Tantalum Capacitors

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Are you sure? I understood that the I=C.dV/dt was responsible for the damage. The higher dV/dt, the higher I. And the longer the dV/dt continues, the hotter the C gets.

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joe

Yes

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If you read the post carefully, you'd see that between power supply and C there is a small R, which gives a tiny reduction of voltage to the IC, and then there is the C // Vcc-GND. So the IC sees the same (or even smaller) ripple from the power supply, EMC suppression is still the same (or even better due to more damping of the ringing) and everything went fine from then on.

Yes. And all those factors are uncertain to a high degree. Therefore it could be advisable to just add a resistor of a known value, if your design permits it.

Indeed, not in that case. But where we had the problem, it worked well.

Indeed.

After finding out the problem, we (finally) read up to the specs and recommendations, found the suggestion to add small R's, did it, and the problem was gone. We did not measure ripple voltage. I guess that would have been useless because the problem was caused by EMC, and the measuring probe would probably have suffered the same problem and not given the right picture.

joe

No, the point is that there is no reason to use a tantalum cap if you're going to blow its ESR with a resistor. Just use an aluminum.

Nevermind!

I sit corrected. ;-)

I was thinking one reason to use the tantalum was because it could pack more capacitance in a smaller space.

I suspect you need to see a diagram of the circuit because you are thinking of a resistor directly in series with the capacitor whereas Joe and John were thinking of placing the capacitor directly across the load (chip) and a resistor between the regulator and the supply pin of the load (chip), (with its capacitor).

The ESR would be high if measured from the location of the regulator, because there would be a resistor between the regulator and the capacitor. The ESR would appear low if measured from the location of the chip which is the load (chip), because the capacitor is directly in parallel with the load (chip). The disadvantage of this arrangement is that the average (DC) load current flows through the resistor which might cause an annoying reduction in the supply voltage at the load.

Nevermind, others have already said what I tried to say.

One other option is Niobium Oxide caps, sold under the brand oxi-caps. Digi-Key has them. I have used several thousand of them on some gear that some users run in hard vacuum, so aluminum electrolytics were out of the question. I have mis-connected a few of the oxi-caps, and can verify they will char a bit, but not burst into flame.

Jon

You can get MLCC capacitors as big as 180uF - but it won't fit in the original space.

They're mostly advertised as SMD, but that certainly won't fit - a few suppliers offer resin dipped leaded versions that you could form the leads to meet the pads.

Aluminium electrolytics of any kind are a no no! - there was much chatter about organic semiconductor electrolytics a decade or so ago, that were claimed to be as good as tantalum, but it lately seems to have gone very quiet on that front.

Tantalum electrolytics are *VERY* intolerant of reverse voltage, even just a little bit makes them go leaky. When the normal supply comes back - if it can shift much current; the tantalum goes off like a match head!

The polymer aluminums are really good. Low leakage, low ESR, don't dry out.

Saw an article about those a while ago - did they ever take off?

The organic semiconductor variety were supposed to be the "dog's nuts" - but all the hoo ha seemed to mysteriously fade away.

Oh boy, I'm going to look uninformed, but polymer Al's are different from "normal" Al's? (I appointed myself capacitor Czar (well maybe just an Earl.) at our company, and now I'm feeling very ignorant.) There look to be polymer Tant's too.

George H.

It crossed my mind that the polymer might be vulnerable to temperature - but apparently its a coiled/sprung polymer that controls the viscosity in motor oil, and that has to survive pretty extreme temperatures.

The polymer caps are dry inside, unlike regular aluminum electrolytics. They have very low ESRs which stay low below 0 degrees C, when aluminums freeze and ESR goes to hell.

Unlike regular 'lytics, they have very low leakage current up to roughly 1.5 or so of rated voltage, when they fail suddenly and permanently.

We use them at switching regulator outputs and when we might have current steps, like to power an FPGA.

Oh, the question then is how low will they go? (maybe someone has stuck them into LN2)

Got it, Thanks.

(the leakage on regular 'lytics is not as bad as the spec sheet. at least near room temp. but I think we've been there before...)

With pic's on wiki, I now see a colleague uses them in an* led strobe circuit. I assume they are more expensive.

George H.

If you can dunk one, I'll send you one.

Maybe 2x the price of regular lytics. 180 uF, 6.3 volts, 47 cents.

Hmm No LN2 at the moment. (I've been swamped with other stuff.) I think these are poly Al's

In which case I can dunk 'em sometime in the future.

George H.

Saying "ell ee dee" is way too much work. Almost as bad as "picofarad".

Besides, to my ear "ell ee dee" is sort of affected-sounding.

Cheers

Phil Hobbs

I think it should actually be L.E.D and I know you know that. If the periods are dropped, as is universal now, it should probably still be pronounced in the same way. If I were in a conversation, my mind would probably be involuntarily put on pause at the use of "lead" so that it could gain understanding from the connotation. But, I would not have that problem with the letters.

But, that just me.