Electro caps derating - Minimum Voltage

[jumbodas]

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 

 

[helius]

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:

5-2 Applying Voltage Effect on Lifetime
Where a capacitor is used at lower than the rated voltage, the
lifetime may not be adversely affected, which means that the effect of the applying voltage is negligibly small, while the effect of the ambient temperature and heat generation due to ripple current is significant.

However, for capacitors of larger size and higher rated voltage contain a larger volume of electrolyte, difference in applying voltages can affect degradation of the oxide layer, other than the diffusion of electrolyte.
Therefore, for screw mount terminal type capacitors with the rated voltage of 350Vdc or higher, the lifetime estimation includes the effect of applying a lower voltage than the rated voltage (derating voltage).

[CatalinaWOW]

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.

[floobydust]

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.

[jumbodas]

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:

5-2 Applying Voltage Effect on Lifetime
Where a capacitor is used at lower than the rated voltage, the
lifetime may not be adversely affected, which means that the effect of the applying voltage is negligibly small, while the effect of the ambient temperature and heat generation due to ripple current is significant.

However, for capacitors of larger size and higher rated voltage contain a larger volume of electrolyte, difference in applying voltages can affect degradation of the oxide layer, other than the diffusion of electrolyte.
Therefore, for screw mount terminal type capacitors with the rated voltage of 350Vdc or higher, the lifetime estimation includes the effect of applying a lower voltage than the rated voltage (derating voltage).

Yes I read this document and it say
Quote
"For a screw terminal type capacitor with the rated voltage of 350Vdc or higher
Lx=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

[helius]

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

[BravoV]

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.

Wonder 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.

[jumbodas]

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

Kv = 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.

[helius]

Wonder if ultra caps also experience the oxide deterioration if keep shorted for long time ?

Super/ultracaps do not have an oxide layer: this is why they are limited to voltages too low to cause electrolysis.