Bloody Tantalums

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Just spent many hours today pinpointing a faulty tantalum capacitor.

It was in the display board of the SC Deep Cycle battery charger.

The charger would run for about 5 minutes, then slowly the 5V rail to
the PIC would slowly drop to 3.5V or so and everything would stop,
obviously.

If they used a simple 7805 regulator, without a series input resistor,
the blue smoke would have been released and made the fault finding far
easier :D

In this instance, obviously (now) the cap began leaking more the longer
volts were applied.

Ray





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Yeah a lot of companies have stopped using the blue smoke indicator.
It takes a smarter more educated fellow to find them. Well done.
Can you charge extra for frustration?

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"Ray"
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 **  LOL !

 Most experienced service techs have the same attitude towards any bead
tantalum cap they see


 * SHOOT  FIRST *    -   ask questions later  ......




......  Phil





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    It ain't only the bead ones .... the axial ones do it too.

Bob

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On Sun, 25 Feb 2007 18:41:19 +1100, Bob Parker

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bead

Bob, are you referring to types such as Kemet hermetically sealed
tantalums? Heaps of these used in mil spec equipment so I would be
surprised if they failed very often at all. The secret to using tants
to to use them only in a well regulated environment where they are not
subject to voltage spikes. They do have extremely low ESR and long
life. I use Kemet hermetics around my linear voltage reg circuits
along with standard aluminium electro's in the appropriate places
without any failures in many years of service. I would agree with the
adage "never to use tantalums" as long as it applied only to the solid
dipped type.

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    I'm thinking right back to some professional Italian
telecommunications gear manufactured in the late 60s I used to work on.
I don't remember the brand of the caps. The technology's probably
changed heaps since then. Apologies for leaving out those 'small' details.

Bob



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On Mon, 26 Feb 2007 14:22:23 +1100, Bob Parker

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tants
not
the
solid
on.
details.

If they were stainless steel cases with glass seal then they were
probably hermetic tantalums.

I will post a pic on abse of a small dual rail 317/337 based linear
mains supply pcb I designed showing where I use the Kemets.

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On Sun, 25 Feb 2007 18:41:19 +1100, Bob Parker

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I can confirm that. I've seen such failures in Control Data BK7 series
disc drives.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

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When I see those rotten bloody things, I replace them with an
electrolytic.

Seen too many of them short internally, and in the 1980's actually saw
one catch fire right before my eyes !






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    The old STC 151 VHF transceivers used to have bead tantalums in the
PA stage. If it was mistuned, it was common to see just two bits of wire
sticking out of the board where each cap used to be ... they exploded!




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"Bob Parker"
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**  He, he  -   seen more than a few charred, black  " peas " roll out of
odd pieces of gear when I tipped them up in my time   !!




.......  Phil



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Funny. They must have been used in the wrong types of situations.
The last Philips transmitters they designed were full of them, and yet
they performed OK. (in most cases)

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    Hopefully the Philips ones didn't put a lot of RF current through
them, and used more appropriate caps for high frequency supply
bypassing? (Guessing a bit here - it was over 30 years ago!)





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Um the Philips transmitters are starting to have issues with them.
Look at the PRM80 series radios. There's a good number of them gone
ferral now...started with tant issues.

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On Mon, 26 Feb 2007 00:54:21 +1100, Tsunami Australia

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Nope, that's all surface-mount aluminium electros .....

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Depends which model. There was a series with tants. There were 2
series of the PRM80.

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On Mon, 26 Feb 2007 15:53:29 +1100, Tsunami Australia

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I'll have to take your word for that.  Every PRM80 I've seen (literally dozens)
had those @#$%& SMT aluminium electros.  Where's Jason when you need him?

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there are several problems. The worst "feature" of tantalums is an
external energy impulse can cause the cap to ignite the tantalum slug,
which is surrounded by manganese dioxide, provideing the oxygen for the
tantalum to combust. The result is a bang (or fire) much, much larger
than the impulse that set it off.

and you can set a tantalum off by applying a fast voltage step (eg 80%
Vrated) from a low impedance source. A good example would be hot
plugging. AVX tell you the amount of resistance you need in series with
the cap to reduce the inrush current below the ignition point (although
they dont refer to it as such). its not usually a problem on SMPS
outputs, as the supply tends to ramp up at a controlled rate.

They also despise over-voltages (c.f. electrolytics, which have a surge
voltage rating), and running at rated voltage is a great way to
seriously reduce lifetime.

Plus of course their ESR isnt that hot....

The first place I worked as an engineer had a design rule "NEVER use
tantalum caps"....

Cheers
Terry

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finger to keyboard and composed:

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Same here. I must have replaced hundreds of faulty tantalums during
the same period.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

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