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But that's not usually how they fail.
They don't get an internal dead short. They develop many tiny layer-to-layer shorts or perforations in the dielectric that usually self-heal.
That's my whole point about why they often don't take the fuse with them. They don't fail like that at all.
Remember this is a spiral of many ultrathin layers of foil and dielectric. There's no real way to all of a sudden develop a "dead short"
There is. Moisture gets in once they crack. Water is conductive.
You seem fully self-convinced.
I'm out of this discussion.
You are totally wrong about that as well. Do some research. You will find that the cracks -> moisture penetration -> turning into a low-ohm resistor due to the conductivity of water explanation dominates.
Your //e PSU clearly had a bad RIFA before that happened.
How well do you understand this type of capacitor? They are pretty cool and very durable devices designed for safely managing bad input power including as you described which should have be good power. So no, a dead short of the capactitor shoudl not blow the cap to smithereens. It should however, in all cases with this design, provide enough time for F1 overheat and blow. If that doesn't happen, the RIFA was clearly bad and should have replaced earlier. The X and Y safety capacitors are designed and rated to handle bad power signals safely and a good X (or Y) caps should never fail as you described. The "safety" label means to mitigate risks of shock and/or fire under some hard to duplicate AC input signals.
The pictures others have shared of cracked RIFA capacitors shows they breakdown. Some still work (still have capacitance) but you can bet the rest of their special traits are long gone. The cracks are the reuslt of a material failure or component abuse. Just like any other cap, a cracked cap is a bad cap and should not be expected to work correctly. Even a pretty looking cap could be bad, the materials used with most of the other X & Y capactitors age very well. The RIFA plastic (potting compound?) not so well at all. Shelf life, for these RIFA X2 caps was 10 years, which means sitting in controlled enviroment waiting to be installed on a board, 10 years. Use time was limited ot 2,000 hours, most likely those have been exhaused over the last 30 yeras. 3 hours a day for a single year is more than 1,100 hours.
Unlike other types of capacitors, when use exceeds the rating on safety caps their behavior can vary drastically from expected (hmmm...). This is why old safety caps should be replaced rather than left until they release their stinky magic smoke.
IMG_4572.JPG
Do you understand what their primary purpose in the PSU is? They are not there for safety and if you remove them the PSU will be just as safe. Perhaps I am misunderstanding what you have written (and I apologize if that is the case), but it sound like you think they are there to manage some kind of "bad input power" comming from the power grid?? Again, this is simply what I am reading from the part I have quoted above.
Also I still don't understand why you keep repeating that the X2 and Y1 caps should never fail or blow up. Of course they should not! Nothing in the PSU should, however RIFAs have a nearly 100% failure rate, so things are not always as they should be. You are basically repeating the same argument, which no one in this entire thread has disagreed with.
I don't get why all the arguments...
RIFA caps are known to fail. Sometimes the fuse goes, sometimes it doesn't. I doubt anyone expected people to still be using these machines nearly 40 years later. A new X2 cap, especially those that aren't made with the kind of hard translucent plastic RIFA used back in the day can normally take quite a bit of abuse without blowing.
Yes, but you may be missing the point that aside from just providing basic bi-directional AC line filtering they do so in a way other caps can't by reducing/eliminating additional risk of shock or fire to users at this input stage of the circuit. Safety is the important part, that's why they need to be able to handle adverse conditions in a predictable safe manner. This is where there are all those safety certification logos on the caps. Do you see these certifications on any of those other caps in the power supply? Even the other RIFA caps on board don't have the safety rating logos.
CVT wrote:
This seems to suggest I'm not doing a good job of making this easy to follow. The reason is the context and application of these devices. Yes they are filters, but if that's all they do then why not throw any ol' .1uF cap rated for 250V and be done with it? Why this type of cap in this one spot in the board? Once it's understood this is to keep people safe, rest should make sense.
I also don't believe I called out X2 other than to say X2 is what's in the Apple supply. The X1 Y1 caps are for industrial voltages greater than 400V and overkill for household voltages. The X2 and Y2 are enough for household voltages which are <400V.
Caps blow up all the time, power supply or other applications, even ceramic caps blow up. It's expected when a cap is pushed byond ratings or has been damaged. There's nothing we can do about line surges. Even under great surges X and Y class caps should never be destroyed. The difference with these caps is they can handle adverse conditions and even recover, it's not a one-and-done cap they heal and keep on going. But, the party can't last forever, they have a limited life and this is where some of where public perception seems to fall short. These can be thought of as caps with nine lives (nine is just a number not an actual value) they take abuse but only so much and then all bets are off. There are risk running these beyond their expected lifetime.
The only reason these caps have such a high failure rate is because people don't understand. They don't understand they have a useful life and being used decades beyond their useful life is not something anyone should be doing with these devices. These are special caps which do filter, but while providing sevearl features that reduce risks to users. These are actually pretty cool, interesting, and useful devices.
No, the only reason RIFAs have such a high failure rate is because of the problematic exposed transparent epoxy used to cover them, which develops cracks over time. Other brands of X2 and Y1 caps where the epoxy is not exposed and not transparent don't have this issue. Even old WIMAs with exposed transparent epoxy seem to fare lot better than RIFAs.
It seems only fair that WIMA is still in business, while RIFA AB is now defunct and if you go to Wikipedia there is a picture of a blown RIFA cap under Legacy. I agree with you that they are interesting, but I would not call RIFA caps "cool" under any of the word's multiple meanings.
Again, I likely didn't make that easy to follow, I was talking about the safety caps being intersting, not the RIFAs specifically.
That said, the RIFA branded X2 found in the Astec products are a curious products for a couple reasons.
The first and most obvious is that clear expoxy casing which is different than other brand does not appear to age well, or maybe was just too sensitve heating cycles after aging, I don't know. Other's casings are less ridgid (more flexible) which could mean less likley to crack (except along the potting seal) Cracks with internal pressure changes allows for dsiplacement during heating. This would also mean something else must fill the void when cooled. Doesn't matter, dry or moist air entering the cap is bad. Kemet is still making the same caps today with the same construction, I'm just not clear if they improved the casing compound, seems easy enough to do... we could ask.
For me, the most interesting thing about these RIFA X2 capscitors is they are made of metalized paper rather than metalized polypropolyene. Internally the paper/plastic materail is the dialetric, with a depositied "metalized layers" actings as electrodes. They may also be an oil (like transformers) around/trhough for heat dissapation and to keep the material from becoming dry and brittle. I don't understand this paper part, it's unclear clear how paper promotes "healing" other than maybe it's been treated with a fire retardant which prevents ignition. Polypropolyene, makes more sense, because that doesn't burn like paper.
So, these safety caps may share design compoents with other layered capacitors, but also have some pretty unique and cool characteristics.
For the majority of my reworks I've used the metalized polypropolyene capacitors. Bbut I've also got a stash of modern Kemet (RIFA) X2 caps.
I haven't decided which is better to use, I've been tesing and expect both have benefits and disadvantaes. So it may be a wash.
The RIFA (aka Ericcson) adevertising promoted paper was more safe than the polypropolyene options.
RIFA - Fire.jpg
Rifa - Shock.jpg
Rifa - Transient.jpg
The increase in capacitance happens from a different phenomenon called dielectric dissolution or erosion. This specifically affects electrolytic capacitors without voltage applied, in other words not in use for decades. The acids in the electrolyte attack the aluminum oxide dielectric and start to etch it off the plates, which makes the dielectric barrier between positive and negative poles thinner. The capacitor equation C=εA/d predicts that capacitance is inversely proportional to dielectric barrier thickness: as the dielectric gets thinner, capacitance will increase. Indeed it is the only way for capacitance to increase, since ε (the dielectric constant) is fixed, and A (plate area) is limited to the size of the foils when they are assembled at the factory.
The other parameter that increases is (electrical) leakage, because the oxide layer is formed only thick enough to act as a dielectric at the capacitor's rated working voltage. Forming a thicker oxide would reduce capacitance; this is the primary reason that the can size corresponds to the CV (capacitance × voltage) rating. The same aluminum foils, in the same can, can be formed electrically into a low capacitance, high voltage capacitor; or a high capacitance, low voltage capacitor. The difference is just dielectric (oxide) thickness. When an electrolytic is charged to over its rated voltage, or after long storage and resulting dissolution, charge "leaks" through it at DC. But this current inside the capacitor is not electrons, because the electrolyte does not conduct electrons, but ions. DC current acts as an electrochemical cell, like a battery, to cause chemical changes to the electrodes—aluminum atoms at the surface of the anode foil lose valence electrons and combine with oxygen ions to form alumina, thickening the dielectric layer. This electrolysis reaction is how the capacitors self-heal.
Aluminum electrolytic cells were originally used as rectifiers. They would be formed for a period of time to grow the dielectric on the anode, after which time what we could call "forward leakage" dropped due to the dielectric, voltage-withstanding effects of the anode, while the "reverse leakage" remained high because the cathode was not formed. Therefore they acted like diodes, passing more current when "reverse biased" and little when "forward biased" (the reverse of how we think about other diodes). The problem is that they wouldn't last long, since the longer they were "reverse biased", the heavier the oxide would grow on the cathode, ruining the effect. They were obsoleted by vacuum tubes and later semiconductors, initially by selenium.
The reason that testing at rated voltage has different results is that the capacitors begin to reform as voltage is applied. This type of reformation has nothing to do with Lutherans, but just means keeping a charge on the capacitor until it self-heals through electrolysis and re-grows enough of its anode oxide to withstand the required working voltage. It is accomplished by putting the cap in series with a resistor on the output of a DC power supply of around the rated voltage. The resistor limits the leakage current to a safe value that won't cause damage. So for a 12 VDC supply, reforming a small cap, leakage would be limited to around 1 mA and a 12kΩ resistor should be chosen. Much larger caps can handle higher currents so 10 mA could be safe. Once the leakage current drops to microamps—which can be measured either using a DMM in current mode in series, or in voltage mode across the resistor—reforming is successful and the capacitor can be returned to service if its ESR is still in acceptable range.
Aluminum Electrolytic Capacitor construction and equivalent circuit.png
Note that in the diagram above, only C1 (the anode foil) is shown with its parasitic elements, but the C2 (the cathode foil) has the same parasitics. And that since C1 and C2 are in series, the total effective capacitance is less than the lower of the two according to the series capacitor equation (same as the parallel resistor equation). So C2 (the capacitor formed by the "native oxide" on the cathode foil) must be higher capacitance than C1, but withstand lower voltage, meaning a very thin oxide dielectric! In NP "non-polarized" electrolytic capacitors, both the cathode and anode foils are etched and formed to make them physically identical, and the effective capacitance is half that of either of the foils alone. That's one reason NP capacitors need larger can sizes. Re-forming them requires charging in both polarities.
High ESR is not caused by the above processes, but by the electrolyte itself drying out or escaping the confinement of the can. It's often said that "the magic smoke escaped" from a failed component, but in this case it's more like the magic liquid. There is occasional speculation that some fluid could be re-injected into the can, but this would need to be done under vacuum to be practical and probably can't return a capacitor to its original specs.
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