Capacitor basics - You're kidding me, right??

[underwurlde]

On 'another' forum, some chap posted up this statement:

....However, he seems to have forgotten some of his basic electronics design concepts!

An electrolytic capacitor is rated at the capacity at the rated voltage ( usually with plus 20% capacitance value ).

If you then consistently only use that capacitor at a significantly lower voltage then it will reform itself over time to give a higher value which is not what the manufacturer intended.

My thoughts were 'what a load of rubbish'.... but is there any truth in this??? I don't claim to know everything about capacitors and I may well learn something here. Who knows!

Cheers,

Andy

[TheSeekerr]

Nope, that really is a load of rubbish. The capacitance is determined only by the physical properties of the capacitor - that is to say, the internal surface area of the opposing "plates" (spirals, in the case of an electrolytic) and the permittivity of the dialectric seperating them.

[SY]

Not so fast. There actually is theoretical merit in this- the physical construction of an electrolytic includes the formed oxide layer. At continuous low voltage, the oxide layer could be thinner than when the cap is run at high voltages.

[pjp]

... and the thickness of the dielectric.
which is determined by the voltage ...

[TheSeekerr]

Actually, that's a fair point. I'm not convinced it would make a significant difference though.

[Mooly]

An interesting practical point relating to (particularly) high voltage electroylitics.
If you take a cap, say a 220uf 400volt used as a reservoir cap that has had many hours of use at near the rated voltage, then disconnect it and make sure it's fully discharged, and then put it on the shelf... and come back to it days later... you will find that residual charge in the dielectric causes the cap to develop quite a resisdual charge.... perhaps many 10's of volts if you measure with a DVM.

[godfrey]

Originally Posted by SY

... At continuous low voltage, the oxide layer could be thinner than when the cap is run at high voltages.

So you'd recommend burning in caps for an extended period at the rated voltage, then?

OK, what have you done with the real SY?
Release him at once and nobody gets hurt.

[pjp]

Originally Posted by godfrey

...

Read the manufacturer's datasheets. Electrolytic capacitors are not stable in storage. The dielectric film degrades.

I don't see why the same thing couldn't happen if run for extended periods at very much lower than rated voltage.

[AndrewT]

Hi,
the manufacturers tell us that the oxide layer degrades with time whether in storage or used at a lower than rated voltage.

The chemistry changes with time and that implies some physical change in the plates and in the oxide layer.
That will lead to variations in the various parameters of the capacitor.

The manufacturers tell us that they reform the capacitor to a set procedure before measuring any of it's parameters.

Do not expect a capacitor to return to specification by burning in. Do it properly. Reform the capacitor before installing it into it's circuit.

[Mooly]

Originally Posted by AndrewT

Do not expect a capacitor to return to specification by burning in. Do it properly. Reform the capacitor before installing it into it's circuit.

This is something I have been doing for a long time... bring the voltage up slowly to 100% of maximum and after a few seconds slowly reduce back to zero... and no shorting them out

I still leaves the problem of caps that see no DC voltage across them at all in use... I wonder what the long term implications are of circuits that use caps this way.