra caps also experience the oxide deterioration if keep shorted for long time ?As I worry the dielectric absorption can make the cap dangerous if left opened and accidentally shorted, especially they're forgotten for quite some time in the storage. Example of my shorted ultra cap.

author: jumbodas
author_rank: Newbie
timestamp: June 18, 2018, 10:20:45 am
content: Quote from: helius on June 18, 2018, 07:46:15 amKv = F(PV,WV) The function is application specific.Definitely Kv ≠ PV/WV in this case as it will simply means PV/WV=0.5 reduces life of capacitor by half. That doesn't sounds right.

author: helius
author_rank: Super Contributor
timestamp: June 18, 2018, 02:08:53 pm
content: Quote from: BravoV on June 18, 2018, 08:06:14 amSuper/ultracaps do not have an oxide layer: this is why they are limited to voltages too low to cause electrolysis.