# What causes a capacitor to fail



## Bwells (Mar 30, 2014)

This is a 12 volt LED strip light power supply with an input of either 120 or 240 volt switchable. It is used for under-cabinet kitchen lights and has worked pefectly for the last three years. It quit so I did some quick tests and it is dead. I ordered another one for 16 bucks, installed it and the wife is happy.
















Before I threw it in the trash, I thought I would take a peek inside and noticed the 5 caps on the far left are toast and one appears to have tried to launch it's clothes. First thought, replace them. Second thought, why did they blow? They are wired in parallel with the output so assuming a cap blows open I should still have 12v at the output, but I don't. There is another problem upstream like a voltage regulator maybe? I don't know but it is quite the nice soldering work and a lot of strange parts all of which are most likely the cheapest available. The caps that blew are 25v 1000uF.
Let me know what you think!


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## gunrunnerjohn (Nov 10, 2010)

Capacitors have a fairly limited life, especially cheap electrolytic capacitors. It could have simply reached the end of it's useful life. An MTBF of 1,000 hours is not uncommon, and most consumer grade have less than 2,500 hours MTBF.

I see at least three of those caps have bulging tops, so they're all bad.


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## Bwells (Mar 30, 2014)

All five are bad. But I assume these are smoothing caps I would still expect 12 volts at the output. Do caps blow open or shorted?
I'm sure these caps are the cheapest available and maybe see voltage for a couple hours every two weeks if that much.
On the trace side, what does the zigzag white line signify, 120 side verses 240 side?


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## ncrc5315 (Jan 25, 2014)

Could have been Capacitor Plague. Had that problem with my Digitrax command station, and two throttles.


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## Bwells (Mar 30, 2014)

So you sent the Plague to me? Shoot!!!


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## gunrunnerjohn (Nov 10, 2010)

Bwells said:


> All five are bad. But I assume these are smoothing caps I would still expect 12 volts at the output. Do caps blow open or shorted?
> I'm sure these caps are the cheapest available and maybe see voltage for a couple hours every two weeks if that much.
> On the trace side, what does the zigzag white line signify, 120 side verses 240 side?


Since this is a switching supply, the caps may be doing a bit more than simply filtering. Normally, you don't need that much capacitance to filter a high frequency from a switcher. The zig-zag appears to separate the primary side from the secondary side of the circuitry.


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## highvoltage (Apr 6, 2014)

gunrunnerjohn said:


> Since this is a switching supply, the caps may be doing a bit more than simply filtering. Normally, you don't need that much capacitance to filter a high frequency from a switcher. The zig-zag appears to separate the primary side from the secondary side of the circuitry.


Agreed. Also, the 120/240 would likely be fed into and then stepped down in the transformer. After that would be rectification, smoothing, and regulation of the 12 VDC.


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## highvoltage (Apr 6, 2014)

Bwells said:


> So you sent the Plague to me? Shoot!!!


It would have to be an old board. Capacitor plague was from 1999 to 2007:

https://en.wikipedia.org/wiki/Capacitor_plague


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## gunrunnerjohn (Nov 10, 2010)

Many switchers actually power the switching oscillator with the high voltage rectified. That's the efficient way to do the job. If you drop the voltage with a transformer, you'd need a 3A secondary, that would be a lot larger transformer than the ones I see on that P/S. The torroid coil is the transformer for the switching supply. Not sure what all the components are, it appears there may be some low current outputs that are supplied by a linear regulator, but that's just a guess.

Believe it or not, some of the defective capacitors have lived in storeroom stock for years and may still be appearing in products of small companies with low production volumes.


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## Bwells (Mar 30, 2014)

There is sticker on the side that says switching supply but I figured that was to switch between 120 and 240. Once set, it needs to transform that voltage to 12 volt DC. Why is there any switching involved in the circuitry? Also, is the torroid coil the gizmo with the exposed winding's? I figured that was a ferrite magnet or something to reduce noise influence. Once again, over my head!!


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## ncrc5315 (Jan 25, 2014)

Bwells said:


> So you sent the Plague to me? Shoot!!!


That's just the type of guy I am! :cheeky4:


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## Lemonhawk (Sep 24, 2013)

My vote goes to the board using old caps that had the plague. Its a big advertising gimmick with motherboard manufactures about their using high grade caps that don't have the plague. Also that switching power supply may be pushing the limits of the caps voltage. The switching PS does not refer to the 120/240 switch but how the power supply works. Long ago (late 70's?) using a switching power supply seemed to be asking for trouble in that they put noise into your circuit that seemed to be impossible to clean up. Now they are very efficient power supplies and we've figured out how to kill all the induced noise, mainly by up-ing the frequency so a cap can easily filter it out. Still when we design new equipment that requires a new power supply I cringe as I know it will be a problem.


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## Bwells (Mar 30, 2014)

OK, so old caps it is and I have some that are 35 volt 1000uF rubycon but that doesn't explain why I don't have 12 volts at the output. I still think there is something before the caps and output that is kaput. The LED at the far left does not light and there is an adjustment for the output (orange thing) that neither have power.
I finally got sick of looking at it so in the trash it went. I know my dad would not be happy as he would fix it no matter what it cost. He loved this stuff!


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## Bwells (Mar 30, 2014)

ncrc5315 said:


> That's just the type of guy I am! :cheeky4:


No problem, the antidote only cost me 16 bucks. Still friends?:smilie_daumenpos:


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## Lemonhawk (Sep 24, 2013)

Trashing it is the right approach (send it to recycling). The chances of damaging the board trying to remove components is far to great and with out a lot know how and special desoldering gear, the chances approach 100%.


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## Bwells (Mar 30, 2014)

I agree but it would be nice to know what happened and why. Kind of a puzzle that you can't solve. We are a trash it bunch like the Yugo, needs plugs, throw it out!
If you look on the trace side, there is a bunch of backup solder on quite a few traces that I assume is to increase amp draw. Cool stuff here, it's a shame to go to the landfill!


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## Lemonhawk (Sep 24, 2013)

Once the caps go there are probably all sorts of cascade failures. I worked for a small company in the 80's and one of the techs managed to get 110v on the pc bus. The boss wondered why we could not just replace a few components and fix it. Yes, some times its best to toss it. The parts would far exceed the cost of the replacement.


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## Bwells (Mar 30, 2014)

Yep, I'll agree. There is most likely more problems than the caps but I just thought I would ask. No big deal and I won't be losing any sleep over it as I will be on a cruise. 5 days of maximum food and booze will keep me quiet!! Adios and hasta Sabado>


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## gunrunnerjohn (Nov 10, 2010)

I personally watched a $50,000 custom test equipment rack go up in flames for the same reason, a poor routing of the 120VAC power managed to get it on the data bus of the whole system. Little tantalum capacitors were popping like firecrackers, it was quite a show. By the time we got the power off, it was reduced to a smoldering ruin!


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## ncrc5315 (Jan 25, 2014)

Bwells said:


> No problem, the antidote only cost me 16 bucks. Still friends?:smilie_daumenpos:


Absolutely, looking forward to being on the receiving end some day:hah:


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## flyboy2610 (Jan 20, 2010)

gunrunnerjohn said:


> I personally watched a $50,000 custom test equipment rack go up in flames for the same reason, a poor routing of the 120VAC power managed to get it on the data bus of the whole system. Little tantalum capacitors were popping like firecrackers, it was quite a show. By the time we got the power off, it was reduced to a smoldering ruin!


No pics?


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## gunrunnerjohn (Nov 10, 2010)

I didn't get pictures, it was at a client's location, no camera or camera phones allowed.

Don't you know, the best stuff always happens when you don't have a camera handy. Since I've been packing a cell phone with a camera everywhere, I never see anything interesting!


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## Massey (Apr 16, 2011)

I personally would have loved to play with that power supply! But then again I have had this habit of fixing electronics ever since the Navy taught me how to fix the radar of the F-14D. 

Good call though on throwing it out. A power supply can be dangerous if you don’t know how to fix them or if they are repaired incorrectly.


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## Lemonhawk (Sep 24, 2013)

I spent a lot of time at Pt. Magu in the 70's doing software for the inertial system on the F14. I think by the time the D version rolled out the CSDC computer and ANS-92 IMU had all been replaced. The CSDC was catchall computer that did a lot of things so I constantly worried about doing something really bad while trying to shoehorn in the inertial nav equations.


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