I'm curious to know, if an electrolytic capacitor is charged using a current source, at some point near its maximum allowed voltage, it will start drawing a large leakage current, so the voltage could stabilize, at a point, if the current feeding it is small enough. Afterwards, what happens? Will this leakage current remain the same, protecting the capacitor from further overvoltage, or it will decrease, letting further voltage on the capacitor?
Also, how does the capacitor itself behave when it starts reaching a voltage that will lead to a failure? Does it suddenly clamp, like a reverse connected zener diode, or current slowly increases? Is there a graph somewhere?
Thanks!
Also, how does the capacitor itself behave when it starts reaching a voltage that will lead to a failure? Does it suddenly clamp, like a reverse connected zener diode, or current slowly increases? Is there a graph somewhere?
Thanks!
Well, lead acid batteries for example have a floating charge that is considered to be safe. Can the electrolytic capacitor behave the same?
the leakage current increases slowly and very approximately proportionally to the applied DC voltage.
As the charging voltage approaches the capacitor's maximum the leakage current begins to increase more quickly than proportional (would that be exponential?)
as the capacitor starts to fail due to overvoltage you may find that the leakage current times the applied voltage has become a high dissipation. This causes overheating in the areas that are passing this excessive leakage and they start to boil/melt and then all hell lets loose and the capacitor explodes.
The above is my layman's guess at what happens, I have never blown up an electrolytic.
I have reformed many electrolytics right up to their rated voltage and a few to 10% over rated voltage.
As the charging voltage approaches the capacitor's maximum the leakage current begins to increase more quickly than proportional (would that be exponential?)
as the capacitor starts to fail due to overvoltage you may find that the leakage current times the applied voltage has become a high dissipation. This causes overheating in the areas that are passing this excessive leakage and they start to boil/melt and then all hell lets loose and the capacitor explodes.
The above is my layman's guess at what happens, I have never blown up an electrolytic.
I have reformed many electrolytics right up to their rated voltage and a few to 10% over rated voltage.
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AndrewT, of course, depends on particular supplier, but many of decent ones keep by design the safe room +10% of the rated voltage.
There is a useful paper on the Panasonic Industrial website about electrolytic capacitors applications and failure modes, with several diagrams and design suggestion to increase life. https://industrial.panasonic.com/cdbs/www-data/pdf/RDF0000/DMF0000COL42.pdf
The overvoltage breakdown is listed at page 18.
The overvoltage breakdown is listed at page 18.
Yes.
You're usually calculating with +10% over voltage of the circuit and 20% safety margin for long term reliability.
My recent test showed that when reforming a screw leaded capacitor, letting it charge with a relatively small, limited (2mA) current, the capacitor stops with the voltage increase at nearly 30% above the rated Vdc. The energy dissipated seems too little to make the capacitor heat, especially with low voltage capacitors.
this is severe abuse of your capacitor.
What you are doing is letting the leakage current increase during excessive voltage until the input current = leakage current. Then the voltage holds steady.
This may have already damaged the ultra thin foils.
As the capacitor reforms it converts the aluminium at the surface into a non conducting compound. This nonconducting film contains aluminium and that aluminium comes from the parent foil, i.e. the foil gets thinner.
Repeat this often enough and you will have created holes in the foil.
Apply some voltage spikes from impulsive interference and that will create more holes in the thinned foil.
"Wet" electrolytics began as surge limiters to protect motors and transformers. Large wet units will reliably absorb huge energy short-term.
Today we only have "dry" e-caps and no short-term reserve.
Yes, the rise is semi-exponential so a little more voltage is a lot more current. A sane designer will always keep a significant safety factor from the maker's rated voltage.
> relatively small, limited (2mA) current, the capacitor stops with the voltage increase at nearly 30% above the rated Vdc. The energy dissipated seems too little to make the capacitor heat
That's a reasonable result. While a large cap "may" have over 2mA leakage at rated voltage, 99% of production will have much less.
Considering the -wide- range of e-caps, it is pointless to cite a current without the uFd or Voltage or can size.
Taking wild numbers: 2mA in a "450V" cap over-volted to 600V (+30%) means 1.2 Watts of heat in the can. For a 1"d 1"h (25x25mm) can of about 3 square inches exposed surface, 0.4W per sq.in. or about 40 deg C temperature rise. Hardly "cool".
The next problem is that leakage increases with temperature. Reasonably double for each 10 deg C rise. If the 2mA is solidly limited, then current will not increase, voltage will decrease, so power and temperature decreases. The system is stable. If current can increase, it will. That increases power and temperature and heat and leakage... run-away.
I'm not over-concerned about damage to thin foils. The oxide layer is nearly mono-molecular. While foils and etchings have become ultrathin, still any practical foil is far-far thicker than the oxide layer, lots of reserve. (Not the same as Metalized Film construction.) I would however worry about the byproducts of oxide destruction contaminating the electrolyte. (Tho I worry more about chloride contamination built into the cap at the factory.)
Today we only have "dry" e-caps and no short-term reserve.
Yes, the rise is semi-exponential so a little more voltage is a lot more current. A sane designer will always keep a significant safety factor from the maker's rated voltage.
> relatively small, limited (2mA) current, the capacitor stops with the voltage increase at nearly 30% above the rated Vdc. The energy dissipated seems too little to make the capacitor heat
That's a reasonable result. While a large cap "may" have over 2mA leakage at rated voltage, 99% of production will have much less.
Considering the -wide- range of e-caps, it is pointless to cite a current without the uFd or Voltage or can size.
Taking wild numbers: 2mA in a "450V" cap over-volted to 600V (+30%) means 1.2 Watts of heat in the can. For a 1"d 1"h (25x25mm) can of about 3 square inches exposed surface, 0.4W per sq.in. or about 40 deg C temperature rise. Hardly "cool".
The next problem is that leakage increases with temperature. Reasonably double for each 10 deg C rise. If the 2mA is solidly limited, then current will not increase, voltage will decrease, so power and temperature decreases. The system is stable. If current can increase, it will. That increases power and temperature and heat and leakage... run-away.
I'm not over-concerned about damage to thin foils. The oxide layer is nearly mono-molecular. While foils and etchings have become ultrathin, still any practical foil is far-far thicker than the oxide layer, lots of reserve. (Not the same as Metalized Film construction.) I would however worry about the byproducts of oxide destruction contaminating the electrolyte. (Tho I worry more about chloride contamination built into the cap at the factory.)
Might not be enough. 😛
For people who like to overvolt their capacitors, I suggest this EOD approved suit:
For people who like to overvolt their capacitors, I suggest this EOD approved suit:
I second what PRR said. The Al2O3 layer's thickness should be of a molecular level. It's a subject worth reading further though.
Thank you folks for this great discussion. I gathered lots of info.
No worries, I have 6 years of experience with pyrotechnics and high explosives. 😀
Hey, that's great! I'd like to have my capacitors explode in the colours blue, yellow and red please! 😛
there is no chemical reaction in theory, but in fact there may any electron path that damage the cap partially on high voltage cap, and if not at least the seal is weaken caused by cycle of pressure changes.
it accumulate minor damage or instant damage at certain level (much higher voltage than allowed) that lead in shorted cap, and only dissipate heat before that level that lead to explosion and dry dielectric (likely open). current is instantly increased then decreased slowly until totally boiled.
True, it is an electrochemical reaction - usually known as electrolysis which is why the cap is called an electrolytic. This attempts to thicken the oxide layer but if it cannot do this sufficiently then gas is formed instead.ontoaba said:
If one forms an Al electrolytic capacitor with at reasonable constant current, I think it should just continue to form even above its rated voltage, so long as breakdown is avoided - shouldn't it ? At some point, the capacitor will break down, of course. I'm not advocating, BTW, just a thought experiment.
The rated voltage is such that it guarantees the capacitor won't break down and will meet reliability specs under set conditions of use.
LD
The rated voltage is such that it guarantees the capacitor won't break down and will meet reliability specs under set conditions of use.
LD
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We used to have a childish fun at my first workplace. When one went out the room, someone plugged in an axial electrolytic into a 24V AC outlet near to the guys seat. When the poor victim returned, the capacitor not immediately exploded but sometimes after a couple of minutes and covered everything with white flakes, smoke and sour odor. He believed it was the circuit he was just working on...
We used to have a childish fun at my first workplace. When one went out the room, someone plugged in an axial electrolytic into a 24V AC outlet near to the guys seat. When the poor victim returned, the capacitor not immediately exploded but sometimes after a couple of minutes and covered everything with white flakes, smoke and sour odor. He believed it was the circuit he was just working on...
...or that it was his fart that acidentally has cought fire? :-D
Best regards!
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