Over voltage on an electrolytic capacitor

What happens to them if their voltage limit is exceeded by a small amount over a period of time?

Do they buldge as they do when you apply a reverse voltage?

Normally they allow about 20% of overvoltage, but it will shorten their lifetime. It depends what did You mean by "period of time": minutes, hours, days,..?

GUMY said:

Normally they allow about 20% of overvoltage, but it will shorten their lifetime. It depends what did You mean by "period of time": minutes, hours, days,..?

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Perhaps I will put it this way. A failed ATX power supply has two bulging 2200uF / 10V capacitors in it.

What is likely to have caused that? The capacitors having too narrow a margin between there max voltage and what is passing through the circuitry? Or a diode or something has failed and they have suffered some low level of reverse voltage.

Age, low cost design and temperature.

Ice-Tea said:

low cost design

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Absolutely! I had an electrolytic capacitor in an ATX supply explode once. When I had a look, I found it's rated voltage was less than the voltage across it in normal use. I think it was a surge that killed it, but still - very sloppy design.

[ot backstory]
I was working at my PC late one night when we had a power failure. I decided to call it quits, and went to bed without bothering to switch off the lights or computer. When the power came back on, I was woken up by a loud explosion and bright lights in my face.:shock:
[/ot]

- - - Updated - - -

A few years later, I got around to fixing it. After cleaning up the mess and replacing the cap it worked fine.

Well it definitely sounds as though replacing those bulging caps will probably resurrect the power supply.

I bought some 25V / 2200u caps today - I was going to get 16V ones but they weren't available. Oh well they will just have to stand above the designated positions a little.

The capacitors in switch mode power supplies get hot because of the very high ripple current. This causes the electrolyte to deteriorate, causing high leakage, high ESR and reduction of capacitance. It is normally the increase in leakage that causes the power supply to fail. When replacing these capacitors always replace them with 105c types as normal capacitors will not last long. Substituting a higher voltage capacitor is a good idea because even though a higher voltage capacitor has a higher ESR and will dissipate more heat, the larger size and surface area will more than offset this causing the capacitor to run cooler.
In answer to the original question of what happens to a capacitor that has their voltage limit exceeded by a small amount over a period of time? They normally go short circuit after a very short period of time, the life time of the capacitor will be drastically reduced, it is common practice to use capacitors rated at twice the expected voltage if you want the equipment they are in to last.

pjmelect said:

it is common practice to use capacitors rated at twice the expected voltage if you want the equipment they are in to last.

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Unless you're talking about tantalums, 'twice' seems a bit excessive, no?

boylesg said:

What happens to them if their voltage limit is exceeded by a small amount over a period of time?

Do they buldge as they do when you apply a reverse voltage?

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Marked voltage rating on capacitor body is maximum safe voltage, capacitor can handle voltage over that but that is specified in manufacturers datasheet.


What can happen in over temperature and over voltage situations :



See these videos:
https://www.youtube.com/watch?v=_WheLp0RdLQ
https://www.youtube.com/watch?v=3b7mjukhTyQ



Rated DC Voltage
Rated DC voltage is the nominal voltage marked on the capacitor, and it is the maximum peak voltage including ripple voltage that may be applied continuously between the terminals and over the rated temperature range. Higher rated voltage capacitors may be substituted for lower rated voltage capacitors as long as case size, DF, and ESR ratings are also compatible.

Rated Surge Voltage
Rated surge voltage is the maximum DC overvoltage to which the capacitor may be subjected at 25ºC for short periods not exceeding approximately 30s at infrequent intervals of not less than 5 min.

Surge voltage Measurement
Subject the capacitors to their rated surge voltage at normal room temperature and through a 1000Ω ±10% resistor (except for capacitances of 2500μF and up, use a higher value resistor calculated as 2,500,000/CΩ ±10% where C is the capacitance in μF). Cycle the voltage ½ minute on followed by 4½ minutes off during which each capacitor is discharged through the charging resistor or equal resistor. Repeat the cycles for 120h. Post test requirements are for DCL, ESR and DF to meet initial requirements and for there to be no evidence of mechanical damage or electrolyte leakage. Electrolyte residue with no droplets or visible flow is permitted.

Reverse Voltage
Aluminum electrolytic capacitors are polarized and must be connected in the correct polarity. They can withstand reverse voltages up to 1.5V. Higher reverse voltage can cause failure by pressure build up and rupture of the capacitor’s safety vent structure. Non-polar and semi-polar devices are available that can withstand reverse voltage.

Transient Overvoltage
Aluminum electrolytic capacitors can generally withstand extreme overvoltage transients of limited energy. Application of overvoltage more than about 50 V beyond the capacitor’s surge voltage rating causes high leakage current and a constant-voltage operating mode quite like the reverse conduction of a zener diode. The capacitor may fail short if the electrolyte cannot take the voltage stress, but even if it can, this operating mode cannot be maintained for long because hydrogen gas is produced by the capacitor, and the pressure build up will cause failure. However, special designs are available that use the overvoltage, zener-clamping effect to successfully protect equipment from overvoltage transients such as lightning strikes.
Capacitors used as bus capacitors in large, high-voltage capacitor banks are less capable of withstanding overvoltage transients because the high energy and low source impedance of the capacitor bank can prevent a momentary partial discharge from self healing and cause it to become a runaway short-circuit failure. For high-voltage capacitor-bank applications use capacitors proven for that use.




:wink:

Quote Originally Posted by pjmelect View Post
it is common practice to use capacitors rated at twice the expected voltage if you want the equipment they are in to last.
Unless you're talking about tantalums, 'twice' seems a bit excessive, no?

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Well I might have exaggerated a bit, but on low voltage capacitors the price difference between different voltage ranges is so small that you might as well use the higher rated capacitor if you have the space. On higher voltage capacitors it becomes expensive and you should give only a 30% margin.

I don't believe that ATX power supply capacitors primarly fail due to insufficient voltage rating. It's rather the usage of cheap capacitors with high ESR or bad quality in general.

Yeah, most likely is excessive ripple current, caused by a too-small inductor, coupled to a cheap capacitor with high ESR and a low temperature electrolyte.
On these supplies, there is so little money to be made by the manufacturer, that they cut costs everywhere.

Capacitor plague (specially read section called "Industrial espionage implicated")
http://en.wikipedia.org/wiki/Capacitor_plague

List of Bad capacitors
**broken link removed**

How to identify Jap electrolytic capacitors
**broken link removed**