In rec.radio.amateur.homebrew Ken Finney wrote:


Silver cased wet slug tantalums DO explode, most contracts that
allow the use of wet slugs require the use of tantalum cased parts.

Some years ago I wandered into one of the design labs to ask some questions
about a new thingy we were doing.

On a bench was a metal trash can with wires leading to a power supply,
thermometer, and a chart recorder.

"What's all this", I ask.

"Getting some real data on stressing tantalums. The trash can is a blast
shield just in case" was the answer.

Just after the guy running the test uttered the words "looks like nothing
bad is going to happen" the cap exploded with the trash can acting as
a megaphone for the bang and director for the shrapnel; everyone around
hit the deck.

We got off the floor and looked up to the acoustical ceiling where there
was capacitor pieces and an alligator clip embedded therein.

When the guy running the test said "Maybe we had better rethink this
design", I decided to go back to my lab and come back on a better day.

We continued to use tantalums (aerospace), but there were a lot more
explosions in the lab to make sure they didn't happen on the shipped
product.

--
Jim Pennino

Remove -spam-sux to reply.

In article , OK1SIP
writes
Hi all,
tantalum caps seem to be too expensive for consumer-grade equipment.
They contain pricey material - silver and, of course, tantalum, so
making them cheaper is impossible. AFAIK they are widely used in
military-grade equipment, where the price is not an issue. Their main
advantages are a longer life (they do not dry out nor leak) and a
bigger temperature range (frost resistance).
About using cheap parts in consumer electronics: At least 80 percent
of failures of certain types of TV sets were caused by dried-out
aluminum caps. The good practice when repairing these sets was: first
check all electrolyte caps by adding a good one in parralel. It was
successful very often.

BR from Ivan
The observations are consistent with the view that electrolytic
capacitor reliability decreases within a short time most other
components have failure modes that take a much longer time to reach the
end of the bathtub.
--
ddwyer

In article , Fred
writes

"Jeroen" wrote in message
...
John Larkin wrote:

I think multilayer ceramics are pushing 100 uF these days.

Yes, but alas, only with zero volts across them. Capacitance
drops precipitously with DC bias. For a cap with Y5V dielectric,
at half the rated DC voltage, there's only 10% of the initial
capacitance left. Most manufacturers don't tell you.

The high k types vary to +-10% and +20-80% from memory if full temp is
allowed for.
High K doped with piezo material, they can be heard to click if hit with
a square wave.

In article , Mike Andrews
writes

Some years ago I wandered into one of the design labs to ask some questions
about a new thingy we were doing.

On a bench was a metal trash can with wires leading to a power supply,
thermometer, and a chart recorder.

[snip]

We got off the floor and looked up to the acoustical ceiling where there
was capacitor pieces and an alligator clip embedded therein.

When the guy running the test said "Maybe we had better rethink this
design", I decided to go back to my lab and come back on a better day.

We continued to use tantalums (aerospace), but there were a lot more
explosions in the lab to make sure they didn't happen on the shipped
product.
Read the data books carefully. There is a formulae relating source
resistance, proof voltage/actual voltage and capacitance.
Note early Plessey button tantalum (very reliable) incorporated liquid
conc nitric? dont blow them up!

--
ddwyer

In article , John Larkin
writes

Wet-slug tants are expensive (do they still have silver cases?) but
don't blow up like the dry ones. The dry slugs coat the sintered
tantalum (fuel) with MnO2 (oxidizer).
As I mentioned elsewhere some wet slug used conc nitric as the
electroyte (from memory 35 years ago) but very reliable.

--
ddwyer

I've seen more than one problem with high-value ceramics causing problems
during operational vibe tests because of microphonics. In our case it
usually seems to be the vibration causing capacitance change rather than
true piezoelectricity, but it happens in any case. The last one that I
remember we replaced the ceramic caps with back-to-back tantalums, in fact.
Worked like a charm.

"ddwyer" wrote in message
...
In article , Fred
writes

"Jeroen" wrote in message
...
John Larkin wrote:

I think multilayer ceramics are pushing 100 uF these days.

Yes, but alas, only with zero volts across them. Capacitance
drops precipitously with DC bias. For a cap with Y5V dielectric,
at half the rated DC voltage, there's only 10% of the initial
capacitance left. Most manufacturers don't tell you.

The high k types vary to +-10% and +20-80% from memory if full temp is
allowed for.
High K doped with piezo material, they can be heard to click if hit with
a square wave.

I read in sci.electronics.design that Bill Turner
wrote (in ) about 'Tantalum
caps.', on Thu, 22 Jan 2004:
Consumer electronics are costed down to the lowest possible level. If
they can use something cheaper, they WILL use something cheaper. A TV
set or VCR has had people go over the design hundreds of times with
BOMs and catalogs, checking to see if they can shave a penny here or a
penny there.

_________________________________________________ ________

Well, maybe. Any manufacturer who has been nailed with thousands of
dollars in warranty costs caused by saving a penny might disagree with
your statement. I've seen it happen.

Epidemic faults were, in my experience, rarely caused by cost-reduction
but either by component manufacturing faults (noisy tantalum caps and
unreliable fuses, for example) or by unrecognized 'gotchas' in the
original circuits (high-Q series resonance in a loudspeaker crossover
filter).
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

I read in sci.electronics.design that OK1SIP
wrote (in ) about
'Tantalum caps.', on Wed, 21 Jan 2004:

tantalum caps seem to be too expensive for consumer-grade equipment.
They contain pricey material - silver and, of course, tantalum,

No. We used quite a lot of tantalum 'bead' caps in consumer audio and TV
until we found the problems they have and we could get aluminium
electrolytics 'with no added salt', so they didn't leak and were much
more reliable. I still have some boards with them fitted.

The problem with Al caps drying out is mainly that people let them get
too hot. They were rated at 75 C or 85 C *max. ambient*, not
'temperature rise'. It's still a problem; we have 'designer' set-top
boxes with no ventilation, and service people put 130 C rated Al caps in
them as replacements; 105 C rated is often not good enough!
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

In article ,
says...

Hi, Frank,

Tektronix was, during that time, strongly discouraging all new designs from
using tantalums. IIRC they had been taken to court over a case in which
a 465 'scope (the original, not the plastic follow-ons) had spontaneously
ignited and had resulted in an expensive fire. Forensics revealed that
a tantalum power-supply bypass cap had started the conflagration. The
drive to reduce tantalum usage was driven primarily by this liability issue,
more than component cost. If you wanted to use a tantalum, you had to
justify its usage to the component/design review committees -- which wasn't
difficult if you had good reasons and your design was solid.

Wow! I didn't know this... Thanks for the neat bit of history.

I will add that most of the tantalums I'm finding in my gear are
localized filters for the power-input traces in 7000-series O-scope
plug-ins.

Everything still works great, though. ;-)


--
Dr. Anton Squeegee, Director, Dutch Surrealist Plumbing Institute
(Known to some as Bruce Lane, KC7GR)
kyrrin a/t bluefeathertech d-o=t c&o&m
Motorola Radio Programming & Service Available -
http://www.bluefeathertech.com/rf.html
"Quando Omni Flunkus Moritati" (Red Green)

Frank Miles wrote:

Tektronix was, during that time, strongly discouraging all new designs from
using tantalums. IIRC they had been taken to court over a case in which
a 465 'scope (the original, not the plastic follow-ons) had spontaneously
ignited and had resulted in an expensive fire. Forensics revealed that
a tantalum power-supply bypass cap had started the conflagration. The
drive to reduce tantalum usage was driven primarily by this liability issue,
more than component cost. If you wanted to use a tantalum, you had to
justify its usage to the component/design review committees -- which wasn't
difficult if you had good reasons and your design was solid.

-frank
(ex-Tekie)

I was there at the time, too. Tantalums were essentially verboten unless
the source impedance supplying the tantalum cap was at least 3
ohms/volt. That's because it was found that the short circuit failure
mode was aggravated by high inrush current, so the source current had to
be limited. One of the chief reasons we had been using tantalums in the
first place is that they have very good bypass characteristics up to
quite high frequencies -- so a single capacitor could handle a very wide
range. When the source impedance was high, the capacitor didn't need to
be so good in the first place, and of course adding a physical resistor
in series with a supply bypass pretty much defeats the whole purpose.
Consequently, the 3 ohms/volt rule pretty much eliminated tantalums as a
viable choice for most applications. Fortunately, it was at just about
the same time that very big improvements were made in aluminum capacitor
technology. As the aluminums shrunk in size, they became much better at
bypassing higher frequencies. So they took over from tantalums pretty
rapidly. There was a glitch for a while, though -- boards were being
cleaned with Freon at the time, and it was discovered that Freon could
migrate past the seals on some or most aluminum capacitors and corrode
the aluminum, leading to poor reliability. The solution adoped by some
manufacturers was to add a rubber seal at the lead end of the capacitor.
That increased the length of the leads between the outside of the
capacitor and the inner body, increasing the lead inductance and
decreasing the capacitor's high frequency bypass capability. . . but
that's just another example of the day-to-day problems an engineer faces
and has to overcome.

Incidentally, I got a Tek 1502 TDR on eBay not long ago. It had a
shorted tanalum power supply bypass capacitor.

A couple of other anecdotes -- A time base plugin I designed had gotten
through the entire extensive pre-production test phases, accelerated
life tests, etc., and was in pilot production. I walked past the
production line technician's bench every day, and began noticing several
tantalum capacitors of the same type in the replaced-component box. They
had come from a sweep circuit I had essentially copied from an
instrument which had been in production for some time. Puzzled, I
analyzed the circuit carefully, and discovered that at an extreme
setting of one control, the tantalum cap could have a very small reverse
voltage applied. I modified the circuit to eliminate the possibility of
any reverse voltage of any level, and the capacitors quit failing.
Servicing data from the instrument I had copied the circuit from showed
noticeably reduced reliability of the capacitor, also. The lesson
learned is that tantalums won't tolerate _any_ reverse voltage. If they
don't fail immediately, a disproportionate number will fail eventually.

The other anecdote involves a QRP rig. As a crude reverse-voltage
protection, I had reverse-connected a 3-watt diode (actually, a 36 volt
zener I had lots of) across the power supply terminals. My battery
supply normally had an-line fuse which would blow. Just before Field Day
one year, the fuse holder broke and I didn't have a spare in the junk
box. I'd never blown a fuse in 20 years of Field Days, so went without.
The battery was a 12 volt, 5 Ah sealed lead acid unit, capable of a few
hundred amps if shorted. As I'm sure you've guessed, that was to be The
Year of the Reverse Connected Supply. The wires to the battery
immediately melted out of their insulation, burning some holes in the
tent floor. I managed to disconnect the battery without getting burned
and before a real fire started, and checked the damage. The rig's
(recently installed) power switch was fortunately off, so the innards
didn't get any reverse voltage. The diode had gotten so hot that the
plastic case had fractured and probably burned -- it was gone. The
diode's solder joints had melted, and the two separated diode leads were
dangling. But there was still a dead short across the terminals -- a
small 6.8 uF dipped tantalum capacitor was also across the terminals,
and it had become such a good short that it hadn't gotten hot enough to
explode. (The power supply wires were something like #24 or #26, so
they'd limited the current.) My guess is that it went short just as soon
as the diode opened, and made a better quality short than the diode had.
The fuse is now back in place (along with new diode and capacitor), so
of course I haven't reverse connected the supply since.

Roy Lewallen, W7EL