Fe Fi Fo Fum, I smell Tan-tal-um - Page 1
Forum section: electronics / projects

author: TerraHertz
author_rank: Super Contributor
timestamp: February 26, 2014, 08:16:53 am
content: There was a shelf up high, and on this shelf there lie... several pieces of test gear 'in-repair' but long abandoned due to... well, forgetting about them mostly.One was the Fluke 332A that may have a new lease of life due to this thread:https://www.eevblog.com/forum/testgear/fluke-332baf-in-the-slaughterhouse/So I thought I'd have a quick look at the others, to see if there's any chance of getting some working now.Next on that shelf is a Racal-Dana 9084 Synthesized Signal Generator. I can't recall what its problem was, but not having any manual at all would have been a factor.Taking the covers off, it looks complete. It's been years since it was powered up, so it goes on the variac and gets a slow mains voltage ramp up, while watching the mains current. So far so good.Turn it off and on at full mains voltage, so it resets correctly. The 7-segment display blinks through some sensible looking init patterns then has just settled down into a steady state when it rapidly fades to blank, together withthatsmell we all know well. Tantalum. Again.Power off. Can't see smoke, but sniffing around the stink is definitely from the rear power supply area. Thankfully. But to get to it, there's some disassembly required.Oh. What I'd expected to be a switch mode supply, is actually a linear. Not even a circuit board, just transformer, bridges, filter caps and five TO3 regulator ICs. Each of which has a tantalum bypass cap. OK, so one of them must be burned.But no, they are all intact looking. Huh?It took an embarrassingly long time to notice there was one more tantalum on that rear assembly.There's a 5MHz oven-stabilized frequency reference on the rear panel, next to the power supply. With one tantalum across its power input pins.Thatone.Now that I look, there are lots of tantalum caps on many of the boards in this system. After I finish cleaning it (while it's apart) and put it back together with a new cap on the Freq. Ref module, if just one more of these blasted tantalums goes, I'll replace ALL of them.Remember when tantalums were first developed, how great it was to get useful filter capacitance values in small components? Now I wish the bloody things had never been invented. They are sooooo unreliable in the long term.

author: peter-h
author_rank: Super Contributor
timestamp: February 26, 2014, 09:02:36 am
content: That generator looks hand-made - built in a garageLook at that rotary encoder.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 26, 2014, 09:14:50 am
content: Tantalums mainly die from power spikes. If they survive one turn-on event, and so on, they aren't going anywhere. They don't dry out or leak over time like electrolytics. As far as I know, they're pretty much eternal in current-limited circuits?Tim

author: TerraHertz
author_rank: Super Contributor
timestamp: February 26, 2014, 09:58:57 am
content: Quote from: peter-h on February 26, 2014, 09:02:36 amMade in 1980, from test stickers. It's fairly typical construction for that time.QuoteYes! That's why I included the pic. An optical spinner wheel, in 1980. Cool. They even added a big brass flywheel and little felt friction brake, to get just the right feel to it. It's really very nice.Btw it's not a position encoder, just direction and speed.Quote from: T3sl4co1l on February 26, 2014, 09:14:50 amNo, they seem to have a 'go short circuit for no reason at all, then burn' failure mode. Some sort of crystal growth through the insulating barrier maybe? Google 'tantalum capacitors failure mode'

author: Unknown
author_rank: Unknown
timestamp: February 26, 2014, 10:59:29 am
content: Quote from: TerraHertz on February 26, 2014, 09:58:57 amNot ex-Telecom, is it? I purloined a couple of encoders that looked exactly like that when they shut the Workshops. I'd only ever seen them in custom made / modified gear, and always thought they were made in-house.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 26, 2014, 02:09:21 pm
content: Tantalums are extremely reliable, probably more so than small aluminum electrolytics over the long term. Problem is, there's a short list of things not to do in application that weren't appreciated when they first came out, and probably for decades after. They don't tolerate any reverse voltage at all. They can only be used in current limited locations. Spikes and dV/DT issues will kill them. They'll die if they go resonant with trace inductance and violate the above. Look at the schematics and see if any of the above is violated and change them to electrolytics if so. Also, it's been found that significant voltage derating is needed for good reliability. I can't remember how much, but 50% is probably a good place to be. All that sounds like a PITA, but I've actually replaced a lot of small electrolytics with tantlum in some test equipment, because I never ever wanted to rebuild it again.

author: jimmc
author_rank: Frequent Contributor
timestamp: February 26, 2014, 04:08:18 pm
content: I couldn't agree more Conrad, I thought I was in a minority of one, I prefer tantalum capacitors!I have used them professionally for over 40 years with no significant failures by obeying a simple rule of thumb...The series resistance should be at least three ohms per volt and/or the rated voltage of the capacitor should be at least three times the applied voltage.Doing both can improve the reliability by a factor of greater than a thousand. (see graph 2 ofhttp://www.kyocera.co.jp/prdct/electro/pdf/technical/rmtant.pdf] http://www.kyocera.co.jp/prdct/electro/pdf/technical/rmtant.pdf, confusingly a,b & c are mislabelled).I would not replace tantalum capacitors unless obviously faulty or misused, they do not have an ageing problem like aluminium electrolytics.(By misused I mean something like a 6.3v capacitor across a 5v power rail which may well fail S/C with much smoke)Jim

author: uwezi
author_rank: Unknown
timestamp: February 26, 2014, 04:21:13 pm
content: I have also the experience that tantalums are - if treated correctly - are more reliable than aluminum electrolytics. However, the big difference is their preferred failure mode: short - while aluminum capacitors just continuously loose capacity, tantalums go with a bang (and stink).Aluminum electrolytics do that only if connected in reverse.

author: uwezi
author_rank: Unknown
timestamp: February 26, 2014, 04:24:21 pm
content: Quote from: TerraHertz on February 26, 2014, 09:58:57 amWhat I find most interesting about the encoder wheel is that the quadrature signal is achieved by the offset between the inner and the outer wheel, rather then placing the optical detector at an offset...

author: free_electron
author_rank: Super Contributor
timestamp: February 26, 2014, 05:20:47 pm
content: they should be called Craptalium ...i hate the damn things.I;ve had t repair many an instrument where a tantalium either went poof or was just simply shorted out.Even if you perfectly treat a tantalium they can be damaged during reflow or wave soldering and become a timebomb ready to go off when you least expect it. a small microfracture in the caseing combined witha tiny bit of moisture and poof. the damn thing spontaneously combusts.there are tantalum caps now that have built in fuses and special electrode constructions to prevent microfractures.Those didn't exist 10 years ago.... and many an instrument that is 10 years old gets tantalium-pest problems.Then there is the problem with the availability of tantalum. Australia was the largest producer until they closed their mining facility. not economical... now it comes from conflict area's.Prices fluctuate wildly on those caps. When we rolled out ADSL en-masse in 2000 the demand for tantalum caps spiked. cap prices quadrupled in a matter of weeks !fortunately this was the time ceramics became available in high volume and value and at a lower price and we designed out tantalum.today , apart from very special applications that require wet tantalum caps , there is no compelling reason to use these anymore. ceramics are cheaper and better. Dry tantalum caps (like the orange and black surface mounted parts and orange / yellow / red/ blue 'drop' thru holes should not be used anymore.Wet tantalum caps are silver cannister type axial mounted parts and very expensive. These will NOT explode.

author: SeanB
author_rank: Super Contributor
timestamp: February 26, 2014, 06:02:18 pm
content: Wet slugs do explode, but it takes some work to get them to do so. Reverse voltage will do it, but running them at insane current will not. I used one as a temp replacement in a PC power supply as i had a 1000uF 10v blow, and I wanted it running and going to the shop to buy one was a week away. So in went a quartet of 10uF 25V ones and I turned on the power. Never changed that and it ran until I got a whole new computer about 5 years later. They ran a lot cooler than the ones next to them as well, and ripple on the rail was about the same as the other rails from the supply. OK, it was a 286 running at 16MHz, but still was pretty high current for the day.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 27, 2014, 03:31:10 am
content: Quote from: Tac Eht Xilef on February 26, 2014, 10:59:29 amThe Racal was an old "Brit-beast",so maybe Telecom bought theirs from the same supplier in the UK.Optical spinners weren't uncommon in the 1980s.They were used in some Ham Radios,& in TV studio equipment.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 27, 2014, 04:27:48 am
content: Quote from: TerraHertzEven HP got sucked in!They,for some unknown reason,used them on the RF input of an AM Modulation Monitor.The Tants didn't like high level RF at all!The replacement caps were OK,so all was ultimately well.The then State Broadcasting Branch of Telecom Aust got caught out,too.The ABC would turn the unattended TV sites off & on,by removing or restoring Field syncs.The old units,which used a sync separator like in a TV,were a bit large,so we built some replacements using 567 tone decoders.The 15kHz line rate one was easy,but the 50Hz one needed a fairly big capacitor.Enter the Tantalum!!---lots of capacitance,nice small size.All went well,until we started to get "fail to start" reports from one remote site.We visited,tweaked the device,& waited---after about an hour,it reported "No field syncs"!By applying heat & "canned cold",we could make it drop out,or come back.Eventually,we "shoe-horned" a big Polyester "Greencap " in its place-----No more "fail to start".Really ruined that nice neat device,though!

author: TerraHertz
author_rank: Super Contributor
timestamp: February 27, 2014, 10:43:35 am
content: Conrad Hoffman and jimmc, those are interesting points. So you're saying if we use tantalum capacitors in situations where there's hardly any need for a capacitor at all, they work fine?The problem is maybe 90% of all tantalum caps ever used in old gear were used for supply rail decoupling. Where the "series resistance should be at least three ohms per volt" rule is ALWAYS broken, by definition. And the caps were typically rated not that far above the supply rail voltage. Because for some reason designers just assumed that if a tantalum had "10V" printed on it, it would be reliable on a 5V rail. Or 16V on a 12V rail, etc.As for ever using them in any new designs (Ha, if I ever did any again), no thanks! Once bitten...Actually, I just remembered one type of capacitor I hate more than tantalums. A particular style of rectangular metalized polyester cap, commonly used in AC mains input filters in old equipment of all kinds, including HP and Tek. The ones where the encapsulating plastic was originally clear, but yellows and crazes with age. Then at some point (especially if left unpowered for a few years) when you apply power there's a breakdown internally in the cap. But rather than just fail quietly it forms a superating eruption of foulness. The arcing area works through the cap body, typically taking a long time to actually blow a fuse. Meanwhile the flame converts the polyester to a vapor that condenses elsewhere as a vile brown stinky slime, which is almost impossible to clean off complicated surfaces like PCBs. As icing on the cake, sometimes when these caps fail and the fuse doesn't blow quickly, the noise of the arc sputtering around in the black pit forming in the cap is the closest thing to a fart sound of any electronics failure mode I can think of.It can take days to get that stench out of a room.Quote from: uwezi on February 26, 2014, 04:24:21 pmAmusing isn't it? It makes me think it may have been one of the very first optical spinners. Somebody had a great idea, but hadn't yet had a chance to sit back and look at the implementation and go 'waaaait a moment, that can be simpler.'The HP 8350A came out in 1981, and that had optical spinner wheels with one ring of optical bars and two sensors at the same radius but spaced to get the quadrature signal. Yet they still used a filament bulb for illumination (damn them!)

author: andtfoot
author_rank: Unknown
timestamp: February 27, 2014, 10:49:35 am
content: This thread makes me want to go and buy a tantalum capacitor to blow up, just so I can discover this smell that is raved about...

author: TerraHertz
author_rank: Super Contributor
timestamp: February 27, 2014, 11:12:40 am
content: Quote from: andtfoot on February 27, 2014, 10:49:35 amIt won't be a genuine _old_ tantalum capacitor smell. You should buy some really expensive piece of gear from the 1970s to 80s, and overvoltage some of its power rails to fully experience it.

author: GreyWoolfe
author_rank: Unknown
timestamp: February 27, 2014, 04:14:22 pm
content: Quote from: andtfoot on February 27, 2014, 10:49:35 amI have to ask, are you single? I'm not and SWMBO doesn't like when things blow up here. Last was an old tube scope that had a minor fire. The major smell took a day and a half to dissipate from the house. SWMBO wasn't amused with me

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 27, 2014, 10:42:14 pm
content: Nobody has mentioned the other thermal indicator. The yellow dipped tants will change color ever so slightly when they get hot. Look for a slight darkening. When they cool, they go right back to bright yellow. I've saved my fingertips several times by looking carefully.