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Old tantalum capacitors (solid type) Would you use them?
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cdev:
What I have done is used the current limiting on my bench supply to apply a low voltage with current limited to just a few ma for a while. (how long is appropriate, though?)

Just because I've seen that suggested here. None of my capacitors have ever gone bad but I am still wary of them. Ive never seen one pop in real life, only on YouTube.

I have thought about bringing my bench supply outdoors along with some of these tantalum caps to see under what conditions they fail, and documenting it.

Until I feel totally confident they will literally never fail under the conditions I use them at, I think I'll still use electrolytics in everything for now.
nctnico:

--- Quote from: Conrad Hoffman on October 23, 2018, 07:30:19 pm ---IMO, if they weren't reliable, they weren't used properly. Tantalums are known to fail due to inrush current.

--- End quote ---
I agree. Over the years I have replaced a lot of tantalums which failed as a short. That is one of the reasons I never use tantalums (the others: conflict minerals and environmental damage due to mining). Also if you connect them wrong they can explode violently. Fortunately I wear glasses.
cdev:
This sounds like a useful test and guaranteed to be educational. How would you do it? I have the usual beginning/midlevel hobbyist tools. I could probably rustle up a chart recording tool. Is there any standard testing protocol?


--- Quote from: T3sl4co1l on October 23, 2018, 06:27:55 pm ---Not a problem.  If you like, you can test a statistical sample, measuring leakage current or noise* under bias, and before and after soldering.  Go up beyond rated voltage if you like, find typical failure points.  Maybe with temp cycle, too.  Would be good practice just to understand their behavior, really. :-+

*Noise is due to self-healing, where a partial short or discharge occurs, locally heating the electrolyte which decomposes it to an insulating compound.  Catastrophic failure occurs if this happens too rapidly on a low-impedance supply, driving runaway self-heating, triggering a thermite-like reaction.

Tim

--- End quote ---
T3sl4co1l:
The description suggests the protocol -- if I were doing it, I'd be interested in the short-term (self-healing) noise voltage, and the average leakage current, with respect to time, voltage, temperature, thermal history and thermal cycling.  (That's a lot of variables, yes; it would be very involved to test all of them exhaustively, all the while taking a statistical sample of components!  Just a few combinations would be fine.)

The noise measurement would look very much like a Geiger counter: the expected mechanism is impulsive, so amplifying the AC voltage, and logging the magnitude of the impulse, and rate (presumably following Gaussian and Poissonian statistics, respectively?), and how they vary with various conditions, would be interesting.

And, actually logging that data as such (say, magnitude and time stamp), would be a good random number generator, but not very useful given the amount of effort required to collect it (essentially a photomultiplier spectrometer -- an impulse size and rate counter, but slower), so simpler methods could be used, like taking the AC RMS voltage over a suitable duration (it might not be something you can measure with a TRMS DMM, but an RMS converter chip and a minutes-long filter would do).

It would then be interesting to correlate noise with DC leakage, and DC leakage with time, and DC leakage or integrated noise against capacitance change (if any).  Capacitance change is quite pronounced in self-healing film caps, but probably not that strong here?  Would be an interesting hypothesis to test.

Standard testing -- and expectation -- for capacitors in general, is applying a voltage via current-limiting resistor, and monitoring the leakage current over time.  There will always be a long time constant due to dielectric absorption (which may not be a time constant at all, but a 1/sqrt(t) -- diffusion -- dependency), but there may be additional effects at work.  They can be detected by teasing apart the different responses stacked on top of each other, assuming simple RC time constants, and/or a diffusion element.

Presumably, there will be some reforming or self-healing effect, i.e., the leakage starts high then falls gradually, at a given voltage; subsequently, that voltage (or below) will retain a low leakage level, but raising the voltage higher causes more leakage, until that level is "formed", and so on.

The test can be accelerated somewhat, by using a low impedance driver.  The current should still be limited to low levels, but the dynamic impedance (change in voltage / change in current, for small changes) can be quite low.  A good example is an op-amp follower, set for a constant input (and therefore output) voltage, and measuring its output current.  That way, as soon as leakage is less than the current limit, output voltage will be forced to exactly the setpoint, eliminating the potentially long or compounded time constant an RC test has.  Downside: output noise voltage is differentiated into noise current (Inoise = C * dVnoise/dt), making this a difficult method to combine with an AC noise test (depending on how much excess noise the DUT has, relative to the amp).

But yeah, the basic test would be logging leakage current (hopefully microamperes) into a plot, for a bunch of individual parts, and raising and lowering the voltage stepwise to see the various effects.  Then the temperature stepwise, and so on.  Take as many steps as you like, each one as long as needed to verify the time constant(s). :)

Tim
coppercone2:
How does this self healing stuff factor into real reliability?
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