Tantalum caps

I've designed some unusual power supplies boards with about 100 (yep, one hundred) 33uF/25V tantalums, for bypassing +/-15V supplies. 100A RMS into the caps. The customer produced more than 2K boards to my knowledge which are running almost 24/24 since 10 years and I've yet to hear of a catastrophe scenario. Of course it was carefully designed :-)

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Thanks,
Fred.

1A ripple each and only 40% derated? With such small capacitors that is like Russian Roulette :-)

You were very lucky. I have seen designs where they weren't. And the shirt of a guy with dozens of tantalum holes in it. Luckily he wore a T-shirt underneath and it didn't go through.

Could you explain that some more? What's the secret trick?

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Regards, Joerg

http://www.analogconsultants.com/

I haven't done a whole lot of analog, but a "rule of thumb" I've used on most of my digital connect-the-dots boards has been about a 10 uF tantalum at the point where the power enters the board, another at the opposite corner, a 1uF tant. for each row of chips, and a preferably .1 uF monolythic at each chip if I have any on hand or can grab a handful from the stockroom when nobody's looking. ;-) Otherwise, .01 discs.

Hope This Helps! Rich

John, why do your power supplies have such high dV/dT?

Thanks, Rich

Because people turn things on and off?

John

I guess you never worked in a repair department :-) Besides that tantalums may explode when mounted wrong. Last year one blew up right in my face when testing a board which just came in from the assembly company. Fortunately I wear glasses otherwise I would have suffered serious eye injuries.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico@nctdevpuntnl (punt=.)
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Yo, Rich, in case you do another one of those: 1uF and 10uF ceramic have been invented quite a while ago. Que estan mucho mejor. Oh, and they are cheap.

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Regards, Joerg

http://www.analogconsultants.com/

Never use a tantalum. They suck. Or, actually, they blow!

I've never seen a blown-up tantalum cap, and until I came hear I'd never even heard of one.

Thanks, Rich

"Rich Grise"

** Most designers shied away from them by the late 1970s - because of bad experiences.

The horrible things are like red flags to repairers, soon as you see even ONE on a PCB you say to yourself " well, there's the problem ".

.... Phil

Yep. We used to use tantalums for everything, until we had a board with a 470uF 16V tant across the power-in jack, for bulk decoupling. The PSU was 9V at a couple of amps but the short-circuit transient curent was much higher, about 18 amps for a few tens of ms IIRC. Fortunately all the "*BANG* + smoke + flames on PSU connection" incidents happened during initial testing rather than in front of the customer. We replaced it with an aluminium electro - no more problems.

Even when run under very benign conditions they can go leaky - we had another board which used the same tant to hold up the supply for some CMOS RAM during battery replacement. Current was limited to a milliamp or so. A few boards came back with no power to the RAM, the cap measured a few hundred ohms. Remove the cap from the board and apply rated voltage (ramped up slowly) from a bench PSU -> nice fireworks display.

These were all AVX TPS series. Anyone have any better experience with other brands?

R.

On a sunny day (Wed, 21 Sep 2011 13:18:35 -0700) it happened John Larkin wrote in :

Well to use your own argument, if you have one across an external power input, directly AT the point where the power comes in, the peak current would be high. If you have some PCB traces first and the cap further away at say a chip for decoupling, then the peak current would be limited by the trace resistance, can make quite a bit difference. I just made this up to counter your argument :-). I have seen exactly one tantalum blow up, in a 19 inch rack, right at the supply input. I have seen hundreds used and used myself without ever having a problem. I stopped using them a lot because they were much more expensive than the normal electrolytics. They make great decoupling, low ESR, are small. I killed some by putting it in the wrong way...

A backwards tant is like a ticking time bomb except you never know how long the fuse is. Could happen in a few seconds, or months later after it gets to the customer.

I was working on a HP 8561E a few weeks ago that had 3 pin dipped tantalums all over - they are impossible to put in backwards. IIRC the center was the + and the two outer leads were the -. (or vice versa, can't remember.)

That was the design rule when I started at Nortel (early '90s), the boards weren't allowed to self ignite under any circumstances.

If you were using a tant you had to have 1 ohm/volt in series.

Nial.

"Nial Stewart"

** Bit like wearing two condoms ....

.... Phil

That first surface mount Tantalum picture is from one of my boards. It was a low ESR input filter for switching power supply. Ceramic cap in parallel cured the problem.

-- Boris

On a sunny day (Thu, 22 Sep 2011 11:12:55 +0100) it happened "Nial Stewart" wrote in :

Maybe a good idea to put a 20mm fuse in the supply line before the caps if the board goes in a rack with a very powerful supply.

I noticed in my PC they put one of these automatic fuses in a sense wire to the VGA connector, that wire changed specification (or pin), and can be a shorted by old equipment. Save you from burned out traces...

The piss poor peanut gallery comment retard shines through with another retarded comment... AGAIN.

You're an idiot. Component engineers test devices BEYOND their ratings.

You have never heard of "destructive testing"?

When we sold HV supplies to NOAA, they bought three, tested all three, and then beat the piss out of one, and only one goes on the balloon, never to be seen again.

But they make sure all three perform within their design spec window very precisely, and beyond it, and far beyond it with one unit.

THAT is how the military and science guys work.

The post where you describe your RF burns and "scars" tells me that there are several things you missed in your education.