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'[OT] From: Fire in the Hole - Tantalums'
2000\04\27@133618 by Dan Michaels

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Alan B Pearce wrote:
.....
>A colleague of mine was even luckier than this. A brand new piece of equipment
>arrived, but failed to work on power up. He had the lid off it, and with the
>power still on was bending over it looking at the circuit board. A teardrop
>tantalum decided just at that moment to give up the unequal struggle and flew
>past his ear!!! He was extremely lucky it was not his eye!!!
>

I wonder if lots of others aren't making the same mistake we
talked about a couple of weeks ago in another thread. Namely,
using tantalum caps for "power buss filtering'.

To re-iterate, for the sake of all those who do the same thing
as I too have done in the past:

"Reliability degrades when they are operated in low-impedance
circuits, and so they are not suitable for power supply
reservoir operations".

ref, p 83, "The Circuit Designer's Companion", Tim Williams,
1996, Newnes pub.

2000\04\27@140930 by William Chops Westfield

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   To re-iterate, for the sake of all those who do the same thing
   as I too have done in the past:

   "Reliability degrades when they are operated in low-impedance
   circuits, and so they are not suitable for power supply
   reservoir operations".

   ref, p 83, "The Circuit Designer's Companion", Tim Williams,
   1996, Newnes pub.

Really?  I see them used this way constantly, even recomended (particular
brands and values, usually) in SPS chip app notes, and advertised for this
purpose (ie by ESR, IIRC.)  Are SOME tantalum caps specifically (re)designed
for such applications, or are some other specific types (perhaps no longer
very common) the ones that are unsuitable there.

(come to think of it, I don't know that I've ever seen tantalum caps widely
used for anything BUT power bypass...)

BillW

2000\04\27@150312 by Dan Michaels

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>
>    "Reliability degrades when they are operated in low-impedance
>    circuits, and so they are not suitable for power supply
>    reservoir operations".
>
>    ref, p 83, "The Circuit Designer's Companion", Tim Williams,
>    1996, Newnes pub.
>
>Really?  I see them used this way constantly, even recomended (particular
>brands and values, usually) in SPS chip app notes, and advertised for this
>purpose (ie by ESR, IIRC.)  Are SOME tantalum caps specifically (re)designed
>for such applications, or are some other specific types (perhaps no longer
>very common) the ones that are unsuitable there.
>
>(come to think of it, I don't know that I've ever seen tantalum caps widely
>used for anything BUT power bypass...)
>
>BillW
>

Someone else mentioned the issue referenced above, ie use in
"... low-impedance circuits...", in another thread recently. But I
know what you are saying. I am still fuzzy on the issue. But then
we just heard 27 'Fire in the Hole' stories about them exploding too.

DanM

2000\04\27@184243 by paulb

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William Chops Westfield wrote:

> Really?  I see them used this way constantly, even recomended
> (particular brands and values, usually) in SPS chip app notes, and
> advertised for this purpose (ie by ESR, IIRC.)  Are SOME tantalum caps
> specifically (re)designed for such applications, or are some other
> specific types (perhaps no longer very common) the ones that are
> unsuitable there.

 It sounds like there are two varieties of tantalums available.  The
ones that you can use, and the ones you can't.  Can probably be guessed
by the trademark thereon.

 Just like all other components.

 Story begins.

 OK, I'm not a professional (I don't make my money engineering PICs
into products).  In fact, I'm a scavenger.  I scavenge PC parts, in fact
I picked up some PSUs at the Wyong Field Day last February.  Just got
them out of storage this week (over the Easter public hols - the only
sort of hols I actually take).  Two are ATX and I want to refurbish them
for testing purposes.

 One of the others caught my attention - it is tagged "internal short"
which to my mind is an easy fault to suss.  I started by buzzing out the
connections to the plug and socket on the back - no immediate short
found.  Proceeded to verify connections between these and the cable tail
to the power switch.  Aha!  Colours don't match properly.  In fact,
there's a wiring diagram on the PSU label which if followed on *this*
particular specimen, places one switch pole across the input plug and
the other across the output socket.

 Say no more!  I haven't tested the unit yet (have to find a switch)
with the wiring carefully rearranged, but I believe I have a $2 power
supply, absolutely new and never used.  I will of course have to open it
and re-connect the wires correctly.

 But that wasn't the real story.  At least once, the fault in a reject
power supply has been the one ohm inrush current limiting resistor, a 5W
ceramic "wire wound" type which sometimes fails silently, other times
with a bang and distributes sand and shards inside the supply cage.

 Why do I quote "wire wound"?  Well, the lesson I learned when I
replaced a resistor which failed silently, when I bought another from
Dick Smith locally, was that the replacement I bought, like the
original, was unbranded and did not even survive one turn-on.

 A *real* resistor with the (Australian) "IRC" (intertwined with an
omega symbol) is to the best of my knowledge happily working to this
day.  I just don't know in which of my machines it is. :)

 It seems if you buy non-components, and that's what are being sold in
profusion at present, you can expect non-performance.

 Story ends - sorry if I told it before within easy memory.
--
 Cheers,
       Paul B.

2000\04\27@215148 by Russell McMahon

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The problem. as I understand it is two fold

- Tantalums fail catastrophically at a voltage slightly in excess of their
rated value.

   Energy of excess voltage spike is irrelevant

- Failure mode is a metallic short circuit

If your power bus never has voltage spikes in excess of the tantalum cap's
rating then fine.
Unfortunately, the world I live in usually can't guarantee this condition.
Mains has transients.
Car battery supplies  has transients.
External noise can induce transients in most anything else sometimes.

As a friend of mine used to say "Tantalum Caps - fastest fuse on two legs" -
really fastest crowbar on 2 legs.

As I have noted here before - with a little luck you can get noise, smell,
smoke, fire and explosion from a single tantalum failure. I have. Lots of
fun if its on the test bench. Worse if its inside some equipment.

I personally would never use a tantalum cap in a high energy circuit.
SOLID Aluminum electrolytic caps don't have this problem and are about as
good.
These are rarely seen but are available.
Philips have some.



     Russell McMahon
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