capacitor failure mode etc ?

2x 250V , 2.2uF yellow epoxy sealed polyester? caps used as intermediary high DC storage in an electric fence unit. Failed as one of these caps is fairly consistently ohmic at 370 ohm. Now repaired 195V dc over them ,peak. Used in the wettest part of the UK , Cumbria, so assumed due to condensation getting inside the cap, no direct rain ingress traces seen. But removing the cap casing and pulling back the leads , no trace of green/corrossion. Heating , with low heat of a hot air gun , ohms drop to 350 and then back up again on cooling. Failure due to damp or HV punch through? . This is PbF solder but is RoHS a consideration inside such caps , ie dendrite formation? If punch through then does the oscillator need knocking back a bit, 2 presets on the board. Replaced both of these caps and another one used in the pulse shaping drive to the "pulse Tx" , the usual backwards mains Tx. Thinking perhaps if unit is in direct sunlight perhaps the pump circuit gives out 300V say. Schematic not seen but HV zeners in that area 270V + 16V plus another , voltage not seen but probably 16V. I was inclined to drop the value of the 270V one but I assume these are fudge values, "selected on test" for nA leakage at less than combined knee voltage of about 300V and has been working for 5 years , then presumably ok to leave as is , as these zeners DVM diode test at least as normal.
Reply to
N_Cook
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high DC storage in an electric fence unit. Failed as one of these caps is f airly consistently ohmic at 370 ohm. Now repaired 195V dc over them ,peak. Used in the wettest part of the UK , Cumbria, so assumed due to condensatio n getting inside the cap, no direct rain ingress traces seen. But removing the cap casing and pulling back the leads , no trace of green/corrossion. H eating , with low heat of a hot air gun , ohms drop to 350 and then back up again on cooling. Failure due to damp or HV punch through? . This is PbF s older but is RoHS a consideration inside such caps , ie dendrite formation? If punch through then does the oscillator need knocking back a bit, 2 pres ets on the board. Replaced both of these caps and another one used in the p ulse shaping drive to the "pulse Tx" , the usual backwards mains Tx. Thinki ng perhaps if unit is in direct sunlight perhaps the pump circuit gives out 300V say. Schematic not seen but HV zeners in that area 270V + 16V plus an other , voltage not seen but probably 16V. I was inclined to drop the value of the 270V one but I assume these are fudge values, "selected on test" fo r nA leakage at less than combined knee voltage of about 300V and has been working for 5 years , then presumably ok to leave as is , as these zeners D VM diode test at least as normal.

As you say, there are several factors involved here. Indeed it could be moi sture ingress or it could be punch through. The punch through could be from a spike or just a weak point in the dielectric that failed under long term stress somewhat below rating. Since this is in a fence charger I would als o consider lightning. I have repaired a couple of fence chargers that suffe red from induced transients from nearby lightning, and in both cases the ca ps had failed to high leakage. One unit was an electromechanical type where the cap across the points had gone leaky, the other was similar to what yo u have and the storage cap was blown.

Neil S.

Reply to
nesesu

high DC storage in an electric fence unit. Failed as one of these caps is fairly consistently ohmic at 370 ohm. Now repaired 195V dc over them ,peak. Used in the wettest part of the UK , Cumbria, so assumed due to condensation getting inside the cap, no direct rain ingress traces seen. But removing the cap casing and pulling back the leads , no trace of green/corrossion. Heating , with low heat of a hot air gun , ohms drop to 350 and then back up again on cooling. Failure due to damp or HV punch through? . This is PbF solder but is RoHS a consideration inside such caps , ie dendrite formation? If punch through then does the oscillator need knocking back a bit, 2 presets on the board. Replaced both of these caps and another one used in the pulse shaping drive to the "pulse Tx" , the usual backwards mains Tx. Thinking perhaps if unit is in direct sunlight perhaps the pump circuit gives out 300V say. Schematic not seen but HV zeners in that area 270V + 16V plus another , voltage not seen but probably 16V. I was inclined to drop the value of the 270V one but I assume these are fudge values, "selected on test" for nA leakage at less than combined knee voltage of about 300V and has been working for 5 years , then presumably ok to leave as is , as these zeners DVM diode test at least as normal.

As you say, there are several factors involved here. Indeed it could be moisture ingress or it could be punch through. The punch through could be from a spike or just a weak point in the dielectric that failed under long term stress somewhat below rating. Since this is in a fence charger I would also consider lightning. I have repaired a couple of fence chargers that suffered from induced transients from nearby lightning, and in both cases the caps had failed to high leakage. One unit was an electromechanical type where the cap across the points had gone leaky, the other was similar to what you have and the storage cap was blown.

Neil S.

++++

I've come across a near lightning struck one before and it knocked out a custom PIC/ASIC with no work around. I would gave expected that more likely than high V caps failing from spikes. This 2006 made unit oddly could have be made in 1976 all discrete , assuming a Programmable Unijunction Transitor for the SCR was available in 1976 . I had asked the owner about lightning in area and he reckoned not. I wonder what other test to determine if punch through or metalisation creep/dendrite ? only for professional interest reasons, I'm reasonably content to return, as is , without any mods. As lightning is "act of God" cannot sensibly mitigate against that. One advantage of old circuits - nearly always repeairable

Reply to
N_Cook

Lead free solder should have nothing to do with hermetic sealing really, un less you are talking the tinning of the leads. Even that is pretty far fetc hed I think. The tinning is very thin and the thermal expansion coeffeicien t of the base wire underneath should be the major factor.

If you want my highly respected carefully considered professional opinion - I think the cheap ass thing just went bad.

Reply to
jurb6006

I've started unwinding this and it would seem to be wide scale electrochemical breakdown. Each turn I unwind the ohmage goes up each time I measure it, now 700 ohms or so. The unwound film looks like a piece of 35 mm projection film of white image and no sprocket holes. Milky clear patches, not clear polyester, between "silvering" that is wavy at the edges, periodicity of the half the length per turn, and a repeat strip like 35 mm film strip, twice per turn. The silvering of each large flat face seems to have disappeared leaving it at the tight bends, twice per turn Of course I don't know what it should look like and will have to break into a new, different make one, to check and also one of the other 2 of the same batch as the failed one , both measure near enough 2.2uF. And try taking pics of it all

Reply to
N_Cook

Here is a pic.

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The leftmost is the bad cap, centre an older but unused Philips also 2.2uF

250V, and the third is from the same batch as the bad, not ohmic up to 30Mohm anyway

The 2 red >s are "frames" 10&11 that show the curving and the graph paper behind where all the "silvering" is absent, so film colours consist of silver, grey colour , hazy white and clear . Centre is good "silver" and crystal clear polyester , only colours, with alternate gutters for the edge connection, metal lead masses retained so ragged edges from my unwinding. Right one shows the same effect on the outer 4 frames but beyond that the metal foil is entire, but going home as the other. Yellow case is the bad TC make and blue is 3 sides of the Philips puter case. The philips actual capacitor volume is much less than the TC so epoxy filling , inside the block casing is complete, but both the others you can see the active film in a number of places , after removing the block plastic casing. This epoxy? inner encapsulation was much easier to break away than the Philips. I suspect the Philips used 100% epoxy and the TC had fillered epoxy so maybe making porous. No green corrossion products seen but I assume capillary migration of condensation through the poor epoxy from the open end with the leads or along the leads which were bright shiney in all of them. So this will be a generic fault with Electric Shepherd ESB55 , first becoming apparent in the wettest county of Cumberland in 5 years of use and then other counties later presumably

Reply to
N_Cook

I can't tell anything from the tiny photo, but a lack of metallization sounds like the cap had the elctrodes burn from constant overvoltage or breakdown/overload/ With such a low resistance, it may not be possible to measure the actual capacitance of the bad caps. It will drop as "self healing" caps start to wear out. Something around 5% loss is considered end of life for many applications.

Are you able to burn test the dielectric to see what it really is?

Reply to
Cydrome Leader

2.2uF

paper

the

epoxy

can

plastic

than

fillered

assume

end

them.

and

I'm assuming, although no water trails seen, that it starts with capilliary action bringing dampness between the outer casing and the capacitor proper as in Philips and TC the epoxy fill does not bond to the outer case , it pulls away easily. Then something to do with the 200V, perhaps the curving edges , maximum width at the middle of the flat faces is something to do with charge distribution. As powdered aluminium is grey I'm assuming the grey colour of the depleted metal is finely divided Al and the white is Aluminium oxide. So you reckon millions of point discharges have erroded away the metal ? sounds logical but would not the plastic show signs of roughening, I will have a look under a microscope. The discharges have to be through the plastic, would dampness enhance that, considering the effect starts at the outer layers , not the inner, it would seem I would not know what to smell for / test for, with burning or do you mean melting and then The melting point of HDPE (High Density Polyethelyne) is about 130 ºC The melting point of LDPE (Low Density Polyethelyne) is about 110 ºC The melting point of PET (Polyethylene terphthalate) is about 250-260 ºC The melting point of PP (Polypropylene) is about 160-170 ºC The melting point of PS (Polystyrene) is about 70-115 ºC The melting point of PVC (Polyvinyl Chloride) is about 75-90 ºC

Reply to
N_Cook

2.2uF

paper

the

epoxy

can

plastic

than

fillered

assume

end

them.

and

by burn test , this sort of discrimination?

formatting link

Reply to
N_Cook

Along the way I seem to have misinterpreted the meaning of "self-healing" as the plastic is punctured and then melts to reseal. But seemingly it is the aluminium that "spark erodes" away to something, what material ? and then cannot discharge to that point again and termed healing. Via microscope lots of random tiny holes in the aluminium in the bad sections, no holes seen in the plastic. Next thing is to identify the insulator film as I assume some plastics are porous or whatever the term is under the presence of water/water vapour to reduce the electrostatic breakdown potential.

Reply to
N_Cook

revised pic on that URL the Philips 373 MKT was polyester as was the yellow TC probably, by that flame test. Yellow flame , sustained after flame removal, no drips, slow progress,brittle remains, although HCl not smelled. This film 15 microns, WVTR permeability of 25 micron films in g/m^2/day polyester 4 Polythene 20 Polycarbonate 170, I didn;t realise this was so "porous" From the boating industry epoxy has much better permeability figure than polyester.

Now just to find the quantative deleterious effect of water/vapour on dielectric strength.

Reply to
N_Cook

polyster is cheap, so that's usually what you'll come across first for a DC capacitor.

Tape and fill caps aren't considered hermetically sealed. If you want that you need a metal can, but those cost more money, so moisture ingress isn't really too surprising- if that's the problem.

in the self healing caps, once the dielecric punches though, the super thin aluminum coating burns up like a fuse, isolating that are with the pinhole though the plastic. Everytime this happens, you lose more and more plate area and capacitance drops.

the low resistance reading is strange though. Even damp nylon probably won't conduct at much, and it really lik to absorb water.

Reply to
Cydrome Leader

you can do that or see how the film melts, tears or handles stretching (changes color etc).

A DC filter-ish cap would likely be polyester, but they have other plastics now these days in addition to the plain old polycarbonate and polypropylene. Polystyrene caps are no longer made last I heard.

Reply to
Cydrome Leader

What happens to the "evaporated" aluminium from all those "35mm frames" ? perhaps some of it migrates and settles at the edges as a slightly conductive coating. It was interesting that as I unwound each layer of 2 "frames" the resistance incremented higher each time on a regular basis. The real fault of these TC is the capacitor proper had to be squashed in the outer shell and hence the moisture path in at the centre of those "frames" as the potting was absent at those points , the silvering remains at the tight curving sections of each turn . I'm amazed at how spectacularly cleanly all the Al has erroded from those frames , the remnant silvering is finely punctured though , looking via microscope. Once an island of silvering emerges with a ring of eroded around it , what mechanism ensures the erosion of the islands?

Reply to
N_Cook

Ask the OEM. They understand the technology. You waste a lot of time on useless things, and this is one of them, unless you're going to manufacture a few million plastic caps every year.

The damned cap is closed, so it is redeposited near the puncture. Where else could it go? In most applications for self healing capacitors, it won't matter.

Reply to
Michael A. Terrell

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