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.

On Thu, 22 Jan 2004 08:54:58 +0100, Jeroen
wrote:

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.


Which opens up the possibility of using them as parametric amplifiers
or modulators. I have a paper somewhere that uses the nonlinearity of
ceramic caps to make a nonlinear transmission line - a shock line -
that sharpens the rising edge speed of kilovolt pulses.

John

Over the last few years I've acquired quite a few consumer electronincs pcbs
including TVs, VCRs, stereos, etc, so when I discovered that I needed a

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.

Computer equipment is a good source for tantalums - motherboards, hard
drive PCBAs, etc. Of course, it will be surface-mount

In article , "Henry Kolesnik"
writes:

Over the last few years I've acquired quite a few consumer electronincs pcbs
including TVs, VCRs, stereos, etc, so when I discovered that I needed a
tantalum to repair some test equipment I was going to salvage a tantalum. I
couldn't find one anywhere, so I assume they're too expensive or too
unrelaible for high end consumer electronics. A couple of the boards were
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.

Tantalum capacitors became a component item about 45 (or so) years
ago and originally favored in spacecraft and aircraft because they could
contain lots of electrostatic storage in a small space with ligher weight.
That was when PCBs were relatively new, quite new in spacecraft
electronics.

It hasn't been until the last decade or so that the cost of tantalum
capacitors has approached the level of improved electrolytic
capacitors of the same value. Tantalums are still relatively
expensive but they are good for SMT due to their smaller size;
its a trade-off between cost and overall system size in that case.

Inherent problems in tanatalum capacitors have been improved
since their initial debut as a component but so have electrolytic
capacitors and their manufacturing methods. One can purchase
FARAD-value low-voltage electrolytics now where once it was
not possible unless one had a room to put them in.

Len Anderson
retired (from regular hours) electronic engineer person

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.

The problems with tantalum are their fragility (we've had exploding caps on
our boards, with one manufacturer's part being fine and another being
horrid), cost, and the relative scarcity of tantalum. Does anyone remember
the Great Tantalum Shortage of a couple of years ago? One of the big
tantalum supplying regions is central Africa, and a combination of wars
reducing supply and increased demand led to some supply problems for a
while -- I remember that at least one of the manufacturers even came out
with a Niobium cap as a substitute.

"Henry Kolesnik" wrote in message
...
Over the last few years I've acquired quite a few consumer electronincs
pcbs
including TVs, VCRs, stereos, etc, so when I discovered that I needed a
tantalum to repair some test equipment I was going to salvage a tantalum.
I
couldn't find one anywhere, so I assume they're too expensive or too
unrelaible for high end consumer electronics. A couple of the boards were
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?
tnx
hank wd5jfr

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).


The problems with tantalum are their fragility (we've had exploding caps on
our boards, with one manufacturer's part being fine and another being
horrid), cost, and the relative scarcity of tantalum.

This is really erratic. One spool of tants will be bombs, another
can't be made to fail by deliberate abuse.

John

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


snip

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).

Boy are they ever. I have some surplus ones, but they have cases that are
more of a silver-gray than that nice yellowish-white look you get from
silver-plated connectors. These are special parts, but if you want a
generally high-performance cap in a (relatively) small package they're hard
to beat.

I worked for a while on a project to make a power-wire networking device.
During testing I accidentally dragged a scope ground across a circuit that
was referenced to the 115V power line, thereby exceeding the tantalum cap's
voltage rating -- er -- "slightly". Little pieces of flaming capacitors
bounced around the lab. After that all of my digital logic (3.3V and lower)
coworkers _never_ messed with my bench.

The problems with tantalum are their fragility (we've had exploding caps
on
our boards, with one manufacturer's part being fine and another being
horrid), cost, and the relative scarcity of tantalum.

This is really erratic. One spool of tants will be bombs, another
can't be made to fail by deliberate abuse.

Interesting. I'll have to remember that. Thanks. In any case when
designing with _any_ electrolytic capacitor it's best to use specify a cap
for 20-50% higher voltage than what you think it's ever going to see,
particularly because many voltage regulators overshoot on power up and the
output cap sees more voltage than you think.

John

"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.

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.

wrote:

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.


Few years ago, my boss came in, laid a printed schematic on my desk, and
said "Put this in PSpice, so we can demo it to a customer. I said OK,
and started entering the design. He comes in a couple of hours later,
and wants to know if it is finished! I go, SaWha? He now tells me that
he is getting on a plane in a few hours, and has a meeting with the
customer in the morning! So I get to it, end up emailing it to him the
next morning, as there were some digital parts not in the standard
libraries, had several components that I had to create, etc. There are
still major bugs in the simulation, etc. That afternoon, he calls, and
says that they were mad because they didn't see what they were looking
for in the simulation. I ask, What were they looking for? And he tells
me, this cap in the power supply keeps exploding! I then realize he is
talking about the tantalum! Since I had just seen a thread like this
one, I knew about tantalums, so I told him that there was no way we were
going to create a special model for PSpice for tantalum caps! Most
designers just knew not to use them as power supply filters!

Got a bad review from him that year. I now know that, when you get an
assignment, ask LOTS of questions, to the actual customer whenever
possible! 8-)

Charlie
Edmondson Engineering
Unique Solutions to Unusual Problems