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X-capacitor failure modes?

Folks,

This afternoon I took the old HP-4191A apart. Yesterday in the middle of some measurements it went pop-pop-pop, loud, like firecrackers. I ran over and turned it off, upon which the popping stopped. An immense amount of light-gray smoke wafted out and it had an odd stench to it. The stench lingered for hours.

I can't find any source and upon powering it up the analyzer worked fine. That was before I changed anything. As if it had repaired itself. The only thing I could see is a crack in the plastic of the X-capacitor. Snipped the thing out, measured, has almost 0.28uF capacitance (27% more than stated) and no leakage current. There is another X-cap and two Y-caps inside a canned IEC receptacle but those can't be the culprits because that is on the line side of the power switch.

Is it possible that an X-cap self-heals to the point where you can barely see a thing, yet release a serious plume of smoke? It is a Rifa GPF-series film cap 0.22uF/250VAC with all kinds of agency logos on there.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg
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I've once had a fluorescent lamp which produced popping sounds for some time and also smelled a bit of burned plastics. Was hard to localize where it came from hence it took some time to find the culprit.

It turned out to be an X-capacitor, which also had a crack in the housing. Had to look carefully, because besides the rather unimpressive crack, it looked just fine.

No idea. I didn't made any measurements on it, just replaced it. Thus I don't know whether it self-healed or not.

Regards, Klaus

Reply to
Klaus Bahner

It's possible.. the old Philips 630VDC (I think the AC rating was

250VAC on those) Polyester caps from that era were explicitly claimed to be self-healing. I don't think I ever accidentally tested it, but I would have expected less drama (but if it formed an arc track internally, then an arc might have become self-sustaining). I would have expected more leakage, but maybe it burned back.

Did it smell more like burning polyester than burning transformer?

If I had it, I would stick it on my "classic" Hi-pot tester and see where it breaks over.

I guess you could dissect it and see what the insides look like,

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward" 
speff@interlog.com             Info for manufacturers: http://www.trexon.com 
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

Judging by the smell I was almost sure it was the transformer. But it's encapsulated and the only place smoke could come out is a small hole where all its wires tunnel out. But there was not smoke residue anywhere.

The X-cap sits right in the open, so that is one of the very few parts that could spew stuff without leaving traces.

Maybe I'll do that, I kept it. But first my wife wants me to barbecue chicken wings, bratwurst and peppers. And when I pulled that analyzer out of the rack she remarked "Don't put that back in before cleaning back there!". It's running for 2h now, sans problems, with a new X-cap. While I cleaned the lab bench, rack and all that.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

Ah, ok, so it could indeed be the X-cap then. I was impressed how much smoke it let off. The whole room (size of a bedroom) was full of smoke.

[...] 
--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

They are Self-Healing to the point that they don't pose a safety risk.

Cheers

Reply to
Martin Riddle

They're supposed to self heal

Smoke has low density, you can fit a lot of smoke precursor into a small space.

if it's got a crack it should definately be replaced, why not do some destructive testing and see if it can withstand the test voltage required and if it makes the right sort of wrong smell.

--
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Reply to
Jasen Betts

Doesn't the UL standard prohibit "excessive" amounts of smoke? But those Rifa parts are pretty ancient.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward" 
speff@interlog.com             Info for manufacturers: http://www.trexon.com 
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

Some smoke is ok, It is the flame and sustained smoking that throw red flags for UL. X caps fail open, so they should not smoke for long.

If Joerg left the power on long enough, he probably would not have seen flames or more smoke.

I would have had a 'Oh sh... moment and turned the power off too" ;D

Cheers

Reply to
Martin Riddle

Presumably his immediate objective was to interrupt the proximate cause of the popping and smoking rather than to fully test those ancient capacitors to UL standards. ;-)

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward" 
speff@interlog.com             Info for manufacturers: http://www.trexon.com 
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

"Joerg"

** Yep.

** I have been in the room twice when one of them blew itself up - RIFA brand both times.

The first one was years ago inside a fan heater and the second in a portable TV late last year.

Filled the room with yucky smelling smoke and set off the alarm.

... Phil

Reply to
Phil Allison

Never heard of Rifa GPF.

In fact, if it's Rifa, and manufactured since 1980, it's part type will begin with the letter P, even after their amalgamation with Evox.

Metalized paper PM (or PZ), metalized polyester PH.

Perhaps you've mistaken the brand?

The 4191A was first released in 1980.

Interesting thing about the older Rifa parts, was that their outer layer was transparent - an overstress or corona can often be seen, as well as heard, but usually only in locations that are subject to the attentions of overly zealous test technicians with manually adjustible hipot equipment.

RL

Reply to
legg

Yes, definitely. But, it is a bit surprising that it still has above rated capacitance. The crack in the plastic is pretty close to a "smoking gun". I have seen this before, at least once.

Jon

Reply to
Jon Elson

Yep. The smoking was quite intense, had to open the windows and leave the lab area to avoid coughing. Plus the odor was really nasty.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

Maybe it's PME. The label says this:

Rifa GPF

40/085/56 PME 271 M 622

Then also "CM1" rotated 90 degrees on the right. No Evox mention on there.

No, says Rifa on there and has the logo.

The service manual is from 1980 but I believe the machine itself was built around 1985. It's a good system. Ok, had its quirks. A big electrolytic in it gave up once. Then they had screwed up the backup battery charger but since I fixed that it doesn't lose calibration data anymore. The plastic component clamp levers for the 16092A probe head are wimpy and, predictably, one snapped in half. Still usable but one has to have strong fingernails. Other than that it has served me well.

This cap developed a crack across one of the flat sides. Considering the loud pops and the amount of smoke I'd have expected a total meltdown but other than the crack I can't see any damage.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

It is rated 250VAC and I gave it 300VAC from my "Australia test setup". No popping, no smell. But who knows, the suspect area might have already vaporized itself.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

formatting link

Some numbers on time to incapacitation and time to death in there.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward" 
speff@interlog.com             Info for manufacturers: http://www.trexon.com 
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

The two Labradors were smart and immediately left the area. They must have read it :-)

Such burning stuff can be dangerous, it has killed many travelers at Duesseldorf airport, via smoke inhalation.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

I had a very nice hand-wired Krohn-Hite filter croak itself a year or so back--the clamp on one of the 'computer-grade' filter caps was loose, and the cap worked its way down through the chassis till it shorted to the bottom of the case. (No fish paper there, for some silly reason.)

This resulted in clouds of transformer smoke filling the lab, which was a shame, because it was a really nice box and the transformer was an irreplaceable custom job. It even had completely separate power supplies for the two channels, to prevent crosstalk.

Good thing I'm nuts about turning all the lab power off when I'm not there--it could have happened any time at all.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 USA 
+1 845 480 2058 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

Still, watch those X-caps in them boat anchors where you now have so many of. I have seen these golden-clear plastic caps in many of them. My lab has pretty good line spike filtering and the cap ran at less than

50% of its rated voltage yet it decided to blow. Problem is, things such as this impedance analyzer can be unattended for long periods of time when you run an experiment. Same with a spectrum analyzer when it runs a baseline scan over a 100MHz swath with 1Hz BW. You don't want to come back from lunch only to see trucks with flashing blue lights surrounding your building because a fire alarm went off. 
--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

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