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'[EE]:Maintaining electrolytic capacitor'
2002\12\16@200750 by Jinx

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I'm using a 47uF cap to filter a 50mV signal on a PIC
comparator pin. Is 50mV enough to maintain the oxide
layer or is this cap going to deteriorate. Web references
to re-forming old or dead caps don't appear to give any
clues as to what voltage is actually needed to keep the
cap alive in the first place. I recall a thread about using
aluminium foil caps close to their rated voltage for best
life. Would a Ta cap be better ?

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2002\12\17@005156 by Spehro Pefhany

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At 02:07 PM 12/17/02 +1300, you wrote:
>I'm using a 47uF cap to filter a 50mV signal on a PIC
>comparator pin. Is 50mV enough to maintain the oxide
>layer or is this cap going to deteriorate. Web references
>to re-forming old or dead caps don't appear to give any
>clues as to what voltage is actually needed to keep the
>cap alive in the first place. I recall a thread about using
>aluminium foil caps close to their rated voltage for best
>life.

Don't worry about it, Jinx.

>Would a Ta cap be better ?

Not really.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2002\12\17@020522 by Vasile Surducan

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let's try to concentrate Spehro's good opinions... :)


On Tue, 17 Dec 2002, Jinx wrote:

> I'm using a 47uF cap to filter a 50mV signal on a PIC
> comparator pin.

Joe, if you dislike filtering using electrolytic capacitors then
integration using OA and not-polarised capacitors are just another
( expensive ) option.



Is 50mV enough to maintain the oxide
> layer or is this cap going to deteriorate. Web references
> to re-forming old or dead caps don't appear to give any
> clues as to what voltage is actually needed to keep the
> cap alive in the first place.


How many years do you offer guarantee for your products ? :)
With or without voltage, a good wet aluminium capacitor does not have a
life greater than 7...10 years at full capacity.


I recall a thread about using
> aluminium foil caps close to their rated voltage for best
> life. Would a Ta cap be better ?


Yes, only if you don't exceed the amount voltage drop ( in your case will
be ok ) The Ta have the avantage of filtering both low and high
frequencies noises, if you will use an inductive electrolytic capacitor then a small
non-inductive, non polarised one is also required ( usually 47...100nF )
connected in parallel with the first one.


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2002\12\17@163947 by Jinx

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> Yes, only if you don't exceed the amount voltage drop ( in your
> case will be ok ) The Ta have the avantage of filtering both low
> and high frequencies noises, if you will use an inductive electrolytic
> capacitor then a small non-inductive, non polarised one is also
> required ( usually 47...100nF ) connected in parallel with the first one.

There's a 47nF ceramic in parallel already. To lower the cost I can
increase the R in the filter so I need a smaller C. Probably will switch
to tantalum. In the overall scheme of things it's not a big cost, but the
voltage being detected and compared by the PIC is an important
part of the s/w, and all being well I'd like to see the units run for many
trouble-free years. Normally I'd have not worried about it, as Spehro
suggested, but I just happened to come across a web page the other
day that traced faulty operation of a piece of equipment to electrolytic
caps de-forming

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2002\12\17@190253 by Spehro Pefhany

picon face
At 10:39 AM 12/18/02 +1300, you wrote:

>Normally I'd have not worried about it, as Spehro
>suggested, but I just happened to come across a web page the other
>day that traced faulty operation of a piece of equipment to electrolytic
>caps de-forming

It simply isn't a problem under the conditions you describe; we have
direct experience with hundreds of thousands of electrolytics under
similar conditions. *If* a cap is stored for 20 or 30 years and then
suddenly subjected to high voltage then you might see this kind of
problem, but CERTAINLY not with 50mV, which is essentially the same
as leaving them in the shipping bag from an electrochemical POV.

As to the life of electrolytic capacitors, if your item is line-powered,
the power supply capacitors will typically fail a couple of times before
small signal parts fail- due to the higher temperatures present from ripple
current and proximity to the transformer and regulator components.
20 year life is not at all unusual for this kind of part. Most likely
the product will be discarded by then. If you're really concerned, buy
quality brand name parts, but with few exceptions (like the ones we got
directly from the China factory in which about 5% had the ENTIRE RUBBER
BUNG MISSING, they are generally pretty decent (those particular ones
dried out pretty fast though).

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
@spam@speffKILLspamspaminterlog.com             Info for manufacturers: http://www.trexon.com
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2002\12\18@030342 by Russell McMahon

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> .... all being well I'd like to see the units run for many
> trouble-free years.

In which case, if using tantalum fuses be CERTAIN that the applied voltage
NEVER exceeds their voltage ratings for any reason. Tantalums have their
place but they have to be protected against their great weakness of failing
spectacularly when over-volted.


       RM

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2002\12\18@095043 by Spehro Pefhany

picon face
At 08:54 PM 12/18/02 +1300, you wrote:
> > .... all being well I'd like to see the units run for many
> > trouble-free years.
>
>In which case, if using tantalum fuses be CERTAIN that the applied voltage
>NEVER exceeds their voltage ratings for any reason. Tantalums have their
>place but they have to be protected against their great weakness of failing
>spectacularly when over-volted.

I've never personally experienced it, but unfortunately they can stand up
long enough under REVERSE bias in a power supply situation to explode,
burn etc. later after the product has been tested and shipped. As the markings
are the reverse of aluminum caps, this has caused problems for some.
Smoke is bad.

They are also quite limited in terms of transient current capability, and
some manufacturers tell you to put an xx ohm resistor in series, which
practically kills their major application as a bypass capacitor. You
MUST never connect them to a low impedance source without some limiting
of the current. More recent ones are better, but the limitation is there
in the fine print.

Nasty and expensive little things, but they are small.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
RemoveMEspeffTakeThisOuTspaminterlog.com             Info for manufacturers: http://www.trexon.com
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2002\12\18@100535 by Alan B. Pearce

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>I've never personally experienced it, but unfortunately
>they can stand up long enough under REVERSE bias in a
>power supply situation to explode, burn etc. later after
>the product has been tested and shipped.

I can attest to this. A colleague was lucky not to loose an eye as he was
bending over a non-functioning piece of equipment just arrived from the OEM
when the teardrop tantalum went whizzing past his ear =8)

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