On Thu, 22 Jan 2004 18:12:20 GMT, "Ken Finney"
wrote:


"John Larkin" wrote in
message ...
On Wed, 21 Jan 2004 22:21:31 -0800, "Tim Wescott"
wrote:

I wouldn't assume that just because your test equipment comes to you
broken
is a result of tantalum caps -- perhaps your sample is skewed by buying
at
hamfests instead of burgling active technology companies? Maybe if you
only
acquired your home entertainment equipment from dumpsters you'd conclude
that aluminum electrolytics are bad?

I recently escaped from a company that does aero (but not space) systems.
They get mounted on aircraft and are expected to survive being shipped in
an
unpressurized cargo hold at 50000 feet. At that altitude a wet aluminum
electrolytic will dry out, but a tantalum will be fine. There are even
wet-slug tantalums for high-altitude applications that will not dry out
at
these altitudes.


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).


snip

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




Sure, any cap will explode if you dump enough energy into it. The
difference is that the dry Ta:MnO2 guys only need a tiny bit of energy
to ignite, then chemically explode on their own. Just a high dV/dT
will set one off.

John

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

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

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

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...

snippety

from my personal stuff purchased new. One example is a MGA Mitsubishi rear
projection TV that operated flawlessly for nearly 20 years of daily use.
Most of my test equipment comes from hamfests and is surplus after becoming
obsolete and non-operative in less than 20 years. That leads me to wonder
what the real story is behind tantalum capacitors. What do the experts have
to say?

The ONLY problems I've ever had with tantalums are whe

(1) The part was defective from the manufacturer.

(2) The voltage rating was consistently exceeded.

(3) The thing was installed backwards (reverse polarity).

I have no less than five Tektronix O-scopes here, all vintage
late-70's to mid-80's. This means not one of them is less than 20 years
old. They all use lots of tantalums, and they all work great, but then
again Tek was (in those days) proud of what they put out, and was most
definitely engineer-driven (which means at least a 20% 'fudge factor'
built into everything they made).

Tantalum caps are very stable and durable, but they are much more
costly than aluminum types. In consumer electronics, the manufacturers
will try to shave every penny they can off the cost of the design, often
contrary to good common (engineering) sense.

Such considerations are (usually) not so critical when it comes to
non-consumer stuff.

Keep the peace(es).

--
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)

In article ,
Dr. Anton.T. Squeegee wrote:
In article ,
says...

snippety

from my personal stuff purchased new. One example is a MGA Mitsubishi rear
projection TV that operated flawlessly for nearly 20 years of daily use.
Most of my test equipment comes from hamfests and is surplus after becoming
obsolete and non-operative in less than 20 years. That leads me to wonder
what the real story is behind tantalum capacitors. What do the experts have
to say?

The ONLY problems I've ever had with tantalums are whe

(1) The part was defective from the manufacturer.

(2) The voltage rating was consistently exceeded.

(3) The thing was installed backwards (reverse polarity).

I have no less than five Tektronix O-scopes here, all vintage
late-70's to mid-80's. This means not one of them is less than 20 years
old. They all use lots of tantalums, and they all work great, but then
again Tek was (in those days) proud of what they put out, and was most
definitely engineer-driven (which means at least a 20% 'fudge factor'
built into everything they made).

Tantalum caps are very stable and durable, but they are much more
costly than aluminum types. In consumer electronics, the manufacturers
will try to shave every penny they can off the cost of the design, often
contrary to good common (engineering) sense.

Such considerations are (usually) not so critical when it comes to
non-consumer stuff.

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)
--

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

"Roy Lewallen" wrote in message
...
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.

How much voltage? We talking tens, hundreds, or thousands of mV?