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'[EE] capacitor types'
2004\09\23@155312 by Bob J

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Up until now all of my projects have used electrolytic capacitors in
the power supply section.  After learning a few things along the way
(like putting a filter cap next to the pic from Vdd to gnd, which are
tantalum that I had in the junk box)  I assume that even with surface
mount stuff that electrolytics are still ok to use in dc power
applications.  What I'm wondering is should I be thinking of switching
to polypropylene for better temperature stability, working life, etc.,
in non-power circuits especially when there are size constraints?  I
guess my real question is, where and under what circumstances should
one use electrolytic caps, where should one use polypropylene caps,
tantalum, etc.?

Regards,
Bob
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2004\09\23@192001 by Robert Monsen

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here is a link you might find helpful:

http://www.radio-electronics.com/info/data/capacitor/capacitor_types.php

Bob J wrote:

{Quote hidden}

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2004\09\24@062534 by Spehro Pefhany

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At 04:19 PM 9/23/2004 -0700, you wrote:
>here is a link you might find helpful:
>
>http://www.radio-electronics.com/info/data/capacitor/capacitor_types.php


A couple of things I'd add.

Tantalums are NOT generally recommended for power supply or bypass
applications. If you must use them, they should be greatly derated in
voltage, but IMHO, it's best to avoid them entirely in such applications.

Ceramic caps now come in much higher values than 0.1uF (1uF/10uF and even
100uF) and are being used to replace some low-value electrolytics,
especially at higher frequencies and where low ESR is important.

The highest performance in analog applications is with plastic film caps.
Mylar (polyester) is not the best, but is very common and relatively
cheap. If dielectric absorption is a factor, then polypropylene is better.
Newer SMT film caps often use a high-temperature film called PPS
(polyphenylene sulfide) which is really quite nice electrically and can
withstand the soldering temperatures. You'll never see an SMT styrene
cap. ;-)

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2004\09\24@065848 by hael Rigby-Jones

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{Quote hidden}

What are they actually good for?  ISTR that they aren't very good for
coupling caps as they exhibit certain non-linear effects.  Seems to me that
they are pretty much obsolete with the advent of cheap low ESR
electroylitics and high value ceramic caps.

Regards

Mike

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2004\09\24@072022 by Spehro Pefhany
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At 12:01 PM 9/24/2004 +0100, you wrote:

> >Tantalums are NOT generally recommended for power supply or
> >bypass applications. If you must use them, they should be
> >greatly derated in voltage, but IMHO, it's best to avoid them
> >entirely in such applications.
>
>What are they actually good for?  ISTR that they aren't very good for
>coupling caps as they exhibit certain non-linear effects.  Seems to me that
>they are pretty much obsolete with the advent of cheap low ESR
>electroylitics and high value ceramic caps.

Not much, IMHO. Timers and coupling. If you have the room, there are
aluminum low-leakage etc. that can replace them in most situations.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2004\09\24@090635 by Russell McMahon

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> Tantalums are NOT generally recommended for power supply or bypass
> applications. If you must use them, they should be greatly derated in
> voltage, but IMHO, it's best to avoid them entirely in such applications.

Agree.
Tantalum make excellent decoupling capacitors ****PROVIDED*** that you can
absolutely guarantee that their voltage rating is not exceeded by even the
shortest duration voltage spike. Almost always you can't. When they do fail
they do it really really well. Some or all of smoke, smell, noise, smoke,
explosion, flame. And they often short to a very very solid short. Best
avoided.

       RM

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2004\09\24@091923 by alan smith

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Tantalums are still usable for some applications, such as timers on one-shots, etc.  Also when doing bypassing, each type of cap exhibits certain behaviors at particualar frequencies so a mixture of ceramic, OsCon and tantalums sometimes provide the necessary response curves across all domains.  OsCon types are really good for low ESR and bulk capacitance.  MLCC ceramics are also excellent for lower ESR but generally do not provide the massive bulk capacitance required for some applications such as switching power supplies.

Watch the dielectrics tho for your applications.

               
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2004\09\24@100949 by Spehro Pefhany

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At 01:04 AM 9/25/2004 +1200, you wrote:
>>Tantalums are NOT generally recommended for power supply or bypass
>>applications. If you must use them, they should be greatly derated in
>>voltage, but IMHO, it's best to avoid them entirely in such applications.
>
>Agree.
>Tantalum make excellent decoupling capacitors ****PROVIDED*** that you can
>absolutely guarantee that their voltage rating is not exceeded by even the
>shortest duration voltage spike. Almost always you can't. When they do
>fail they do it really really well. Some or all of smoke, smell, noise,
>smoke, explosion, flame. And they often short to a very very solid short.
>Best avoided.
>
>        RM

It's worse than that. They can and do fail catastrophically when peak current
into or out of them exceeds a fairly low value. They are okay in very limited
situations- perhaps on the regulated side of a 78L05. Otherwise, you may get
enough reliability by the recommended (for example) minimum 3:1 voltage
derating.
For example, on a 10V circuit, use a 35V minimum cap, and it *might* be okay.

They should not be used directly across batteries, high current power
supplies,
or other low-Z sources without a resistor in series (which makes them lousy
bypass caps). See all the fine print in the manufacturer's data. The one I'm
looking at right now (from a major manufacturer of tantalums) says:

"Do not use tantalum electrolytic capacitors in power supply circuits".

You don't get much plainer talk than that out of manufacturers.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2004\09\25@012038 by hilip Stortz

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actually, for a very long time tantalum caps were the standard for power
supply decoupling, especially in analog circuits where they are still
often useful.

however, high value ceramics have come out, and are better for power
supplies (at least for switching power supplies) and are probably now
also still useful in a lot of analog applications.  i don't think
tantalums were ever big for digital stuff other than possibly on old
large ram chips.  tantalums are still great for many things, and ceramic
caps are still more expensive in the several uf to tens of uf range. given equal price, ceramics would be preferred, just as they or some
other cap will displace aluminum electrolytic caps some day, as the
"super caps" have for very high value low voltage caps.  note however
that many of the new high value ceramic caps have a poor temperature
performance/value change, i think tantalums still have them beat there
as well.  i believe you can also get tantalums with more precise values
than you were able to get ceramics in previously and possibly still
without paying more.  cap technology has always been one of those
strange, ugly and complex situations.  i've used plenty of tantalums in
power supplies etc. and never had problems with any of them when they
weren't put in backwards, but i've had that same problem with aluminum caps.

Michael Rigby-Jones wrote:
>
> >{Original Message removed}

2004\09\25@073432 by Howard Winter

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On Sat, 25 Sep 2004 01:04:39 +1200, Russell McMahon wrote:

> > Tantalums are NOT generally recommended for power supply or bypass
> > applications. If you must use them, they should be greatly derated in
> > voltage, but IMHO, it's best to avoid them entirely in such applications.
>
> Agree.
> Tantalum make excellent decoupling capacitors ****PROVIDED*** that you can
> absolutely guarantee that their voltage rating is not exceeded by even the
> shortest duration voltage spike. Almost always you can't. When they do fail
> they do it really really well. Some or all of smoke, smell, noise, smoke,
> explosion, flame. And they often short to a very very solid short. Best
> avoided.

Dammit!  Anyone want to buy a box of mixed-value tantalums?  :-(


Howard Winter
St.Albans, England


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2004\09\25@102140 by Bob Ammerman

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Many years ago I built a Z80 single-board computer called the (Ferguson) Big
Board. This 8" x 10" board contained everything one would typically find in
an S100 bus system of the same time period. There were many tantalum caps
used for power supply bypass. I remember this because I installed one of
them backwards :(

Bob Ammerman
RAm Systems

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2004\09\25@103739 by Dave VanHorn

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At 09:07 AM 9/25/2004, Bob Ammerman wrote:

>Many years ago I built a Z80 single-board computer called the (Ferguson) Big
>Board. This 8" x 10" board contained everything one would typically find in
>an S100 bus system of the same time period. There were many tantalum caps
>used for power supply bypass. I remember this because I installed one of
>them backwards :(

I kinda remember the big board.  Mine was an Ampro Little board. 4 MHz Z80, with SCSI hard drive interface. Mounted on top of a floppy. Ran ZCPR  

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2004\09\25@115602 by Morgan Olsson

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Howard Winter 13:34 2004-09-25:

>On Sat, 25 Sep 2004 01:04:39 +1200, Russell McMahon wrote:
>
>> Tantalum make excellent decoupling capacitors ****PROVIDED*** that you can
>> absolutely guarantee that their voltage rating is not exceeded by even the
>> shortest duration voltage spike. Almost always you can't.

?  Almost always we do, as other parts like semiconductors also react to very short spikes.  What is different?

I have never had any tantalum blow, and on things I repair the problem of dried electrolytics is more frequent than shorted tantalums.  Also, i have replaced a handful of cracked multilayer chip caps, but unly a couple tantalums.  I dont have the same kind of experineces like you seem to have...

/Morgan
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2004\09\26@043804 by Russell McMahon

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>>> Tantalum make excellent decoupling capacitors ****PROVIDED*** that you
>>> can
>>> absolutely guarantee that their voltage rating is not exceeded by even
>>> the
>>> shortest duration voltage spike. Almost always you can't.

> ?  Almost always we do, as other parts like semiconductors also react to
> very short spikes.  What is different?

> I have never had any tantalum blow, and on things I repair the problem of
> dried electrolytics is more frequent than shorted tantalums.  Also, i have
> replaced a handful of cracked multilayer chip caps, but unly a couple
> tantalums.  I dont have the same kind of experineces like you seem to
> have...

It's not just my experiences, but I have personally seen the failure modes
that others tell about.

The key problems are these:

- When a tantalum capacitor is connected to a "high energy" source (such as
a power supply rail) and is subject to an over-voltage spike of relatively
low magnitude, the capacitor can (and often does) fail short circuit and
allow destructive high energy dissipation in the capacitor. It's not the
spike energy that causes the destruction - all the spike needs to do is
start the initial breakdown and the power supply does the rest. Other types
of capacitors are nowhere as near as sensitive to fault initiation by over
voltage and do not have the catastrophic failure mode. eg Aluminium wet
electrolytics (the standard electrolytic capacitor) can take significant
overvoltage or limited reverse polarity without catastrophic breakdown.

- The failure mode is often (usually?) a very hard short circuit. A bead of
metallic tantalum forms between the leads. This clamps the power supply to
ground and can be quite hard to find. On a board with many tantalums used
for power supply decoupling with one only shorted to ground,troubleshooting
without suitable tools can be difficult. (A current tracer or very low ohm
meter maybe required - or unsolder each one in turn and test it :-(  ).

- As well as short circuiting on failure the capacitor may do some or all of
smell REALLY bad, smoke, shriek, emit a jet of fire or explode. I have seen
a single tantalum capacitor do all 6! :-)

A tantalum capacitor connected to a high energy supply can be thought of as
a randomly operated power supply crowbar.
If you want a product that generates service calls out of warranty, use
tantalum decoupling caps and set the warranty period as short as possible
:-).



       RM


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2004\09\26@084437 by Bob Ammerman

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> At 09:07 AM 9/25/2004, Bob Ammerman wrote:
>
> >Many years ago I built a Z80 single-board computer called the (Ferguson)
Big
> >Board. This 8" x 10" board contained everything one would typically find
in
> >an S100 bus system of the same time period. There were many tantalum caps
> >used for power supply bypass. I remember this because I installed one of
> >them backwards :(
>
> I kinda remember the big board.  Mine was an Ampro Little board. 4 MHz
Z80, with SCSI hard drive interface. Mounted on top of a floppy. Ran ZCPR
>

My big board drove two 8" floppy drives ($500 each). It was mounted in a 11"
high rack mount case and included a honking big linear power supply. I
actually built a 3' x 8' desk that had a  built in rack to mount the
computer. In fact, that is the desk that I use to this day.

Bob Ammerman
RAm Systems

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2004\09\26@092111 by Russell McMahon

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>> >used for power supply bypass. I remember this because I installed one of
>> >them backwards :(

> high rack mount case and included a honking big linear power supply. I

Combination of those two can lead to vast dissipation in the tantalum and a
really good explosion :-).



           RM

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2004\09\26@112700 by Morgan Olsson

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Russell McMahon 10:11 2004-09-26:
>- The failure mode is often (usually?) a very hard short circuit. A bead of metallic tantalum forms between the leads. This clamps the power supply to ground and can be quite hard to find. On a board with many tantalums used for power supply decoupling with one only shorted to ground,troubleshooting without suitable tools can be difficult.

To detect trace- component shorts I use to set a power supply to a couple hundred millivolts, one amp current limit, then inject it using hook probes at strategic places, and detect which way the curent goes by measuring voltage drop separately using microvoltmeter at two points along the traces.

When measuring delicate cirquits, place a decent schottky to violtage limit the power supply.  I remember once repairing an old HP calculator, and when i was finished i shut of the soldering station, and then the calculator died permanently.  Researching it I found that the bench power supply (an old Mascot 7xx IIRC) was sensitive to the glitch that comes when shutting off the soldering transformer, outputting a spike of up to 15V, and the calculator was designed for 3V.  That evening I ripped out the power supply card and exchanged for for a couple 78xx!

>- As well as short circuiting on failure the capacitor may do some or all of smell REALLY bad, smoke, shriek, emit a jet of fire or explode. I have seen a single tantalum capacitor do all 6! :-)

Seen that on a mosfet that was supposed to shunt regulate 400VDC large capacitor ;)  Then that conducting plasma fed high voltage through the air to the controller board above.  More popping... Grand strike...

>If you want a product that generates service calls out of warranty, use tantalum decoupling caps and set the warranty period as short as possible :-).

I only have had returns on semiconductor failures, and on several years old designs the power supply electrolytics.  Never a tantalum - but att design time i am as careful about their rated voltage as I am with the semiconductors, and never allow a design that might have spikes.

Anyway, if affordable and there is room, the dry aluminium caps can often be an reliability upgrade from both smala aluminium caps and tantalums.  But also sometimes large ceramic caps can do the same.

/Morgan
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2004\09\26@113732 by Richard Benfield

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Are you sure it forms a bead ?
I used to use tantalum electrodes on copper vapour lasers because if I
remember correctly tantalum had a melting point of over 3000C (that's C not
F!)



{Original Message removed}

2004\09\26@172445 by William Chops Westfield

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On Sep 26, 2004, at 1:11 AM, Russell McMahon wrote:

> - When a tantalum capacitor is connected to a "high energy" source
> (power supply rail) and is subject to an over-voltage spike of
> relatively low magnitude, the capacitor can (and often does) fail
> short circuit and allow destructive dissipation in the capacitor.

Perhaps the problem is no so much tantalum itself, but the trend
to put as much capacitance as possible in a package by fine-tuning
the voltage rating (dielectric thickness, presumably.)  Those 4V
470uF tantalums are very impressive, I'm sure, but perhaps it was
a bad idea to expect them to work on your 3.3V power supply rail.

Likewise, I wonder if the older tantalums (16V and higher?) had
the same failure mode, or whether they were just inherently made
with more leeway in their voltage spec...

And then there's the fact that it's hard to tell whether those little
SMT packages sprinkled all over the power supply rails are tantalums,
or something newer (niobium, aluminum, etc) - WHEN I WAS A BOY...
you could tell whether a cap was tantalum just by looking at it.

If your (reasnoably overrated) tantalums are exploding on your
power supply rail because of over-voltage spikes, perhaps you
have more serious problems than the failure characteristics of
the caps themselves...

BillW

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2004\09\26@210120 by Russell McMahon

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> Are you sure it forms a bead ?
> I used to use tantalum electrodes on copper vapour lasers because if I
> remember correctly tantalum had a melting point of over 3000C (that's C
> not
> F!)

I'm sure that they often make a very very very good short circuit. This is
standard lore. Whether it is pure metallic tantalum I don't know.


       RM

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2004\09\26@210121 by Russell McMahon

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> On Sep 26, 2004, at 1:11 AM, Russell McMahon wrote:
>> - When a tantalum capacitor is connected to a "high energy" source
>> (power supply rail) and is subject to an over-voltage spike of
>> relatively low magnitude, the capacitor can (and often does) fail
>> short circuit and allow destructive dissipation in the capacitor.

> Perhaps the problem is no so much tantalum itself, but the trend
> to put as much capacitance as possible in a package by fine-tuning
> the voltage rating (dielectric thickness, presumably.)  Those 4V
> 470uF tantalums are very impressive, I'm sure, but perhaps it was
> a bad idea to expect them to work on your 3.3V power supply rail.
>
> Likewise, I wonder if the older tantalums (16V and higher?) had
> the same failure mode, or whether they were just inherently made
> with more leeway in their voltage spec...

It's a fundamental tantalum capacitor problem, at least as implenmented. It
has applied for decades.
Wet Al has a fuzzy failure mode. Tantalum is sharp and brutal. I have seen
and heard and smelt Al cap failures, but they are quite different from
tantalum ones, and AFAIK the failure mode is almost always open circuit.

> If your (reasnoably overrated) tantalums are exploding on your
> power supply rail because of over-voltage spikes, perhaps you
> have more serious problems than the failure characteristics of
> the caps themselves...

Not ever spiking them will help heaps - but it's a fundamental problem.


       RM

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2004\09\27@043623 by Howard Winter

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Russell,

On Mon, 27 Sep 2004 12:56:11 +1200, Russell McMahon wrote:

> It's a fundamental tantalum capacitor problem, at least as implenmented. It
> has applied for decades.
> Wet Al has a fuzzy failure mode. Tantalum is sharp and brutal. I have seen
> and heard and smelt Al cap failures, but they are quite different from
> tantalum ones, and AFAIK the failure mode is almost always open circuit.

I think there's a bit of an "it depends" on this!  Al caps often have an "X" scribed in the top of the can to
provide an "easy break" in case of excess pressure.  Many years ago (in a Computer Room far, far away... :-)
we had a big power supply capacitor (about the size of a baked bean can) explode, spraying the inside of the
cabinet with electrolyte and creating a cloud of smoke/gas that triggered the fire-supression system, filling
the room with CO2 (those were the days! :-)

The whole PSU had to be replaced, and if the cabinet had been open there would have been injuries to anyone
nearby.  I don't know what the failure mode was but presumably it was overheating caused by something - and a
short-circuit would be the Usual Suspect!

Burst electrolytics are by no means unknown, and the "dodgy electrolyte" problem has increased the instance of
this on PC motherboards.

Cheers,


Howard Winter
St.Albans, England


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2004\09\27@054821 by Alan B. Pearce

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>>If you want a product that generates service calls out
>>of warranty, use tantalum decoupling caps and set the
>>warranty period as short as possible :-).
>
>I only have had returns on semiconductor failures, and
>on several years old designs the power supply electrolytics.
>Never a tantalum - but att design time i am as careful
>about their rated voltage as I am with the semiconductors,
>and never allow a design that might have spikes.

I have been following this discussion with interest, as about the only large
value capacitors available for space use are tantalums. Thing is though,
that anything for use in space has to have a derating factor of 2 applied,
i.e. you can run a capacitor at only 50% of its voltage rating, a resistor
at only 50% of its wattage rating, and so on. If you don't apply this sort
of rule, and NASA or ESA catch on, and you haven't applied for a waiver,
along with some pretty good reasons for the waiver, then you are in for some
pretty lengthy meetings.

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2004\09\27@064458 by Howard Winter

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Alan,

On Mon, 27 Sep 2004 10:50:12 +0100, Alan B. Pearce
wrote:

> Thing is though, that anything for use in space has to
have a derating factor of 2 applied, i.e. you can run a
capacitor at only 50% of its voltage rating, a resistor
at only 50% of its wattage rating, and so on.

Why is this, I wonder?  Just to give a better chance of
never exceeding any ratings?  

Is there any justification for the 50% or is it just a
number out of a hat?

Cheers,


Howard Winter
St.Albans, England


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2004\09\27@072616 by Russell McMahon

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Summary: It IS as bad as I said ... :-)
               (Read the manufacturers' design guides referenced below).

_________________

{Quote hidden}

I've been privileged to experience a few Al cap explosions first hand -
including being showered with the contents. And I've seen the remains of
various others. But Tantalum is in a class of its own :-). A real
fire-cracker type explosion on occasion, plus sometimes real flame in a jet
over quite some seconds - certainly enough to set other things on fire in
the right circumstances. The Al caps that fail usually appear to "boil", and
the case rupture can indeed be reasonably violent. This is usually due to a
very old leaky cap finally giving up or because a cap has been severely
over-rated over time. eg the two caps used in series across rectified mains
to form half of a bridge switch in many PC psu's tend to fail nicely,
partially  because one getting well out of spec as it ages places extra
stress on the other one.

But none of this is the main point I think. What matters is that even a new
tantalum can be triggered by quite minor low energy excursions above rated
voltage whereas aluminium electrolytics seem to need either gross abuse or
prior degradation under arduous service. It's a case of ever present
inherent mechanism versus exceeding design life.

As someone noted on this thread recently - at least one manufacturer says
not to use them in power supply circuits.
All that said, tantalum capacitors have excellent characteristics and are
extremely reliable PROVIDED THAT you manage to design to avoid their special
weaknesses. Unlike wet aluminium electrolytics which should be run very near
their rated voltages, tantalums should be operated at no more than half
their rated voltage in high energy circuits.

Decided to do some Googling:

Here is an excellent 1 page summary on derating for failure reduction.
(Raytheon reliability labs). Nice picture of the inside of a "used" tantalum
capacitor :-) !
Note that the failures addressed here are lifetime ones AFTER inrush
currents have been reduced by adding a series resistance of 3 ohms per volt
!!!!

   http://www.reliabilityanalysislab.com/tl_hd_0312_ReliabilityOfTantalumCapacitors.asp

Excellent paper on failure modes and failure avoidance (make that superb
:-) )

       http://www.kemet.com/kemet/web/homepage/kechome.nsf/vapubfiles/KO-CAPUpd/$file/KO-CAPUpd.pdf

Nippon Chemicon precautions and design guide
Confirms all points I have raised.

   http://www.chemi-con.co.jp/pdf/catalog/ta-e1002g/ta-precaution-e-040129.pdf

Excellent NEC tantalum capacitor design guide

   http://www.nec-tokin.com/english/guide/cap/pdf/notes.pdf

Matsuo application note for tantalum capacitors - substantial amount on
voltage derating.

   http://www.ncc-matsuo.co.jp/pdf_e/app_tan.pdf

Dummy's intro to tantalum caps
Tells you what they are and the good points with NO mention of the bad
points

   http://www.cabot-corp.com/cws/Businesses.nsf/CWSID/cwsBUS02272001074813AM8998?OpenDocument&SITE=Tantalum_and_Performance_Metals


       RM.


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2004\09\27@074317 by Russell McMahon

face
flavicon
face
>> Thing is though, that anything for use in space has to
> have a derating factor of 2 applied, i.e. you can run a
> capacitor at only 50% of its voltage rating, a resistor
> at only 50% of its wattage rating, and so on.
>
> Why is this, I wonder?  Just to give a better chance of
> never exceeding any ratings?
>
> Is there any justification for the 50% or is it just a
> number out of a hat?

If it was applied to everything randomly it would indeed be a number out of
a hat. But it would also be a reasonable start in most cases. And in some
cases it would make things worse. eg aluminium electrolytics (sounds
familiar for some reason) should be run at near their rated voltage. Their
lifetime is reduced by running at lower voltages. Worst you can do is store
an Al-El (not an Israeli Airline) at higher temperatures while not powered
at all!

Reading the tantalum cap design guides that I just gave references to shows
that voltage derating by factors of from 3 to 5 times may be advisable in
some cases. So a factor of 2 there would be too low.

As far as resistor wattage goes, I generally try to keep resistors below
half wattage in "normal", let alone space use. A 5w free air cooled resistor
at 5w is a very hot resistor indeed.



               RM

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2004\09\27@074731 by Alan B. Pearce

face picon face
>> Thing is though, that anything for use in space has to
>>have a derating factor of 2 applied, i.e. you can run a
>>capacitor at only 50% of its voltage rating, a resistor
>>at only 50% of its wattage rating, and so on.
>
>Why is this, I wonder?  Just to give a better chance of
>never exceeding any ratings?
>
>Is there any justification for the 50% or is it just a
>number out of a hat?

I don't know the specific reason for that number, but with aluminium
electrolytics, back when I was an apprentice, we were always told that you
need to have a margin on voltage ratings. For resistors, it is a bit more
obvious - there ain't no gas to conduct heat away :))

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2004\09\27@075412 by Wouter van Ooijen

face picon face
> Is there any justification for the 50% or is it just a
> number out of a hat?

IIRC it's a figure from experience: start at 0% and increase over the
years untill no more failures can be attributed to this aspect.

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu



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2004\09\27@092649 by Morgan Olsson

flavicon
face
Alan B. Pearce 13:49 2004-09-27:
>For resistors, it is a bit more
>obvious - there ain't no gas to conduct heat away :))

Yes that 50% rule seem silly if it is general, but I guess it is just a rough path.

A 0402 SMT resistor is almost entirely colled by conduction to the PCB, so space or not should not matter much.

A larger air cooled resistor in vacuum becomes cooled only by radiation, so have only a few percent power rating, but heavily dependent on its surface radiaiton emitting properties, and environment.

Even if inside a manned spacecraft with air, the power is much decreased as the air will not circulate by heat when there is no gravity...

I think you have to revive component power capacity entirely for the actual application *then* use that 50% rule.

/Morgan
--
Morgan Olsson, Kivik, Sweden

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2004\09\27@101056 by ISO-8859-1?Q?Ruben_J=F6nsson?=

flavicon
face
When You work with intrinsic safety the component shall not operate at more than two-thirds of their rated maximum current, voltage and power.

Regards / Ruben

> > Is there any justification for the 50% or is it just a
> > number out of a hat?
>
> IIRC it's a figure from experience: start at 0% and increase over the
> years untill no more failures can be attributed to this aspect.
>
> Wouter van Ooijen
==============================
Ruben Jönsson
AB Liros Electronic
Box 9124, 200 39 Malmö, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
rubenspamspam_OUTpp.sbbs.se
==============================

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2004\09\27@113922 by Wouter van Ooijen

face picon face
> Yes that 50% rule seem silly if it is general, but I guess it
> is just a rough path.

IIRC it is used as a minimum for V-rating derating. Again IIRC there are
differnt factors for other deratings, and for specific cases (like you
mention, heat transfer in vacuum) a specific calculation has to be made,
often with a specific derating applied to the calculated value.

Wouter van Ooijen

-- -------------------------------------------
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consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


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2004\09\27@152408 by Dwayne Reid

flavicon
face
At 05:34 AM 9/25/2004, Howard Winter wrote:

>Dammit!  Anyone want to buy a box of mixed-value tantalums?  :-(

Sure!  I'll take them!

But seriously, we've used somewhere between tens of thousands to hundreds
of thousands of tantalum caps over the past the past 20 years with almost
zero failures.  Those failures that did occur were because the cap was
installed backwards.

There can indeed be problems when you abuse tantalums but if they are on
the output side of a low current regulator or zener supply, those problems
simply don't occur.

The flame and loud noise failures mentioned earlier happen when you have a
huge power supply driving the card.  I've seen a few of those failures
myself over the years - always on older computer-type stuff with a 50A or
100A power supply powering the whole system.

The systems we build operate from a simple zener regulated supply or from a
78L05 or LP2950.  These are all inherently voltage and current
limited.  And tantalum capacitors work just fine under those circumstances.

The main reason we still like using tantalums is temperature: they work
down to -40.

dwayne

--
Dwayne Reid   <@spam@dwaynerKILLspamspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

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2004\09\27@184535 by Paul Hutchinson

picon face
> -----Original Message-----
> From: KILLspampiclist-bouncesKILLspamspammit.edu [RemoveMEpiclist-bouncesTakeThisOuTspammit.edu]On Behalf
> Of Dwayne Reid
> Sent: Monday, September 27, 2004 3:24 PM
>
<snip>
>
> But seriously, we've used somewhere between tens of thousands to hundreds
> of thousands of tantalum caps over the past the past 20 years with almost
> zero failures.  Those failures that did occur were because the cap was
> installed backwards.
>
<snip>

This has been my experience as well. Over the past 25 years I've designed
tantalum caps into nearly all our products and have not had any failure
problems even on 20 year old instruments.

I never use tantalums in unregulated circuits and I always specify a voltage
rating that is 50% to 100% higher than the absolute maximum voltage of the
circuit. This level of voltage de-rating I also use for electrolytic caps so
that they work for decades.

Paul

=========================================
Paul Hutchinson
Chief Engineer
Maximum Inc.
http://www.maximum-inc.com
=========================================

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2004\09\27@191419 by hilip Stortz

picon face
small (aluminum) electrolytics can make nice firecrackers as well.  i've
seen a tech blow up 4 of them right in his face, until one hurt his ears
more than the others (me and his boss were a good 20' away, and they
were all very loud!).  at that point, he finally checked the wiring of
the harness he just tried to use (i fail to see how any one could
replace the cap after the second pop and be surprised when it poped
again without realizing that the cap was not the problem well before
blowing up 4 caps...).  any small part that can fail shorted can pop
rather nicely. to-92 transistors can as well, and so can dips under the
right conditions.  
i've used tantalums in my limited work, and they survived being hauled
around in the backs of jeeps off road in russia without any problems,
and all the power supplies were fairly good sized linear supplies.  on
the other hand, i have had one backwards on a board before, usually the
best way to find them is to apply power and carefully feel for heat
coming off the board, you can burn your' fingers, and as mentioned they
can pop, goggles might be smart.  i liked tantalums a lot, but if there
are better caps now that aren't too expensive, and it's not going in a
vehicle i'd use whatever is cheapest.  if it's going in a vehicle, then
i guess i won't use tantalum caps.

of course, fuses and current limits are always nice when a part gets put
in backwards (or hot plugged as one of my bosses insisted on doing with
everything that didn't always fry when hot plugged).

Russell McMahon wrote:
>
> >> >used for power supply bypass. I remember this because I installed one of
> >> >them backwards :(
>
> > high rack mount case and included a honking big linear power supply. I
>
> Combination of those two can lead to vast dissipation in the tantalum and a
> really good explosion :-).
-----

-- Philip Stortz--"In Germany they came first for the Communists, and I
didn't speak up because I wasn't a Communist. Then they came for the Jews, and I didn't speak up because I wasn't a
Jew. Then they came for the trade unionists, and I didn't speak up because I
wasn't a trade unionist. Then they came for the Catholics, and I didn't speak up because I was a
Protestant. Then they came for me, and by that time no one was left to speak up."
-- Martin Niemöller, 1892-1984 (German Lutheran Pastor), on the Nazi
Holocaust, Congressional Record 14th October 1968 p31636.

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2004\09\27@220115 by Russell McMahon

face
flavicon
face
Tantalums are indeed fine capacitors provided you can tame the applied
voltage or available enerrgy.
But there is analternative that has much the same advantages and lacks the
main disadvantage.
See below


       RM
_________________


A friend says:

Use solid aluminium capacitors (eg Philips/BC 128-series) - as good as
tantalums but without the need to derate for voltage and without the
anti-social failure mechanism.

I have used 10's of thousands of 47uF 6V3 SAL units on +5V rails in
industrial environments (at elevated temperatures) and NEVER had a failure.


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