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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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24th Dec 2016, 12:29 pm | #141 |
Dekatron
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Re: Another RIFA X2 failure...
I have seen dozens of cracked Rifa's and have always removed them. I had some Japanese made suppressor caps that were ceramic discs and have never seen any failures amongst those. It must be a mainly European problem using Rifa caps.
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24th Dec 2016, 1:13 pm | #142 |
Dekatron
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Re: Another RIFA X2 failure...
The problem with these capacitors is simple.
The design life of most equipment they are found in is less than half the life of the capacitor. There is no commercial reason for improving them in this throw away world. I have always said that they should have a do not use after date on them. |
24th Dec 2016, 2:18 pm | #143 |
Dekatron
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Re: Another RIFA X2 failure...
I wonder; is there a distinct advantage to impregnated paper dielectrics compared to polypropylene? There must be some reason to keep making them - there must still be a market. Or would the manufacturer argue that they've sorted all the problems with them, thanks to a new formulation epoxy coating that will never crack?
You would have thought that their problems are so widely known today that from the manufacturer's point of view, to keep making them would be pointless at best, or cause reputational damage at worse. Perhaps I'm missing something |
24th Dec 2016, 4:01 pm | #144 |
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Re: Beware Rifa Class X Capacitors!
For the record, I had an exploding Rifa recently in a Wolf drill - in this case a dual-gang special but in the same encapsulation. Not too bad, though, just a bang, loads of smoke and a smell of burning paper. Because there's not much space, I've replaced it with an identical NOS Rifa, but will keep an eye on it.
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24th Dec 2016, 7:39 pm | #145 |
Dekatron
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Re: Beware Rifa Class X Capacitors!
Well Mr Parcel Force delivered me a nice Vishay one this morning. After some scrubbing with IPA, swearing and manipulation, it has been installed and the scope is running entirely cold and happy now. No odour either! It used to get quite warm before.
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24th Dec 2016, 7:49 pm | #146 | |
Dekatron
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Re: Another RIFA X2 failure...
Quote:
[It's from "rhin", a nose, as in rhinoceros] Yes, basically I sniffed around (on occasions using a drinking-straw as a 'nose extension') to locate the burned part! |
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24th Dec 2016, 9:19 pm | #147 | |
Octode
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Re: Another RIFA X2 failure...
Quote:
I guess we will have to wait 20 years to find out if they have cracked the cracking
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25th Dec 2016, 2:16 am | #148 | |
Dekatron
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Re: Another RIFA X2 failure...
Quote:
As others have mentioned, the equipment in which they were fitted probably was designed for a limited operational lifetime anyway -- or else would be subject to regular servicing, on which occasion any suspect capacitors could be replaced. (Perhaps with Vishay, formerly BC Components, "blue block" types, which never seemed to give any trouble.)
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If I have seen further than others, it is because I was standing on a pile of failed experiments. Last edited by julie_m; 25th Dec 2016 at 2:26 am. Reason: Added paragrph 3 |
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25th Dec 2016, 3:14 pm | #149 |
Nonode
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Re: Beware Rifa Class X Capacitors!
Hi
Those Rifa capacitors crack even if they've never been used. I had a whole bunch of them in with other NOS components and every one of them was significantly cracked. They all exhibited considerable electrical leakage too. I think what may happen is that slight corrosion on the leads compromises the seal through the resin thus allowing in moisture which then causes the dielectric to swell and then bursting the resin case hence the cracks. The bin is the best place for these unfortunately. Regards Symon. |
25th Dec 2016, 4:05 pm | #150 |
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Re: Beware Rifa Class X Capacitors!
I think I've got quite a number of new unused ones in my junk boxes. I'll stick any I find on the megger.
OK, it's the wrong season just now, but we could collect a load together in time for November and build a set of sequencing switches for a mains-operated fireworks display. Video it and stick it on youtube. David
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Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
25th Dec 2016, 7:28 pm | #151 |
Dekatron
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Re: Beware Rifa Class X Capacitors!
My link in post#74 no longer works.
It is two NOS WIMA ones that split wide open while laying in a drawer. |
25th Dec 2016, 11:04 pm | #152 | |
Dekatron
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Re: Beware Rifa Class X Capacitors!
Quote:
While there still was some sort of an excuse to produce them 20 years ago, there isn't one now. RIFA still brags about their good qualities, WIMA is a bit more quiet about them but also still produces them (those have a marginally better reputation than RIFA, will fail in 21 years in stead of 20 or so). Btw, 20 years can be anything from 3 to 30 years or so. That probably isn't the whole reason. I think polypropylene capacitors are cheaper now. Also, both the shelf life and working life are limited and semi predictable for paper capacitors. You might be on to something regarding the use by date. |
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25th Dec 2016, 11:12 pm | #153 |
Dekatron
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Re: Beware Rifa Class X Capacitors!
For reference I replaced this one with a polypropylene one. I'll post back in a couple of decades if it blows 7up
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26th Dec 2016, 12:00 am | #154 |
Rest in Peace
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Re: Beware Rifa Class X Capacitors!
I don't now believe the Rifa cap failures are age related as I did before, I installed a new input card in a Mitsubishi modular PLC at work on Christmas eve, the cap that I could see through the vent slots into the plastic case deconstructed itself immediately with a loud bang. Once again I'm glad that I wear glasses.
Andy. |
26th Dec 2016, 6:13 pm | #155 |
Dekatron
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Re: Beware Rifa Class X Capacitors!
That's pretty awful. Perhaps this is a plague coming like the electrolytic plague.
Definitely agree with safety glasses. |
26th Dec 2016, 8:33 pm | #156 |
Nonode
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Re: Beware Rifa Class X Capacitors!
Hi.
Further to post #151, I wondered whether an alternative to the start of failure could just simply be that the resin has flaws in it during manufacture and once the cap is then open to air then the paper dielectric swells creating more cracks. Once the cap has acquired enough moisture and it is then connected to the mains then the rest is obvious. It would be interesting to do some tests on the structure of these Rifa caps. One basic check could involve a microscope to examine the condition of the lead out wires looking for any corrosion that may be present. I also thought about doing an accelerated test involving high humidity which may reveal the formation of cracks in the resin case. Rifa themselves probably know the actual failure mechanism. I can't speak for their modern replacements which have a different shape resin case. Perhaps they've finally "cracked" the problem! Regards Symon. |
27th Dec 2016, 7:15 pm | #157 |
Octode
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Re: Beware Rifa Class X Capacitors!
When working in the automotive electronics business in the '70s, I was given the task of evaluating Rifa capacitors.
One of my standard tests was repeated heat cycling over the permitted temperature range whilst applying the maximum allowable pulling force on the leads. A large number of these capacitors failed this test before I would have moved on to my "dishwasher" test and other electrical fatigue tests designed to test the integrity of the metallising terminations. I consequently disallowed their use in my employer's equipment deeming them in my report as being "inappropriate for our applications". The failure mechanism is due to a mismatch in the coefficients of thermal expansion between the covering epoxy moulded casing and the foil/dielectric element. The epoxy is rock hard, giving rise to locked-in stress reversals as the unit is cycled, fracturing the encapsulent and allowing the ingress of moisture. Catastrophic failure follows. Devices from another manufacturer in Holland using a compliant dipped covering passed my tests. I earned my salary that month. Leon. |
27th Dec 2016, 8:41 pm | #158 |
Nonode
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Re: Beware Rifa Class X Capacitors!
Leon, That's a very interesting account of your experience in testing those Rifa capacitors. So it seems the problems were known about around forty years ago.
One thing that puzzles me is the formation of cracks in unused components. I speculated on possible corrosion on the leads causing failure of the seal. It seems then it is mainly due to the composition of the resin having an incompatible coefficient of expansion compared to that of the dielectric. So if the capacitors were stored in variable temperature extremes then this could accelerate their failure. If anyone has some of these NOS capacitors stored in a sealed bag in a low humidity and fairly constant temperature environment then it would be interesting to see if they have developed cracks. My limited experience on these caps seems to suggest that poorly stored examples ie in damp conditions, are far more likely to crack which is what would be expected. EDIT: Regarding the rock hard resin casing, it always seemed strange that such a composition was acceptable. It reminds me of a brittle toffee. No need to get the toffee hammer out as they're already cracked! I can't think of any other manufacturer's X2 safety caps failing in this manner. Regards Symon. Last edited by Philips210; 27th Dec 2016 at 8:47 pm. Reason: Additional text. |
27th Dec 2016, 10:02 pm | #159 |
Dekatron
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Re: Beware Rifa Class X Capacitors!
When I was at Plessey in the 1970's we used to use some special resilient Araldite for encapsulation. As well as being resilient when cured, it had a well-defined dielectric constant, vitally important when encapsulating assemblies that incorporated tuned circuits that used printed circuit inductors.
I still have an encapsulated "Home Office" item I made at the time using some of the left-overs that is in excellent condition. |
27th Dec 2016, 10:03 pm | #160 |
Dekatron
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Re: Beware Rifa Class X Capacitors!
I wonder if it's because of the cap being an oval shape inside the case. Don't know if they wind them that way or make them round and squash them to the oval shape. My experience with oval shaped caps (remember those brown Wima ones) has not been good and many failed. I recall when the Wima ones were new they were sold on the basis that the shape allowed close fitting on tag strips/boards before PCB's came along and then they were fitted into squared off cases to fit onto a PCB.
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