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Author Topic: When Tantalum caps fail, are they supposed to light up like a match?  (Read 1943 times)
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Danger Boy
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Just wondering...   smiley

I have two tantalum caps rated at 16V across a 7805 which steps a 12V input down to 5V.  As I was working with my project, all of the sudden the current it was drawing went, for no reason that I can ascertain, from about .06A to 5A - the limit of my power supply.  As this happened, one of the caps lit up like a match, almost immediately,  it really charred the other cap next to it and the socket.  I checked for shorts - nothing.  I then desoldered and replaced both caps and now everything is fine.  Is this common for a cap to fail to an internal short and then light up like that?
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SE USA
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it is for tantalum caps

keep in mind the stripe is the + side unlike its aluminum brethren 
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Danger Boy
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it is for tantalum caps

keep in mind the stripe is the + side unlike its aluminum brethren 

I've never wired one backwards before, the brand I have is pretty obvious in polarity, but the proof that I did it right or wrong was long gone a second after it went up.   smiley-grin
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Tantalum caps presented a high failure rate and catastrophic failure mode where I worked. I've seen about 10 burst into flames, fizz, and sputter, making huge amounts of smoke. A Japanese corporate official was inspecting an instrument once when the power supply regulator board had a cap go out. He required the root cause of such a failure be known, and once understanding the issue with tantalums, ordered all products to design out the use of those caps.
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Danger Boy
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Too bad.  They are convenient and I have a bunch from Tayda.  First problem so far...

http://www.taydaelectronics.com/capacitors/tantalum-capacitors/10uf-16v-radial-tantalum-capacitor.html
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A Japanese corporate official was inspecting an instrument once when the power supply regulator board had a cap go out. He required the root cause of such a failure be known, and once understanding the issue with tantalums, ordered all products to design out the use of those caps.

He should have fired people on the spot.

Not putting tantalum caps in power supplies is Electronics Engineer 101.
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Last time I had a failure it was because I accidentally used a 10v one in a 12v circuit.  It went with a very loud bang.  The insides of the enclosure (I'm glad it was in one) is still peppered with bits of tantalum capacitor and scorch marks...
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Yes I have worked in places where using tants were banned.
You can't get UL approval if anything if you use them in the supply.
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Cumming, GA
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I have used them with success but I ALWAYS pay attention to polarity and voltage rating.  They are conveniently small when compared to other electrolytic types and offer long term component stability... but when they go... you can compare it to fireworks.
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Lacey, Washington, USA
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Tantalum caps don't survive high ripple currents, generally. Small, low ESR, very stable, but high ripple currents tend to cause failures.
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A Japanese corporate official was inspecting an instrument once when the power supply regulator board had a cap go out. He required the root cause of such a failure be known, and once understanding the issue with tantalums, ordered all products to design out the use of those caps.

He should have fired people on the spot.

Not putting tantalum caps in power supplies is Electronics Engineer 101.


That's good for even the layperson to know. To be fair to the engineers, I don't believe they were actually on the supply board. I do seem to recall tantalums on the regulator board, post regulation, and perhaps on supply lines on other boards in the system. I still see many tantalums in the designs today though...I think in those cases there may be practical application. I have a few that I would likely use on a regulated supply to a project board. I hope that is not bad design...
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Tantalum was used as filaments in light bulbs before tungsten. The melting point is over 5000 degrees F.
See the following warnings:
http://www.livestrong.com/article/103794-dangers-tantalum/

Powdered tantalum ignites spontaneously on contact with air. Tantalum capacitors use powdered tantalum to make a large surface area for large capacitance. Avoid sparks on tantalum, it burns rapidly.
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Quote
When Tantalum caps fail, are they supposed to light up like a match?
No, No, No!!  They are supposed to explode like a firecracker!!! smiley-grin smiley-grin

Where I work we us ship-loads of 'em.  (We do not build power supplies.)   I've worked here over 10 years and I've NEVER seen one fail in the field.   I've had a few explode during test or burn-in, but as far as I know they were all backwards.  (Sometimes you can't tell after the explosion. smiley-wink )

There have been reports of "bad batches" from certain manufacturers...  Many years ago there were motherboards failing left-and-right.   But, I've never personally experienced that anywhere I've worked.  IIRC, when that happened we banned that particular capacitor-manufacturer, and so did just-about everybody else, and I'm pretty sure they went out of business or quit making tanatlums since nobody would buy their product.


One of my breadboards has a burned-spot from when I installed an op-amp chip backwards...   Same thing...  The top burned-off and it looked exactly like a match when you strike it.   It didn't explode, so it didn't exactly startle me, but I think my eyes kind-of bugged-out because I wasn't expecting THAT to happen!  smiley-grin smiley-grin

P.S.
I just remembered...  I've had a couple of regular electrolytics fail on a home project.    They were probably over-voltaged...  Maybe 1000uF/16V on a "nominally" 15V circuit.   One was on the positive power supply and one on the negative...  Since they both failed at the same time, obviously age had was also a factor.   The thing hadn't been powered-on for a few years, and when I turned it on I didn't see what happened because they were inside the enclosure.  It made an "expensive noise" (as my dad used to say smiley-grin ) and some really nasty smoke came out...  I had to go outside 'till the smoke dissipated.    (It wasn't actually an expensive failure....  I replaced the caps and it was good as new.)
« Last Edit: August 14, 2013, 03:55:39 pm by DVDdoug » Logged

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Sorry, but I have to rescue poor Tantalum, he is a very misunderstood metal.

The Tantalum (which makes up the anode) isn't the issue, it's the cathode.  

Only traditional Tantalum with MnO2 as their cathode ignite on failure.  MnO2 (Manganese dioxide) is also used in Sparklers...

Newer Tantalum-Polymers can't ignite on failure.  Additionally, they are far (far) more reliable than their older MnO2 counterparts, so they are much less likely to fail anyway.

I have two tantalum caps rated at 16V across a 7805 which steps a 12V input down to 5V.
If you ran the boards through reflow, and check the catalog/datasheets carefully, you'll see that you need to de-rate by 50%.  During reflow (and even hand soldering of leaded parts), CTE mismatches between the soft tantalum-pentoxide (dielectric) and the hard MnO2 cause cracks to form in the dielectric layer.  By applying less than their rated voltage, you are less likely to activate one of those weaknesses.  (On the other hand, if you can apply voltage with limited current, you can re-heal any weaknesses.)


Tantalum caps don't survive high ripple currents, generally.
This is actually a myth / misunderstanding about the failure mode.  The failure is activated by voltage.  It is exaggerated by current. Polymer-Ta is VERY good with high ripple currents.  In fact, for some voltage/cap combinations, you will have less ESR than a like-valued ceramic.

The real limiting factor with Ta-MnO2 and high ripple current is the very high ESR of the MnO2.  That causes heating issues, obviously.  

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Fireworks are banned here....


I'm ordering these now smiley
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