Electrolytic Capacitors
I wasn’t aware that it was quite as bad as a capacitor plague as wikipedia.org points out; but it really is that bad. The more I started looking at things that had failed around here, the more I began to see their point. It was only the electrolytics that were failing.
I had been struggling with finding a decent method of determining when a capacitor is bad or not. Its a difficult and time consuming process when they don’t show obvious signs of bulging or leaking as the photo above shows. This led me to watch Dave Jones of eevblog.com fame discussion of electrolytic’s. He mentioned something about his Bob Parkers ESR (Equivalent Series Resistance) meter and a google search opened up the flood gates about failing electrolytic caps and loads of information. BTW, he is really funny and if you want to hear a rant on Electrolytic s then you need to watch that video. 🙂
Electrolytic Capacitor Life is nicely identified by chem-con.com . In a nutshell, they fail more often then not because of excessive temperature. Secondary and more predictable are voltage, high duty cycle, etc. I found that interesting because if I fail to see any hard visual evidence then I am going to be looking at the caps that are closer to high temperature locations on the circuit board. It gets even more interesting because according to some sites – “The service life of electrolytics is approximately halved for every 10 degrees C increase in temperature and, surprisingly, many are only designed for a few thousand hours at their maximum rated temperature and ripple current. (A year is 8766 hours!)”. That is way too low to be using these caps in home appliances.
So what is ESR and why would I need to know about that? In a nutshell, electrolytics are ‘wet’ devices in the sense that their operation depends on a water-based electrolyte, soaked into a strip of porous material between the alumin(i)um foil plates. This completes the ‘outer’ electrical connection to the alumin(i)um oxide dielectric, which coats the anode foil. Unhappily this layer of electrolyte has electrical resistance which, along with the (negligible) resistance of the connecting leads and alumin(i)um foil plates, forms the capacitor’s Equivalent Series Resistance. Under normal conditions the ESR has a very low value which stays that way for many years unless the rubber seal is defective, in which case the electrolyte’s water component gradually dries out and the ESR creeps up with time. Bob Parker, www.ludens.cl, and wikipedia.org do a much better job of explaining this than I and have built special circuitry to identify these out of spec capacitors. As many have pointed out, even if they appear to be within tolerance when you measure their capacitance you still don’t know how they will behave under load.
I also found this great article when researching ESR titled, “Capacitor Testing, Safe Discharging and Other Related Information”. So now I have the why, how, and when.
Next step is to purchase a ESR meter. I think, I am going to with the kit from Bob Parker at http://members.ozemail.com.au/~bobpar/esrmeter.htm
I also found this initially but mention it here only in passing as it applied to other types of Capacitors and I thus far all I am seeing fail are electrolytic.
Failures during use
Randomly occurring failures in capacitors during their use are the most important source of failures in capacitors. But if capacitors are properly selected, they are also the least common.Often the useful life of capacitors is longer than the application itself. However, it is very important in a critical application such as an air bag or automotive braking system that the components do in fact fulfill their useful lifeSeveral factors can prevent them from doing so. When capacitors are in use, energy surges and high temperatures cause different kinds of failure. In any design, it is important to know how a capacitor will react to a surge or high temperatures to determine the most suitable component for the application.
High energy
Capacitors react to energy surges in various ways. In most cases, the effects of a low-energy surge aren’t severe. Conversely, high-energy surges in most capacitors can be catastrophic. In metallized film capacitors, a low-energy surge can cause a reduction of isolation. However, these devices are also self-healing, significantly limiting any damage. If the energy is extremely high, however, a complete failure can occur. High currents can cause this failure by evaporating the connection between the metallization and the end contact. To avoid this, film/foil capacitors with infinite dU/dT, or complex series construction metallized capacitors with high dU/dT, should be used in high-current applications.
Electrolytic capacitors also have a self-healing ability, although to a lesser extent than film capacitors. In electrolytic capacitors, the dielectric can crack in both low- and high-energy surges.
When the electrolyte touches the aluminum through the crack in the dielectric, a reaction occurs that rebuilds the dielectric. The leakage current will increase to drive this self-healing effect. If, in reaction to the surge, the foil is punctured, venting may occur and the capacitor will dry out.
In ceramic capacitors, surges with low energy and high voltage can increase current leakage. Thermal stress can crack the dielectric and may also result in increased leakage or shorts. A high-energy surge may crack the ceramic and let in moisture, providing a conductive path.
In electrochemical double-layer capacitors, electrolysis decomposes the electrolyte if the voltage rating is exceeded. This generates gas, which increases internal pressure. If the pressure gets too high, the case will vent.
Improper voltage derating can damage tantalum capacitors; most tantalum manufacturers recommend derating the voltage down to 50% to 66% of rated voltage. Reverse voltage will also damage a tantalum, as will extreme thermal shock from out-of-control mounting profiles or heating from excess ripple current.
High temperature
Temperature is of great concern to any capacitor. On a circuit board, capacitors should not be mounted close to heat sources.
This applies to most capacitors, but especially to aluminum. A radiation shield between the cap and the hot component prevents the hot component from accelerating failure mechanisms, which can be simply a shorter lifetime (or faster parameter drift), or the opening of the pressure relief vent in extreme cases. To avoid failures in high-temperature applications, the designer should use capacitors with lower losses, a larger size, or a higher temperature rating.
Wear-out
At the beginning of a component’s life, failures per hour are very low, but they are random. During a wear-out period, failures increase per hour and become more predictable. The wear-out mechanism reaches a limit where devices will fall out of specification for the application.
Drifting parameters vary by technology and by conditions in the application. It is important for designers to know how capacitors react during wear-out, as it may be a factor, depending on how long their application is designed to last.
There is no wear-out mechanism for solid aluminum or tantalum capacitors, which is a major advantage over wet aluminum capacitors. Ceramics will have capacitance loss due to oxide vacancy migration.
Film capacitors will have some oxidation of the metal conductors, increasing the dissipation factor. For aluminum polymers, ESR impedance increases due to polymer degradation. For electrolytic and double-layer capacitors, there will be impedance and ESR increase due to electrolyte loss. ?
For more on capacitors, visit http://www.electronicproducts.com/passives.asp.
