Fire!!! (Tantalum capacitor - no trains were harmed in the making of this post)

[craigolio1]

So a while back I acquired some 470 uf 16v caps from eBay. AVX is the brand I believe. 

On the advise of someone here I opted to test some prior to installing them in my first Loksound build.

For the first one I set my power supply to 15v. It’s a giant analogue dinosaur with a knob and a dial readout so I connected my DVM up and turned the knob until it read 15v. The cap immediately caught fire.

For the next one I set it to 14v. That cap lasted about 15 min.

So I’m pretty much decided that these caps are not going in my loco.

But, 20v caps are really expensive. (Not nearly as expensive as my locomotive.). They cost $10 on Digikey.ca. So before I pony up the dough for some of these am I doing something wrong? Should I be running a motor or put a resistor on the PS as well?

Edit: if the voltage is THE issue I could series two of them which would yield a 235uf 32v combination. I have the room for that and it would allow me to use what I have.

Any insight is appreciated. Thanks.

Craig

[peteski]

I believe that the larger the capacitance is, the greater are the odds of it blowing up.  Plus, as you noticed, the prices are through the roof.

Don't use such high value caps - use 150uF ones rated for at least 20V, and just gang them for larger capacitance. They are much cheaper too.  These caps should never be used as close to the rated voltage as you used.  We have discussed this here in the past.  Also, caps from eBay are usually poor quality.

As an alternative, polymer tantalum caps seem to be less prone to blowing up, but I doubt they come in such high capacitances, and they are pricier than standard tantalum ones.  I have not used them, but others here have.

[jdcolombo]

Hi Craig.

I've used 220uf 16v tantalum chip caps in my N-scale diesel installations since I started, and have had only one failure; since I typically wrap the caps in kapton tape, the failure didn't do any damage to any other component or the shell.  But I get my caps from Digikey, not e-bay.  Could you have gotten "seconds" - caps rejected because they are way out of tolerance?

My layout is run via Digitrax boosters set in the "N scale" mode.  This mode puts out about 12v to the track.  A typical keep alive cap is connected to the decoder after the bridge rectifier, which typically drops the voltage another 1.4v.  So my 16v caps end up getting about 10.5v for charging.  Maybe that's why I've never had a problem, though I remember running several engines at one of our NTrak shows where I forgot to set the booster to the "N scale" position, and ran them all on the HO setting.  That setting puts out about 14v to the track, so the caps were probably getting 12.6 as the charging voltage.  Again, no problems.

Today, however, many of us have switched to using tantalum-polymer caps instead of the regular tantalum caps.  These caps are more tolerant of higher voltages and also don't blow up like regular tantalums when they fail.  They are safer.  Also a bit more expensive.  Here is a tantalum polymer 220uf 16v cap from Digikey that is 7.3mm x 6.1mm x 2mm thick, $3.50 each in quantities of 10:

https://www.digikey.com/product-detail/en/avx-corporation/TCN4227M016R0070E/478-13376-1-ND/10063045

If you can find room for two regular tantalum chip caps, you should be able to find room for the cap linked above.  That's what I would use these days, though I've got about 40 diesels with regular 16v chip caps in them running around on my layout.

John C.

EDIT:  As Peteski notes, if you want to use 20v regular tantalum chip caps, get the 150uf ones.  The package size is about the same as the 220uf 16v.  Two of them in parallel gives you 300uf, which should be enough to avoid momentary dropouts in sound. 

[RBrodzinsky]

I've been using these poly-Ta 220uF 25V caps, similar pricing as the 16V, with no concerns for voltage https://www.digikey.com/products/en?keywords=478-12518-1-ND

I still have a few 330uF ones, but they are no longer available

[jdcolombo]

I think Rick and I linked the same cap!

John C.

[RBrodzinsky]

I think Rick and I linked the same cap!

John C.

Same family, different voltage.

[jdcolombo]

Oops.  Missed the voltage difference.

John C.

[Point353]

So a while back I acquired some 470 uf 16v caps from eBay. AVX is the brand I believe. 

On the advise of someone here I opted to test some prior to installing them in my first Loksound build.
For the first one I set my power supply to 15v. It’s a giant analogue dinosaur with a knob and a dial readout so I connected my DVM up and turned the knob until it read 15v. The cap immediately caught fire.
For the next one I set it to 14v. That cap lasted about 15 min.

So I’m pretty much decided that these caps are not going in my loco.

But, 20v caps are really expensive. (Not nearly as expensive as my locomotive.). They cost $10 on Digikey.ca. So before I pony up the dough for some of these am I doing something wrong? Should I be running a motor or put a resistor on the PS as well?

Edit: if the voltage is THE issue I could series two of them which would yield a 285uf 32v combination. I have the room for that and it would allow me to use what I have.
Any insight is appreciated. Thanks.

1) You didn't buy the capacitors from eBay. You bought them from a seller via eBay.
Is that particular seller reliable and were the parts genuine?

2) General recommendation for tantalum capacitors is that the applied voltage be no higher than 80% of the rated voltage.
So, no higher than 12.8V for that 16V capacitor.

3) Putting two capacitors in series is an option.
It's recommend that you put a resistor (of about 100KΩ value) in parallel with each capacitor.

[Steveruger45]

As I recall for tantalum caps you should apply around only 50% of the rated voltage.
And upto 80% for the poly tantalum variety.

[craigolio1]

Awesome info everyone. Thanks for weighing in. I put two of my caps in series and they’ve been sitting on the power supply pretty much since I made this post. So I’m confident that they’ll be fine now for this install.

I’m going to see about getting some of the caps in one of the links.

Craig.

[Steveruger45]

Your two 470uF caps in series will give you now an effective 235uF and each cap will share the total potential difference across the series group. Vtotal = Vcap1 +Vcap2. So, if total potential difference is 12v, each of your 470uF caps will be exposed to 6v

Here is a handy link to do the math for either combining caps in series or parallel
Like just about everything else in this world now there’s an app for that.😀

https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-series-and-parallel-capacitor

[peteski]

To me it makes  no sense for our application to  hook up pairs of capacitors in series.  When using multiple caps  I would use 2 caps with lower capacitance and higher working voltage in parallel.

And I agree that that safe operating voltage is more like 50% (not 80%)  for standard tantalum caps.

[Point353]

As I recall for tantalum caps you should apply around only 50% of the rated voltage.

And I agree that that safe operating voltage is more like 50% (not 80%)  for standard tantalum caps.

https://www.avx.com/docs/techinfo/VoltageDeratingRulesforSolidTantalumandNiobiumCapacitors.pdf

[peteski]

https://www.avx.com/docs/techinfo/VoltageDeratingRulesforSolidTantalumandNiobiumCapacitors.pdf

Excellent info, but it still doesn't make me follow that advice.  I just like being extra cautious. 

[Steveruger45]

Excellent info, but it still doesn't make me follow that advice.  I just like being extra cautious.

Found this one on Kemet site, page 5 talks about failire rates and influencing factors etc etc.
https://sh.kemet.com/Lists/FileStore/Derating%20Guidelings%20for%20Tantalum%202011%20(3).pdf
I will continue to use max applied voltage of 50% Of rated  for tantalum and 80% for poly tantalum caps too. And still do a burn in test on my 18v power drill battery before use in a model.