I had a tantalum cap blew up on me... - Page 1
Forum section: electronics / projects

author: technix
author_rank: Super Contributor
timestamp: December 01, 2017, 09:13:03 am
content: The cap is supposed to be 47uF/25V type, but it blew up with tons of smoke when I applied 12V to it. They are mounted the right way.What is at fault here?

author: fcb
author_rank: Super Contributor
timestamp: December 01, 2017, 09:19:08 am
content: Wrong way round??

author: Psi
author_rank: Super Contributor
timestamp: December 01, 2017, 09:21:39 am
content: Yep. Wrong way around.In their wisdom they decided to make the package marking positive for tantalums. Unlike other caps

author: technix
author_rank: Super Contributor
timestamp: December 01, 2017, 09:22:41 am
content: I think I have mounted them the correct way.

author: Psi
author_rank: Super Contributor
timestamp: December 01, 2017, 09:27:27 am
content: It will be 1 of 4 things1) wrong way2) overvoltage3) dodgy fake tantalum4) installation damage due to boardflex or other force that cracked the cap internally

author: tautech
author_rank: Super Contributor
timestamp: December 01, 2017, 09:33:34 am
content: Quote from: technix on December 01, 2017, 09:22:41 amSMD ?They've caught me out too.

author: technix
author_rank: Super Contributor
timestamp: December 01, 2017, 05:46:13 pm
content: Quote from: blueskull on December 01, 2017, 09:38:22 amI think I found the culprit here: a design problem that masked another, and the dodgy cap of the week syndrome.The cap blew up on my M193v4 STM32 dev kit board. One of the "bugs" I fixed in M193v4 is I have thickened the traces leading to the VIN jack from 10mil to 30mil. The thinner VIN trace on previous revisions happened to provide some inrush limitation due to its higher resistance.After me interrogating the supplier they did sell me sub-par brand caps since I did not specify a brand when ordering them. I have tested them with my power supply and they seem to be the correct voltage rating (did not blew up at 20V with a current limit of 50mA.) Apparently this brand of tantalum caps cannot handle current surges as well as other brands, as my 12V wall wart is capable of up to 2A current.I am running low on the stock of blank M193v3 PCB anyway, so in the next revision of the board I will replace the 47uF 25V tantalum cap here with an aluminum one (or just omit it?) The 220uF 6.3V one on the 5V switch mode regulator output will stay though, as the TPS563201 switch-mode chip does not ramp up current very quickly, and the USB inputs are fused at 500mA.

author: bsudbrink
author_rank: Frequent Contributor
timestamp: December 01, 2017, 05:51:13 pm
content: Old stock by any chance? In the vintage computer collector circles, old tants are known for their "antics". I personally have had them:1) disappear with a bang (I had to inspect the circuit board with a magnifier to figure out where it had been)2) turn into a miniature volcano complete with lava3) turn from yellow to black before my eyes in a second, then crack in half.

author: technix
author_rank: Super Contributor
timestamp: December 01, 2017, 06:19:47 pm
content: Quote from: bsudbrink on December 01, 2017, 05:51:13 pmMaybe? How to tell?

author: cdev
author_rank: Super Contributor
timestamp: December 01, 2017, 06:55:02 pm
content: See attachments!

author: floobydust
author_rank: Super Contributor
timestamp: December 01, 2017, 10:27:59 pm
content: If you have multiple rails, like +5V and -5V, during power up and power down one rail comes up/falls faster compared to the other and a tantalum can see a reverse voltage blip that damages them.Infant mortality also happens, some (new) tants just die when you power them up.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: December 01, 2017, 11:22:16 pm
content: Happened to me today too! 2917 47uF 20V when 12V was applied incorrectly (correct polarity but higher than 6V design voltage)I think it was the inrush current that killed mine... it is now a smelly 2 Ohm resistor with black burn marks on the side.I violated this rule... "When specifying regular tantalum capacitors for use at the input, a minimum voltage rating of 2 X (maximum dcvoltage + ac ripple) is highly recommended."

author: james_s
author_rank: Super Contributor
timestamp: December 02, 2017, 12:00:27 am
content: I've had one blow up in an IBM PC/XT I was working on once. I flipped the power switch on and it made a click and shut down. I tried the same thing a couple more times while putting my ear down close to various areas inside the machine to identify the source of the click, then the last time I flipped the switch *BANG*! At least it was then obvious where the source of the problem was.

author: CopperCone
author_rank: Super Contributor
timestamp: December 02, 2017, 12:18:44 am
content: I use 35 for 15v rails.I treat it the same as an electrolytic, I derate by ~50 percent.Ive seen failed 25v ones (open) on 15v rails, in old equipment, so I figured 35v to be better.hard to justify using a 50v part for 15v rail.

author: langwadt
author_rank: Super Contributor
timestamp: December 02, 2017, 01:16:51 am
content: Quote from: bsudbrink on December 01, 2017, 05:51:13 pmI have an old pair of glasses somewhere with a nice chip right in the middle of one eye from an exploding tant

author: Old Don
author_rank: Regular Contributor
timestamp: December 02, 2017, 01:33:54 am
content: Tantalum capacitors gave me a great deal in "bad" S-100 RAM boards many many years ago. I purchased 5 dead 64K RAM boards and repaired all 5 with 5 new tantalum cap's. One per each board on the -5VDC supply. The OEM had the mask marked with the - lead going to ground and the + lead going to -5V's and all were stuffed as per board markings.Reverse the new cap's and the boards all worked.

author: texaspyro
author_rank: Super Contributor
timestamp: December 02, 2017, 05:58:51 am
content: I used to own a company that made products for major OEMs. IBM required the use of 3-leg tantalum caps that could not be inserted backwards. I've seen manufacturers that had Lexan "blast shields" in front of their test stations to deflect tantalum shrapnel.

author: technix
author_rank: Super Contributor
timestamp: December 02, 2017, 07:30:04 am
content: Well I am revising my boards to use ceramic caps over tantalums now... I just hope the power converters can take the much lower ceramic ESR.

author: texaspyro
author_rank: Super Contributor
timestamp: December 02, 2017, 08:29:48 am
content: Quote from: technix on December 02, 2017, 07:30:04 am

author: Niklas
author_rank: Frequent Contributor
timestamp: December 02, 2017, 12:46:30 pm
content: According to an FAE from Kemet, the recommended 50% voltage derating in the datasheet comes from the microscopic cracks in the insulating layer that can form during the soldering process. We have a 35V rated tantalum on the output of a 12 V regulator in an automotive product. No failures on 500k units. The previous version had a 16 V rated cap that was prone to burn, especially during hot start. PCBA already heated, short power off followed by a new power on with inrush.Don't look too much on the steady state ratings of the power supply. During power on with the output switch it is a question of series impedance from the output capacitor in the PSU to the tantalum capacitor. Also the voltage over the tantalum is 0 at the start, similar to a resistor with only the ESR visible. I measured 6 A in a short peak with a 1 ohm series resistor from a PSU with 1 A rating.

author: H.O
author_rank: Frequent Contributor
timestamp: December 02, 2017, 03:22:42 pm
content: Quote from: james_s on December 02, 2017, 12:00:27 amExactly how my DS4000 behaved (url=https://www.eevblog.com/forum/testgear/rigol-ds4000-power-supply-details/msg1290400/#msg1290400]see here). As soon as I tried to turn it on it clicked and shut down the powersupply so I replaced it and the new power supply blew the crap out of the apparently faulty cap making it quite obvious where the problem was. A 35V rated cap across the 20V input of SMPS.

author: james_s
author_rank: Super Contributor
timestamp: December 05, 2017, 12:00:36 am
content: Quote from: technix on December 02, 2017, 07:30:04 amJust keep in mind that some types of ceramic capacitors have significantly reduced capacitance if operated with DC across them. That bit me a couple times before I learned of that trait, it's not something that is as widely known as it probably ought to be.

author: texaspyro
author_rank: Super Contributor
timestamp: December 05, 2017, 12:17:25 am
content: Quote from: james_s on December 05, 2017, 12:00:36 amA MLCC cap can "lose" 90% of its capacitance if operated near its rated voltage. Generally, the effect is worse the "denser" (capacitance per unit of volume) the cap is. Try to use the largest footprint package that you can. Read the data sheets (but it is not always spec'd there).

author: tautech
author_rank: Super Contributor
timestamp: December 05, 2017, 12:26:02 am
content: Well worth a watch:https://www.eevblog.com/forum/chat/capacitors-explained-by-james-lewis-of-kemet/

author: TimFox
author_rank: Super Contributor
timestamp: December 05, 2017, 06:16:45 pm
content: For applications where this decrease in capacitance is important, be sure to read the data sheet.Note that "MLCC" construction is not sufficient to identify the capacitor. The actual dielectric (e.g., X7R or Z5U) is more important to look at both voltage co-efficient and temperature co-efficient of the capacitance.

author: floobydust
author_rank: Super Contributor
timestamp: December 05, 2017, 08:53:00 pm
content: No, don't replace tantalums with MLCC's unless you take special care.Tantalums are moderate ESR and MLCC's are low ESR, so aside from getting ripped off on capacitance(MLCC 10uF effectively less than 1/2 that), their extremely low ESR causes ringing and sometimes instablility with LDO's.The analog great Jim Williams, always added an ~1ohm in series with MLCC to dampen ringing.

author: cdev
author_rank: Super Contributor
timestamp: December 05, 2017, 10:20:38 pm
content: Some more helpful data to prevent incorrect installation. Also its useful to derate them heavily.