Exploding Tantalum capacitors

[nickelplate759]

I suspect that one of the things one gives up in exchange for the remarkably low  price is any expectation that the devices have passed even a cursory QA process, so the advice to QA it yourself is well taken.

[jdcolombo]

I only use caps I buy from Digikey, usually AVX-branded.  Buying unbranded cheapie stuff from China is asking for trouble. 

I've had two of these fail.  One several years ago, which damaged the side of a 2-8-0 tender.  This was almost certainly a defective cap.  The other was about six weeks ago, when I changed out one of my Digitrax boosters, accidentally put it in 0-scale mode, and poof went a cap in one of my PA-1's.  That in turn caused a system short which shut down the track power, so nothing else happened.

As I've said many times, I keep my track voltage at 11.5v via the voltage pot adjustment on the Digitrax Chief, running it in N-scale mode.   Since the caps are fed from the decoder's bridge rectifier, this means they are probably charging at something around 10v.  Other than the one failed cap several years ago that I'm pretty sure was defective from the outset, I've never had a cap explosion at these voltages since starting my sound installs in 2013.

These days, I also wrap my tantalum cap "sandwiches" in kapton tape.  This is what I did on the PA-1 that had the failed cap from my "O-scale" voltage error, and the tape did a good job of containing the cap explosion.  The tape itself melted, but there was no damage outside the melted tape, and even the other two caps in my 3-cap "sandwich" survived.  So in addition to being careful about drive voltage, wrapping the caps in a couple of layers of Kapton tape might be useful.  Using a Zener to limit charge voltage probably would also be a good idea, and wouldn't take much space.

It would be nice if one could use a "safer" cap or a commercial keep-alive, but it's just not possible with N-scale hood diesels.  A 20v 100uf tantalum is about the same size as a 220uf 16v cap, but I've found that about 400uf of capacitance is the bare minimum needed to avoid sound dropouts with relatively clean track and wheels.  So you'd need 4 of the 20v caps, and good luck finding space for that in a typical N-scale hood diesel.  On the other hand, two 220uf 16v caps and a zener diode to limit charge voltage should be doable space-wise.  For my steam locos, I've started using a TCS KA-1 or KA-3 keep alive; with bigger steam (e.g., Berkshire size and above), the tender usually has enough space to wire one of these.

John C.

[Steveruger45]

Hi Guys,
 I have just completed my first esu loksound micro select install in my Atlas SD50 which is now my favorite loco  after following the many great tutorials on this site.  I installed 2 x 220mF x25v tantalum caps for the keep alive. I use an nce power cab. Should I be scared of these caps  blowing up?
Thanks.

[peteski]

Hi Guys,
 I have just completed my first esu loksound micro select install in my Atlas SD50 which is now my favorite loco  after following the many great tutorials on this site.  I installed 2 x 220mF x25v tantalum caps for the keep alive. I use an nce power cab. Should I be scared of these caps  blowing up?
Thanks.

Always be afraid, very afraid!   
But if you got them from a reputable source (and f they were manufactured by a well-know electronic parts manufacturer) then the risk should be minimal.  Using the 25V rating is very smart on your part. But where did you find 220uF tantalum rated for 25V?  Those are usually hard to find and expensive.

If possible run your model without the shell for a few minutes. If one of the caps will have early failure, at least it will not melt the shell.

BTW, mF means milli-Farads. If the "µ" symbol is not used for micro-Farads (because it is not easily found on the keyboard) the lower case "u" is generally the accepted substitute. Like "uF".  For example 220µF is 0.22mF. 220mF would be 220,000µF.  That would give you a nice keep-alive capacitor value which would run the loco for few seconds with no track power.

I'm not trying to bust your cookies - just pointing out proper usage of the units describing capacitance.

[Steveruger45]

Thanks Peteski
Yes, I meant micro farad or uF.  Although I have seen MF used too for the same thing. You are right of course, as I recall from the decades ago electronics course I did at school the ' u with the tail'  is the correct SI symbol, it just isn't on my iPad keyboard. Anyhow, I got them from eBay and yes it was a Hong Kong vendor and the reason I went with 25v to try and build in some factor of safety.  so afraid I am. I am thinking that doing a proof test as mentioned by another on this thread is a great idea and something I plan to do.

[Point353]

But where did you find 220uF tantalum rated for 25V?  Those are usually hard to find and expensive.

Mouser presently has 600 of an AVX 220µF/25V part in stock. Price is about $5 each.
http://www.mouser.com/search/ProductDetail.aspx?R=0virtualkey0virtualkeyTCN4227M025R0100

[peteski]

Mouser presently has 600 of an AVX 220µF/25V part in stock. Price is about $5 each.
http://www.mouser.com/search/ProductDetail.aspx?R=0virtualkey0virtualkeyTCN4227M025R0100

Thanks!

[nstars]

Thanks for the link, I just ordered them and a price of €3.91 is reasonable, especially as I don't have shipping costs.

Marc

[jdcolombo]

Mouser presently has 600 of an AVX 220µF/25V part in stock. Price is about $5 each.
http://www.mouser.com/search/ProductDetail.aspx?R=0virtualkey0virtualkeyTCN4227M025R0100

Uh-oh.  Going to have to get some of these.  Case dimensions are nearly identical to the 16v ones I've been using - height is 1mm more, but I can usually work around that.

Nice find!

John C.

[RBrodzinsky]

Cool! I have been using some of the Kemet 20V 150uF caps in my latest installs. These are half the thickness of those, but half again as wide. But that extra capacitance and higher voltage make these attractive. Might have to come up with some creative packaging.

[Point353]

Mouser presently has 600 of an AVX 220µF/25V part in stock. Price is about $5 each.
http://www.mouser.com/search/ProductDetail.aspx?R=0virtualkey0virtualkeyTCN4227M025R0100

It should be noted that the case style for this part has "undertab" type leads that are not readily accessible with a conventional hand soldering iron tip when the part is mounted on a printed circuit board, as compared to a "j-lead" style case with exposed terminals/tabs on the sides.
AVX recommends using a hot air pencil type soldering iron for hand soldering of this part.

[peteski]

It should be noted that the case style for this part has "undertab" type leads that are not readily accessible with a conventional hand soldering iron tip when the part is mounted on a printed circuit board, as compared to a "j-lead" style case with exposed terminals/tabs on the sides.
AVX recommends using a hot air pencil type soldering iron for hand soldering of this part.

Good point, but that shouldn't be a problem as most modelers solder wires to the solder pads.  I do uses PC boards sometimes but I also do have full assortment of SMD rework tools.

[woodone]

I have been using a 100Uf 20 volt capacitor for a long time. I use 2 or 3 depending on the room I have to work with.
I have had a few blow up? Like I said I use 2 or 3 wired in parallel. The ones that blew up were only 1 of the 2 or 3. So I would guess the spec QC was not up to par. on the ones that blew. These were on units that I did DCC sound installs in N scale on. I test all functions and program most of the CV's before putting the shell back on. The ones that failed were ones that had been run and tested only to fail at a latter time.
Going to get some of those 25 volt caps to try in the future.

[peteski]

The ones that failed were ones that had been run and tested only to fail at a latter time.
Going to get some of those 25 volt caps to try in the future.

Unfortunately fiery failure is always possible with tantalums.    I think that it might even be possible that the heat from soldering the leads/pads (especially multiple times) affects the tantalum slug making it more failure prone.  That is just my thought - no proof either way.

I prefer to use at  least 20V caps (for safety margin) but then John seems to be fine with ones rated foe 16V (with his track voltage at 11.8V).  We really don't know the cause of those failures. Maybe the weakened cap will fail even at much lower voltages.

[woodone]

Well the ones I have been using have come from well known suppliers. But in this day and age you really don't know where the items are manufactured. I would guess that most of this small electronic stuff comes from China regardless who's name is stamped on it. Really hard for me to understand how we ( the USA) can have things made over seas and then shipped here for less than they can be made here. But that is really
 another matter.