Exploding Tantalum capacitors

[peteski]

Now that I got your attention...

Modelers often use tantalum capacitors with their DCC decoders.  Some use the voltage ratings of their capacitors which is rather close to the actual voltage which will be present across the caps. Last think you want is for your expensive model locomotive to melt or burn up because of a failed tantalum cap.

I always preach using caps rated quite a bit higher than the maximum circuit voltage.  Why?  Because Tantalum caps (unlike the more popular aluminum electrolytic caps) are very sensitive to over-voltage.  They also have a very violent failure mode - they literally burn up!    I have worked for many years as an electronic technician and I have seen my share of exploding tantalum caps (one of which came flying off the circuit board like a missile, narrowly missing hitting me in the eye.

The problem is that the failure voltage can often be less than their maximum rated voltage.  This depends on several factors.  I found a very informative tutorial explaining all of this.  There is a lot of technical stuff in it (like operating caps at elevated temperatures) which doesn't usually apply in the way we use them in models, but if your are willing to look at the tutorial it will explain how even soldering the cap can lower its failure voltage.

Here is the tutorial - check it out.
http://dkc1.digikey.com/us/en/TOD/Kemet/tantalumcapacitors/tantalumcapacitor.html

[Chinapig]

I've had one go.  It was a 220uf rated at 16v.  Blew a hole through the side of my Walthers 0-8-0 yard goat - I could have cried.

This site is very helpful and describe using a 16v Z-Diode to provide protection against over voltage.  Use Google translate and see about halfway down the page.  I know very little about electronics but Wikipedia says, "A Zener diode is a diode which allows current to flow in the forward direction in the same manner as an ideal diode, but also permits it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage".

Ted

[Chinapig]

Ooops, forgot the link!

https://www.1001-digital.de/pages/basteln-bauen/elektronisches/anleitungen/pufferschaltung-fuer-16-v-smd-kondensatoren.php

[peteski]

Ooops, forgot the link!

https://www.1001-digital.de/pages/basteln-bauen/elektronisches/anleitungen/pufferschaltung-fuer-16-v-smd-kondensatoren.php

Ted,  you could have edited your original post to add the link. 

To me a 16V Zener diode with a 16V Tantalum cap is not really all that useful since it clamps the voltage way too close to the cap's rated voltage.   I would recommend using even lower voltage Zener diode to be safe.

[reinhardtjh]

Interesting coincidence as JameCo Electronics sent out a newsletter recently talking about popping capacitors. Although these aren't the tantalum caps, the "pops" are fairly violent.  One must take some care.

Popping a Capacitor: What Not to do With Your Caps

For just the video of the fun:

And some capacitor educational material - Circuit Notes: How to Read a Capacitor

[Chinapig]

Very good point about the Zener voltage being too close to the capacitor voltage, Pete.  As the track voltage is nominally 12v DCC, a Z-diode rated at 12v would seem ideal.  I'm looking at one now, that is rated at 12v Zener voltage, 5mA Zener Current and 500mW Power.  Do those parameters look about right?
Ted

[C855B]

Hmm. It's not all that complicated. Ever since I worked at NASA and the engineers in my department steered me straight about electrolytic caps in general - i.e., they go "boom" -  I have designed with tantalums at double the circuit voltage, minimum. No fancy equations or protection circuits required.

I realize that "double the voltage" makes it inconvenient in N scale because size goes up with voltage rating. However, like Pete I've seen my share of popped caps. Not pretty. Actually, when diagnosing a failed circuit the first thing I look for are bulging or split tantalums. Just 'cuz.

[peteski]

Interesting coincidence as JameCo Electronics sent out a newsletter recently talking about popping capacitors. Although these aren't the tantalum caps, the "pops" are fairly violent.  One must take some care.

Yes, if improperly used, aluminum electrolytic caps can pop. But it takes a lot more severe abuse to pop one fo those than a tantalum cap.  Plus, unlike tantalums, these simply vent gas (boiling electrolyte). Tantalums burn up (lots of heat and actual flames)!  That is very bad for locomotive's plastic shell. 

Aluminum electrolytics are actually designed to vent non-destructively, and they did in great numbers on PC motherboards several years ago. My own PC had that happen and I fixed it by replacing bunch of those caps.  Here is more info on that, if you're interested.
http://en.wikipedia.org/wiki/Capacitor_plague

[Doug G.]

Bang! And make absolutely sure you have polarized tantalum caps in circuit correctly + and -.

They are not being liking the wrong way.



Doug

[peteski]

Bang! And make absolutely sure you have polarized tantalum caps in circuit correctly + and -.

They are not being liking the wrong way.

Doug

That is the very basic rule of any polarized electronic component.    Electrolytic caps also don't like being reverse-biased. But instead of a BANG and flame they usually just go PFFFFT!

[bradb]

I always preach using caps rated quite a bit higher than the maximum circuit voltage.

I couldn't get your link to load but I always derate my capacitors to 50% of the listed value - that seems to be a good rule of thumb.

This is a cool link about capacitor type and failure modes: https://www.sparkfun.com/news/1271

Regards,

Brad.

[peteski]

I couldn't get your link to load but I always derate my capacitors to 50% of the listed value - that seems to be a good rule of thumb.

This is a cool link about capacitor type and failure modes: https://www.sparkfun.com/news/1271

Regards,

Brad.

Perfect!

[LKOrailroad]

I can attest to the exploding tantalum capacitor issue. Long ago I built a circuit that was heavily laden with tantalum caps. It was the first time I had ever used them. I read the polarity wrong. Had every one of them installed backwards. When I turned it on for the first time it was like a Chinese New Year show. Quite violent in fact. Never made that mistake again!

[mmagliaro]

Always good advice, Peteski.  Tantalum caps are touchy.  Yes, choose a part with a good cushion on the voltage rating.  I agree.
My own "funny story" (it's only funny because my head and eyes were not in the line of fire) comes from the high voltage
electrolytics used inside vacuum tube guitar amplifiers (a.k.a. "the only guitar amps worth having")
Those things usually have high voltage power supplies running at 400 - 600 volts to supply the vacuum tubes.

In my case, the power supply had 4 or 5 electrolytics wired in parallel.    A high voltage electrolytic that fails can really be dangerous.
The explosions shown in that Jameco video are nothing compared to what happens at 400 volts.

Something that plagues electrolytics is age.  Even if they are never reversed or run above their rated voltage, they can fail
spectacularly when they get old.   You can be playing away, and then suddenly hear a loud "BANG" come from inside the guitar cabinet,
and then lots of hum. --- for example 
When I took it apart, I was mighty glad those caps were housed inside a metal sub cabinet inside the amp, which contained the
damage.  They blew up like sticks of dynamite (or, at least he way I imagine a stick of dynamite would look if it blew up inside
my amp)

Needless to say, I then went through the entire amp and replaced every electrolytic in it.
It was, after all, a 30-year-old unit by that time.

[peteski]

While browsing the Interwebs I ran into some info which I think is a fit for this thread. I'm quoting the info from http://jbwid.com/art82h.htm


There are more energy-dense capacitors available, like SMD Tantalum capacitors... but they're considerably more expensive than aluminum electrolytic capacitors. I've seen prices in excess of $20 apiece and wondered what special mojo they contained. I suspect that they're manufactured to very tight tolerances for very demanding applications. I wouldn't put model railroading in that category.

Fortunately, the factories in China produce much cheaper versions that probably have inferior specifications, but are affordable and can be bought on eBay in quantities of less than 5000. (The low price does make me wonder what you're giving up, according to the law of "no free lunches".) Steamlined Backshop also has them for a higher price, but with a much swifter delivery in the USA.

    CAUTION: Tantalum capacitors can burst into flames and pop! That's not just hearsay, I witnessed it in my second attempt to make a Keep-Alive circuit: 10 bargain-priced eBay Tantalum capacitors from China in parallel, correct polarity and wiring double-checked. Tested on a programming track. Within a second or so of turning on power, one of the 10 blew up: A sharp pop sound, with a long flame shooting out of the side; caught on fire. Although startled, I put out the flames quickly.



Collateral Damage: Charred, melted plastic compartment walls and floor board in an area about 1.5 cm in diameter. The debris on the right side is melted into the plastic, on the other side of the two adjacent Tantalum capacitors that didn't explode. These things get seriously hot when they fail! Fortunately, I didn't have the body shell on. The decoder was on the other side of the melted plastic wall and worked afterwards. The body shell fit back on, and the charred interior area isn't visible behind the frosted windows. Just lucky, I suppose.



 Causes: I can only speculate. Defective/marginal capacitor? Too much heat during soldering? 16-volt rated capacitor, without a sufficient safety margin for DCC? Tantalum capacitors have very low tolerance for voltages exceeding their rating, which quickly starts them down the path to thermal runaway.

Suggestions: If possible, use aluminum electrolytic capacitors instead; use capacitors with a generous voltage safety margin; measure individual capacitors with VOM for correct polarity markings and proper capacitor behavior; solder quickly without dwelling too long; stress test assembled circuit outside of train frame with adjustable DC power supply before installation; cross your fingers! Or buy a commercially-produced Keep Alive circuit and trust that it's safe.

Maybe there's a good reason why the Tantalum capacitors that Digikey chooses to sell are so expensive?

This example makes me think that testing the caps, by applying a DC voltage (few volts lower than the rated voltage) to them for a minute before using them in the circuit will weed out any weak or defective ones. If someone does not have a variable DC power supply I think that even a 9V battery should do the trick.