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Old 1st Jul 2015, 8:57 pm   #1
Goldieoldie
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Default Electrolytic capacitor working voltage

Hi all.

What determines the working voltage of a electrolytic capacitor? ie how does a 10 volt capacitor 'know' it's only suitable for 10 volts and not say 16 volts?

Cheers Pete
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Old 1st Jul 2015, 10:31 pm   #2
Peter.N.
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Default Re: Electrolytic capacitor working voltage

Its mainly down to insulation. If you put 200v across a 10 volt capacitor you would expect it to fail but 16v across a 10v cap you might get away with. Manufacturers will always use the lowest voltage they can reliably get away with as the capacitors are smaller and cheaper.

Peter
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Old 1st Jul 2015, 11:28 pm   #3
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Default Re: Electrolytic capacitor working voltage

Interesting web pages on the subject, note the different etching for high and low voltage.

http://industrial.panasonic.com/lecs...ABA0000TE3.pdf

http://www.nichicon.co.jp/english/pr...f/aluminum.pdf
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Old 2nd Jul 2015, 10:34 am   #4
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Default Re: Electrolytic capacitor working voltage

The thickness of the electrolytically created oxide film determines the working voltage. But it also determines the capacity too (inversely).

It is important to realise that this is a bit of a moving target.
When made, the film is built up to give the required capacity and breakdown voltage. The applied voltage then maintains the film as any defects get reformed by electrolytic action. A side effect is that with time the voltage rating of a capacitor tends to become whatever voltage is usually on it (and left unused that decreases towards zero). The capacitance will also vary although not as much as the voltage because the voltage rating is determined by the weakest spots.

When I was a child I did a load of research on this without realising it. I was studying the electrolytic rectifier as described in early wireless books, used for battery charging on the new-fangled AC mains. I eventually realised that this device is essentially a electrolytic capacitor with no initial film so on one cycle it resists voltage by forming the film and on the other the film is destroyed and it conducts.
One thing discovered is that chloride ions totally destroy the film. I notice in the above document that chloride is used in the etching process to increase the plate area. I hope they wash them really well because the smallest bit of chloride is death to an electrolytic!
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Old 2nd Jul 2015, 10:43 am   #5
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Default Re: Electrolytic capacitor working voltage

Quote:
Originally Posted by GMB View Post
..........A side effect is that with time the voltage rating of a capacitor tends to become whatever voltage is usually on it.....
I was not aware of that GMB but it makes complete sense. Thanks for that little nugget of information.
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Old 2nd Jul 2015, 12:58 pm   #6
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Default Re: Electrolytic capacitor working voltage

Quote:
Originally Posted by GMB View Post
..........A side effect is that with time the voltage rating of a capacitor tends to become whatever voltage is usually on it.....
With that in mind I've often wondered whether an electrolytic, fed by a silicon or directly heated valve rectifier where the HT voltage is exercised through a range of higher than normal running voltages before everything else warms up, has a longer life than one where the voltage reaches the same point and stays there.

Jim
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Old 2nd Jul 2015, 1:50 pm   #7
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Default Re: Electrolytic capacitor working voltage

Quote:
Originally Posted by GMB View Post
The thickness of the electrolytically created oxide film determines the working voltage. But it also determines the capacity too (inversely) ...
This is true in general of course. But in the case of an old electrolytic capacitor I suspect that the film thickness must vary quite a lot from place to place. I say this because I've measured, at very low voltage, the capacitance of unreformed ones and found it to be surprisingly close to the value printed on the outside of the can. So most of the film must be close to the correct thickness. In the unreformed state though the insulation is usually both electrically weak and leaky. Could this be because it has just a few weak/leaky spots whose area isn't large enough to change the capacitance much ?

Cheers,

GJ
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Old 2nd Jul 2015, 2:26 pm   #8
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Default Re: Electrolytic capacitor working voltage

That's exactly it and is has consequences.

Because the leakage occurs at spots where the film has rotted the most, any current that flows is flowing through a very tiny area. This causes massive local heating which tends to further disrupt the bad spots and makes them worse. This is the mechanism whereby they can fail completely (because otherwise they would reform).

The other problem is that reforming the oxide layer is only one of the possible reactions that can occur. The unwanted one is creating free oxygen gas, which I think occurs at a slightly higher voltage - so if you keep the current low you just reform the film. At higher current you get gas as well and including at the other plate.

The older the capacitor the bigger reserve of electrolyte it is likely to have. Any gas generated represents a loss of electrolyte. Loose enough and the ESR starts to rise. Worse, if you had enough current flowing to generate both hydrogen and oxygen gas then if the pressure of that builds enough and distorts the metal the resulting spark will detonate it.

So changing from valve to silicon rectifier is undoubtedly adding stress to the capacitor as the valve warm-up is a nice controlled quick top-up to the oxide film.
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Old 2nd Jul 2015, 2:49 pm   #9
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Default Re: Electrolytic capacitor working voltage

I had an electrolytic reservoir/filter capacitor that was around 70 years old in a radio and worked fine, it was fed from a 5Z3 rectifier, in the end I recapped the radio with a view to longer term reliability.

Lawrence.
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Old 2nd Jul 2015, 4:51 pm   #10
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Default Re: Electrolytic capacitor working voltage

I was aware of the 'working voltage memory effect', but not of its mechanism.
Thanks for the information.
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Old 2nd Jul 2015, 6:59 pm   #11
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Default Re: Electrolytic capacitor working voltage

When I worked in an electronics factory, we had problems on the production lines with the cheap capacitors management insisted to buy causing units to fail final testing the first time they were tried, but be perfectly fine on re-test. The cure (if the test sequence could not be arranged so the timing test came last after all functional tests, by which time the capacitor should have reformed) was to pre-form the most critical capacitor. I invented an improved pulse forming rig; which used the capacitor directly in an oscillator built around a TDA2030 power op-amp. A quick press of the board onto the two spring-loaded test probes re-formed the capacitor in situ more quickly than the previous effort (two rows of SIL socket soldered onto a strip of breadboard, rigged up to a PSU; insert several caps, switch on PSU, manually solder a capacitor into each board before boxing and testing).
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Old 2nd Jul 2015, 7:49 pm   #12
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Default Re: Electrolytic capacitor working voltage

Only a little off topic, in the USA one can purchase multi voltage electronic ballasts for fluorescent lamps, typicly designed to work on any voltage from 120 nominal up to 277 nominal, or perhaps from 100 volts actual up to 300 volts actual.

It is widely believed that once a ballast has been used on one voltage that it wont work on a different voltage.
This is only partially true, a relatively new but slightly used ballast can be used on any voltage, and moved from any voltage to any other within its range.

If however a ballast has been used for years on a 120 volt circuit, then it is apt to go bang if subsequently used on a 277 volt circuit. I strongly suspect that the reservoir capacitor that is downstream of the input bridge rectifier has "got used" to operation on the lower voltage and fails explosively when used on the higher voltage.

I wonder if anyone has tried re-forming by gradually increasing the supply voltage from 120 up to 277 ?
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Old 2nd Jul 2015, 9:53 pm   #13
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Default Re: Electrolytic capacitor working voltage

Applying a specific voltage to an electrolyic to reform it is not a great idea - better to limit the current and see what voltage you get.

They behave a bit like a zener diode but the breakdown voltage increases as the reforming process proceeds.
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Old 3rd Jul 2015, 12:39 pm   #14
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Default Re: Electrolytic capacitor working voltage

I can remember replacing metal rectifiers with the original silicon diode the BY100 in the '50s. This resulted on a terrific peak voltage before the valves warmed up and a loud cracking noise from the electrolytics but I don't recall any ever failing.

Peter
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