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Old 8th Dec 2018, 2:30 pm   #21
MrBungle
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Default Re: Would you trust these caps?

This is exactly what they’re designed to do. They are sacrificial. They protect the device and eat up some of the noise going into it.

Healing here means it’s still a capacitor and not a short or open after a transient. This it’s still able to perform its duty. Capacitance of course will decline because every flashover means the plate size is reduced permanently. So when you design with them you usually over specify the capacitance by 3-5x.

Ceramics go short then explode. Much worse outcome.

Stuff with these things tends to have a service life of around ten years and that's plenty to eat up all the transients over time. We only hate the things usually because we own the stuff after a decade. I’ve learned to respect the sacrifice and just replace them.
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Old 8th Dec 2018, 3:07 pm   #22
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Default Re: Would you trust these caps?

Yes I remember those blue ceramics in a certain Phillips chassis tv that used to burn up very well!
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Old 8th Dec 2018, 3:28 pm   #23
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Default Re: Would you trust these caps?

Quote:
Originally Posted by Argus25 View Post
Quote:
Originally Posted by MrBungle View Post
Wise move.

The foil capacitors are self healing so they will take more than one transient.
Self Healing is a manufacturer's euphemism for progressive death. With each hit the paper and foil disintegrates, lowering the value of the capacitor.

Recently I checked all the X2 capacitors in my three air con unit control PCB's, the 1uF ones are already below 0.5uF and still falling.

I also set up an X2 capacitor in series with a 110V 2 watt mains clock motor with a pilot light (to run it from 230V), after about a year, the light dimmed off and the clock stopped. The capacitor was down to about 1/2 its original value. I replaced it with a 2kv rated poly cap, no problems since, it doesn't need to "heal" because its not unwell and not damaged by voltage transients, because its voltage rating is higher than any transient in the application (except perhaps a lightening strike, but they are only well defended by a GDT).
A 2kV or even 1kV rated polypropylene impulse rated capacitor will probably never fail in such an application. Manufacturers of X2 capacitors know this as well and some(!) warn against using non-series rated X2 capacitors in series with the mains. Vishay/BC has a short whitepaper about this. Apparently the X2 capacitors that are rated for series usages, consist of 2 capacitor elements that are themselves configured in series.
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Old 8th Dec 2018, 3:32 pm   #24
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Originally Posted by MrBungle View Post
Stuff with these things tends to have a service life of around ten years and thats plenty to eat up all the transients over time. We only hate the things usually because we own the stuff after a decade. I’ve learned to respect the sacrifice and just replace them.
That's solid reasoning for replacing mains suppression capacitors periodically, but I still don't understand why you would use more expensive capacitors that fail after a hard to predict period (I'm pretty sure they can fail with little usage but well within 10 years) in an often violent way.

Also, there's lots of equipment we fully expect to be around after 10 years, especially if it sees little usage. Measuring equipment and sewing machines are two categories where conservative or unknowing designers often specified Rifa that are especially prone to see premature paper capacitor failure.

Last edited by Maarten; 8th Dec 2018 at 3:41 pm.
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Old 8th Dec 2018, 4:38 pm   #25
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Default Re: Would you trust these caps?

The issue with the RIFAs failing is the encapsulation material degrades and lets moisture in. The capacitor has a paper dielectric and this gets damp, turns into a resistor, dissipates power and goes kawoosh. The new Kemet ones have a different encapsulation epoxy. The bit inside the capacitor is top notch. The bit that kept the inside In wasn’t

Measuring equipment gets inspected regularly unless you’re one of those horrible companies that can’t be bothered and sticks “for indication only” on everything and let’s it rot for two decades. Sewing machines for sure.

My mother’s food mixer blew up a few months back with a RIFA going off. Took 30 years. That’s not bad for anything going across the line.
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Old 8th Dec 2018, 6:48 pm   #26
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Default Re: Would you trust these caps?

That’s interesting research, Hugo. Useful to know what I vaguely suspected is actually true.

And yes, I did wonder by what mysterious mechanism caps could be ‘self-healing’. It sounds like a marketing ploy.
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Old 8th Dec 2018, 7:03 pm   #27
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Default Re: Would you trust these caps?

As I was just literally replacing one this afternoon here’s an old vs new RIFA comparison for reference:

Click image for larger version

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Old 8th Dec 2018, 8:37 pm   #28
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Default Re: Would you trust these caps?

Hi, from what is written above it looks like RIFA's are considered OK now.
Has anyone any idea when they were improved?

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Old 8th Dec 2018, 8:39 pm   #29
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Default Re: Would you trust these caps?

They redesigned them when everything went lead free. Not sure when that was precisely.
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Old 8th Dec 2018, 8:57 pm   #30
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Default Re: Would you trust these caps?

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Hi, from what is written above it looks like RIFA's are considered OK now.
Has anyone any idea when they were improved?

Ed
Given their horrible history, I would want to see the "New! Improved!" variety of RIFAs get a decade or so's survival-history stats before I'd consider deploying them.

A bad reputation is easy to earn but takes a long time to live down.
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Old 8th Dec 2018, 9:48 pm   #31
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This is exactly what they’re designed to do. They are sacrificial. We only hate the things usually because we own the stuff after a decade. I’ve learned to respect the sacrifice and just replace them.
Yes I agree, but the word (and marketing ) of a notion like "healing" implies there is some form of repair process when it is just that there is progressive destruction of the film and insulation.

So if there is a component added to a pcb with some life-cycle maybe 10 years, it is merely another example of built in obsolescence.

I regard these capacitors in a similar light to say a soldered in pcb battery, or perhaps an internal battery backed up non volatile ram, or perhaps an electrolytic capacitor too. They are used by manufacturers knowing full well they will set a limit to the life of the device or appliance and that device will definitely be in land fill before the decade is out, albeit some of it recycled.

For those wanting to save our planet from pollution and E-waste, it is much better if our appliances last longer, as they once did when there was less life-cycling and superior quality components and built with the service person in mind and with support documentation. Everyone seems to forget about the entropy increases of manufacturing new goods from scratch and the recycling of old ones. We are encouraged to throw away the old and buy the new.

One reason the X2's are popular though is their small size for the capacity and voltage ratings. As with any electronic part, it will last longer if it is physically bigger. Imagine a transformer the size of a house only being asked to deliver 50W. Probably (if protected from weather) it would still be working 1000 years later. The same for massive resistors, capacitors or practically any part you can think of. Clearly a 2kV rated cap will be over double the physical size of a standard X2.

So maybe we just need a simple equation for the capacitor: Size/Lifetime = constant. This also applies to electrolytic caps where it has been observed by many that the physically larger ones have a much longer life. I have particularly noticed this in vintage computers with analog power supplies where the very physically large and large uF value (>10,000 uF) caps are as good as new over 40 years later. And of course the worst electrolytics with the shorter life are tiny surface mount types.

The only way to improve X2's is to use thicker insulation, in other words increase their voltage ratings and they would last longer and be physically bigger.

Last edited by Argus25; 8th Dec 2018 at 9:56 pm.
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Old 9th Dec 2018, 12:23 am   #32
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Measuring equipment and sewing machines are two categories where conservative or unknowing designers often specified Rifa that are especially prone to see premature paper capacitor failure.
I think I told this story before. Many years ago I was sleeping in a room where there was an Electric Singer Sewing Machine, it had a foot pedal speed controller. It was switched on at the wall.

In the middle of the night a Rifa capacitor cross the foot pedal controller system (inside the foot pedal) spontaneously shorted out, probably after a voltage spike. The machine started on full speed. I woke up wondering what the heck was going on and it really freaked me out. After I investigated it, of course it all made sense in the light of day, but half asleep in the middle of the night, when a machine does that, it was quite disconcerting.
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Old 9th Dec 2018, 1:32 am   #33
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Default Re: Would you trust these caps?

This isn't at all unusual. Singer rheostat pedals from the 30s were fitted with two TCC interference suppression caps. My 1934 Singer 201K has this arrangement, and one day when I forgot to switch it off at the mains I found it running at full speed. I changed the original caps for X2s and all was well.
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Old 9th Dec 2018, 1:14 pm   #34
Maarten
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Originally Posted by MrBungle View Post
The issue with the RIFAs failing is the encapsulation material degrades and lets moisture in. The capacitor has a paper dielectric and this gets damp, turns into a resistor, dissipates power and goes kawoosh. The new Kemet ones have a different encapsulation epoxy. The bit inside the capacitor is top notch. The bit that kept the inside In wasn’t

Measuring equipment gets inspected regularly unless you’re one of those horrible companies that can’t be bothered and sticks “for indication only” on everything and let’s it rot for two decades. Sewing machines for sure.

My mother’s food mixer blew up a few months back with a RIFA going off. Took 30 years. That’s not bad for anything going across the line.
Sounds like every marketing trick in their book. RIFA have been bragging about the superiority of their encapsulated paper caps forever and making up excuses why they are really better than ever before (without mentioning they were no good to start with). Even if they changed the resin, which I doubt, the mechnical construction is still problematic, as is the failure mode. The failure mode is inherent to encapsulated paper capacitors, of which the insides are possiby top notch compared to some other random paper capacitor but not compared to modern foil equivalents. So my question remains: why would you ever want to use them, if at best they are theoretically as good as their cheaper replacement (they aren't) and even then if they fail, they will do so in a nasty way?

P.S. 30 years is indeed possible for such a capacitor, and if you draw up a graph of the average lifespan it may well be that most will live those 10 years you mentioned as a recommendation. That doesn't really address my problems with using them, but I can see why it would be tempting to do so if there would be any other small advantage or percieved advantage, even if only because they look so pretty (my first impression when I saw a few of them in a Swedish Philips set).

Last edited by Maarten; 9th Dec 2018 at 1:25 pm.
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Old 9th Dec 2018, 2:00 pm   #35
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Default Re: Would you trust these caps?

This is all good and that but I've seen the Epcos MKT X2 catch fire in an HP server power supply as well. And that was under three years old with huge upstream transient filters.

This is high energy stuff. It's going to have a working life. Stay in the curve and you're fine.
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Old 9th Dec 2018, 3:58 pm   #36
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Default Re: Would you trust these caps?

I had a similar experience with a RIFA in the pedal of a sewing machine which went with a loud bang followed by a horrible smell... The machine didn't start to run at full speed though as some have described. It had been purchased in the 1970's and I had 'rescued' it when I cleared out my dad's house. Although the problem could easily had been resolved with a replacement cap, the experience left such an impression on the other half that she insisted on not having the thing in the house again. It wasn't that reliable when new, so I guess it was just as well to let bygones be bygones.

It seems that I am not the only one to be unconvinced about the "self-healing" terminology. As per the preceding comments, I had always assumed that a transient or spike that exceeds the design tolerances is likely to damage the foil surfaces/dielectric to some extent and I also could not imagine any mechanism by which this damage might self repair, so I always took the expression to be marketing hype. I hadn't quite thought of it as "progressive death" either but I suppose that progressive deterioration is by nature a feature of any capacitor, just that some deteriorate quicker than others depending on materials used and the application in which they serve.

Last edited by WaveyDipole; 9th Dec 2018 at 4:16 pm.
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Old 9th Dec 2018, 4:00 pm   #37
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Default Re: Would you trust these caps?

An analogy. Your body is self healing. To a point.
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Old 9th Dec 2018, 4:21 pm   #38
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Default Re: Would you trust these caps?

Capacitors were mentioned, and as night follows day the topic has reverted to RIFAs!
These capacitors have been fitted by a wide range of manufacturers, both domestic and professional, for many years and seemingly still are. Their designers must have reasons for using them and the manufacturers must be aware of relevant statistics. They can't all be wrong surely?
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Old 10th Dec 2018, 12:29 pm   #39
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Default Re: Would you trust these caps?

Sorry, my fault! I started the original discussion about unknown HV ceramics purchased from eBay that were the subject of my concern, but then, as an aside, asked whether such HV caps would be suitable to replace RIFA Y-class caps. I received clear advice on that (they are not suitable) for which I am grateful, but it seems that my question triggered a diversion to the general subject of RIFA's.

Regarding the original unknown brand ceramics, a member has offered to soak test them at up to 2kV and I have sent him a couple of samples of each of the 471s and 472s. I will report back on the result.
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Old 12th Dec 2018, 2:13 am   #40
Maarten
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This is all good and that but I've seen the Epcos MKT X2 catch fire in an HP server power supply as well. And that was under three years old with huge upstream transient filters.

This is high energy stuff. It's going to have a working life. Stay in the curve and you're fine.
Agreed, other dielectrics and brands inevitably fail sometimes. MKT (polyester) is by the way a worse dielectric for applications that see any kinds of peak voltages than fresh paper. That's why Philips still made a few types of paper caps after the introduction of their mustard coloured MKT product line in the late 1950's.

Ceramic capacitors weren't really suited as a full range alternative to paper capacitors back then either. I think ceramic Y-capacitors were only introduced in the early 1970's and pulse rated ceramic capacitors such as about which this topic was started probably even in the 1980's.

@AC/HL: After roughly 1980, they all were wrong, but didn't necesarilly know or care. After all, faillure within warrantee would have been reasonably low.

Last edited by Maarten; 12th Dec 2018 at 2:20 am.
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