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February 25, 2007, 7:13 am

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

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

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.

Re: Bloody Tantalums

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

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

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.

Re: Bloody Tantalums

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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