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'[OT] Tantalum Heat Sensitivity'
2000\04\05@105919 by Dan Michaels

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There was a recent discussion regarding failure modes in
electrolytic caps here, and their sensitivty to heat.

Does anyone know about the relative susceptiblity of
tantalum caps to heat?

TIA,
- Dan Michaels
Oricom Technologies
===================

2000\04\05@173613 by Circuit Supply Incorporated

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General temperature range of solid tants is minus 55C to plus 80C, and if
derated they can handle 125C. Leakage current does vary with temperature.
Tants are not suitable in RC timing circuits, triggering, or phase shift
networks due to dielectric absorption. Not recommended in applications
subject to spikes or surges. Other than leakage, solid tants are not
particularly temperature sensitive, and they have one of the lowest
capacitance vs temperature characteristics among the various electrolytic
types.
Polarized units are sensitive to DC or peak AC values exceeding roughly 2
percent of their rated DC voltage.. i.e. not very forgiving of overvoltage.
Non solid type tants have wide tolerance values and similarly are not
suitable in timing circuits. They have a wide cap. range up to 1200 uf and
working V up to 350 WVDC.
Temp range is -55C to +85C and again leakage current varies with
temperature.
Hope this helps some. Terry
----- Original Message -----
From: Dan Michaels <spam_OUToricomTakeThisOuTspamLYNX.SNI.NET>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Wednesday, April 05, 2000 10:57 AM
Subject: [OT] Tantalum Heat Sensitivity


{Quote hidden}

2000\04\05@175117 by Spehro Pefhany

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At 05:34 PM 4/5/00 -0400, you wrote:
>General temperature range of solid tants is minus 55C to plus 80C, and if
>derated they can handle 125C. Leakage current does vary with temperature.
>Tants are not suitable in RC timing circuits, triggering, or phase shift
>networks due to dielectric absorption. Not recommended in applications
>subject to spikes or surges.

Yes.
You really have to be sure that the surge current is limited somehow
when used in bypass applications, otherwise they can fail catastrophically.

See the fine print in any decent tantalum cap data sheet/catalog for the
details. This generally means they should not be used across the power
supply unless you can guarantee that the current is limited to less than
the maximum rating. Derating the operating voltage can allow higher surges.

(They can be used in non-critical timing etc. circuits, just as low-leakage
aluminum e-caps can.)

Best regards,
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2000\04\05@175927 by Randy A.

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Dan:

I have used Tantalum Caps in quite a few projects and one of the applications
is the transient suppression capacitor across a 7805 etc. voltage regulator
that was running pretty much at full capacity 90% of the time therefore was
getting quite warm.  The tantalum cap was right next to the regulator/heat
sink and I never experienced any problems.  As far as I know that piece of
hardware (which was fabricated around 6 years ago) is still running with not
problems.  It is also inside a dustproof enclosure with NO ventilation, due
to some special requirements of the environment in the plant.

I hope this helps but as to the actual specs., I couldn't give you those.
Best thing to do there is to check the manufacturers sheets.

Randy A.

2000\04\05@231821 by Sean Breheny

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Sphero,

What do you mean by "the current is limited to the maximum value"? Are you
talking about inrush current which charges the caps? I know of a good
number of apps which recommend tantalum caps for bypassing because of their
low ESR.

Sean

At 05:48 PM 4/5/00 -0400, you wrote:
>See the fine print in any decent tantalum cap data sheet/catalog for the
>details. This generally means they should not be used across the power
>supply unless you can guarantee that the current is limited to less than
>the maximum rating. Derating the operating voltage can allow higher surges.
>

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2000\04\06@003604 by Spehro Pefhany

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At 11:15 PM 4/5/00 -0400, you wrote:
>Sphero,
>
>What do you mean by "the current is limited to the maximum value"? Are you
>talking about inrush current which charges the caps? I know of a good
>number of apps which recommend tantalum caps for bypassing because of their
>low ESR.

Yes, that's exactly correct.

A *minimum* series resistance as high as 2-3 ohms per volt is specified for
some capacitors. In some cases this can be reduced (for some suppliers)
to 0.1 ohm/volt but this may require some special circuitry for soft-
start (series MOSFET etc.).

In general, if you are using a low current regulator 78MXX, or 78LXX
and there is no mechanical contact etc. it's probably ok, particularly
if the cap is derated a bit. Under other conditions it may well not be.

Ripple current is a different (orthogonal) limitation, which is usually
a little more up-front in the data sheets.

Best regards,
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Spehro Pefhany --"it's the network..."            "The Journey is the reward"
EraseMEspeffspam_OUTspamTakeThisOuTinterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

2000\04\11@131549 by Dan Michaels

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Many thanks to Terry, Spehro, Randy for replying on this matter.
I did also find this statement in a book on analog design, re
tantalums:

"Reliability degrades when they are operated in low impedance circuits
so they are not suitable for power supply reservoir applications ...."

Although tantalums are vastly heat-superior, my original idea of
using them to replace some aluminum electrolytics, which filter a
7805 and are located somewhat near the 7805's heatsink, is probably
is not the best idea [I've ever had].

best regards,
- Dan Michaels
Oricom Technologies
===================
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