Aluminum Electrolytic vs Tantalum Capacitors
bmac...@fox.nstn.ns.ca
to procure than electrolytic capacitors. Some older designs specify
tantalum and I would like to replace with aluminum electrolytic. What
are the disadvantages in doing this? Also are there any good reference
materials that compare the different types of capacitors?
--
Bruce MacIsaac
bmac...@fox.nstn.ns.ca
John Guy
using. Generally, for decoupling power supplies, a tantalum cap (2.2 to 10 uF)
in paralell with a 0.1uF ceramic cap work very well. The problem with both the
tanatalum and the electrolytic caps is that they don't behave like capacitors at
high frequency. Therefore, the additional ceramic cap for decoupling. Additionaly,
some of the more careful designers will put a ferrite bead in series with the
supply line, separatimg the two caps, with the ceramic cap on the component side
of the bead. This dampens potential problems from the relatively low self
resonant frequency of the tantalum caps. I believe Horowitz and Hill discuss
cap types in depth, and include a nice selection chart.
Frank Miles
> Greetings. Is it just me or do tantalums have this nasty habit of
> failing more often in shorted mode than open (like electrolytics)?
>
Yes, shorting is a common failure mode with tantalums. These days, the small,
high-quality aluminum electrolytics are very nearly as good as tantalums,
so there is rarely a need to spend for the Ta. Once upon a time, aluminum
electrolytics had fairly large equivalent series inductance in comparison
with tantalum electrolytics. This is no longer the case with most "small"
Al's, as you may verify yourself if you have access to some high speed TDR
equipment. There may be some small differences in series resistance which
might be important in IF strip design (though I haven't had any problem
tracable to this).
OTOH tantalums do have somewhat more consistent behavior in reverse-bias
(not the recommended mode), and don't dry out over the decades like aluminum.
As far as the tantalum shorting problem: the manufacturers used to specify
that you have at least 3 ohms/volt of series resistance between the power
supply and the capacitor in order to allow the self-healing mechanism to work.
For most uses, this requirement greatly reduces the applicability of the device.
In practice, I have found that different batches of Ta capacitors have
different ability to save themselves from the pin-hole failures. One company
I once worked for ended up repairing almost all products built with a
particular batch of Ta C's --- these had not had the protection R built in;
so this is not idle speculation! More modern Ta C's may have a smaller
protection R requirement; and as far as I know, no similar requirement exists
for Al C's. Anyone have more up-to-date info?
-frank
Filip M Gieszczykiewicz
> Tantalum capacitors appear to be more difficult and more expensive
> to procure than electrolytic capacitors. Some older designs specify
> tantalum and I would like to replace with aluminum electrolytic. What
> are the disadvantages in doing this? Also are there any good reference
> materials that compare the different types of capacitors?
Greetings. Is it just me or do tantalums have this nasty habit of
failing more often in shorted mode than open (like electrolytics)?
What's even weirder is that tantalums short so well that they
blow surrounding circuits rather than themselves! Electrolytics
usually do the "rocket" if they short and then "open" :-)
Take care.
--
+-->Filip "I'll buy a vowel" Gieszczykiewicz | E-mail: fi...@smi.med.pitt.edu
| http://alpha.med.pitt.edu:9000 for new sci.electronics FAQ V2 & my home page
| Enjoy your job, work within the law, make lots of money : Choose any two.
| Making money with CS and spending it on EE, robotics, SCA, & dreams. NOT(PC)
Bob Martin N6MZV
fi...@alpha.smi.med.pitt.edu (Filip M Gieszczykiewicz) wrote:
> <bmac...@fox.nstn.ns.ca> writes:
> > Tantalum capacitors appear to be more difficult and more expensive
> > to procure than electrolytic capacitors. Some older designs specify
> > tantalum and I would like to replace with aluminum electrolytic. What
> > are the disadvantages in doing this? Also are there any good reference
> > materials that compare the different types of capacitors?
Tantalum (slug tantalum) caps often have a lower esr (equivalent series
resistance) than other caps. Makes them better for high frequency use,
especially in switch mode supplies. They also can pack more capacitance
into a smaller volume.
> Greetings. Is it just me or do tantalums have this nasty habit of
> failing more often in shorted mode than open (like electrolytics)?
Tantalum caps definitely have less of a sense of humor than other types,
and usually register their disgust by transcending this mortal plane. You
need to pay attention to the voltage rating on tantalum caps.
The good news is, it's usually easy to spot what went out.
> What's even weirder is that tantalums short so well that they
> blow surrounding circuits rather than themselves! Electrolytics
> usually do the "rocket" if they short and then "open" :-)
Like I said- very low series resistance; especially when they short out!
How well surrounding circuitry survives tantalum tantrums is another issue.
73-
--
Bob Martin * r...@newton.apple.com
N6MZV * N6MZV @ N0ARY.#NOCAL.CA.USA.NOAM
Clifton T. Sharp
> Greetings. Is it just me or do tantalums have this nasty habit of
> failing more often in shorted mode than open (like electrolytics)?
At one old job, we had a high current power supply (originally labeled
"Zotz 1", but mistakenly and affectionately pronounced "zotsi") which we
used to trace shorts on PC boards. Most often what would happen is that
the short would draw high current for 10 seconds or so, then there'd be a
"ftszzzzzzzzzshshshshshshshsBAM" sound and a tantalum would take flight.
It was fun. :-)
> What's even weirder is that tantalums short so well that they
> blow surrounding circuits rather than themselves! Electrolytics
> usually do the "rocket" if they short and then "open" :-)
Ever get one where the paper unrolls and makes like a streamer?
I had one blow in my first kit because it was put in backwards (just
as pictured on two different pages of the kit instructions). This was in
about 1965, and it resulted in PCBs all over the kitchen ceiling, my hair,
the refrigerator...
--
Cliff Sharp When you starve with a tiger, the
WA9PDM tiger starves last.
cli...@indep1.chi.il.us --The Second Law of Reality
Stuart Friedberg
Clifton T. Sharp <cli...@indep1.chi.il.us> wrote:
> Ever get one where the paper unrolls and makes like a streamer?
>
> I had one blow in my first kit because it was put in backwards (just
>as pictured on two different pages of the kit instructions). This was in
>about 1965, and it resulted in PCBs all over the kitchen ceiling, my hair,
>the refrigerator...
I had a similar experience. As a early teen, I had just finished off a
Heathkit 400V power supply (tube era, folks) on a bench in the basement
and applied power for the first time, when my mother came down the
basement stairs and asked how things were going. Right on cue, the
top blew off one of the electrolytics, spraying nasty stuff all over
the basement ceiling. I think I said something intelligent like
"I think I should turn this off now." :-) Mom was very understanding. :-)
Stu Friedberg (stu...@sequent.com)