[sdiy] capacitor ID

[harry]

Yes... Tony is a (self-proclaimed) nut... or at least I agree that I would
refer to him as such   ;^P

Tantalums have the advantage of extremely LOW ESR (equivalent series resistance)
a parasitic (as in parasite) that you wish your caps did not have.  They are
extremely
small, so they can be closer to the ICs (if done right) and the series inductance

(another parasitic) can be kept small.

Because Tony likes aluminum electrolytics... he uses bigger ones in part to
compensate for their much larger ESR and series inductance...  and because
(as he said) 1uF is just too small, unless there are a lot of them in total....

Tantalums have become very expensive (tantalum is a rare element these days...)
and higher values and voltages go up exponentially in cost !!!

And they are really unpleasent when they die... mostly going shorted and getting
very hot... often hot enough to melt the solder on their own leads (if the supply
will
deliver enough current...)

But (nay-say) I disagree with Tony that they go for "no reason".  Almost evert
time
I have seen a tantalum go POP... it is the result of being installed
backwards....
or subject to reverse polarity, even in transient.  Maybe a bad ground situation,

a power up sequencing issue, a power supply plug that can short when inserted...

I agree, double the supply voltage.  If you use +/-15 then 35V is correct. There
is always the possibility a circuit fault might put the difference of the supply
(30V) across a cap, and you like to limit subsequent damage.

So lets go with like a 10uF/35V and if you want to switch to an
electrolytic...well
OK. It will save a pile of money and be only a little bigger.

OTOH lytics are known to fail as well   ;^P

H^) harry

Tony Clark wrote:

> > >    Definately a tantalum (read _EVIL_!) 1uF 35V.  Use Electrolytic in
> > > stead if you really want to save yourself some future headaches!
> >
> > what's bad about them? quality? tolerance?
>
>    Well of all the capacitors I've used, tantalums seem to be the worst.
> They all have this wierd ability to spontaneously short out for no
> apparent reason.  Sure other caps aren't without their faults, but most
> only go out of spec after a time, which is why we have the wonderful term
> "recapping".
>
> > >    Looks like tantalums again, so it's just like the first set, 1uF.
> > > Again, you might want to find something else to replace these with.
> > > More than likely these are doing nothing more than filtering the supply
> > > line to the IC's, so it would do to add a beefier part here anyway (you
> > > really don't want these shorting out the power line!).  Again I'd go with
> > > electrolytic.
> >
> > beefier as in 'different material' or as in 'higher value'?
>
>    Higher value.  Higher voltage ratings aren't necessarily a good idea
> as larger voltage ratings usually mean poorer performance at lower
> voltage levels.  That being said, typically aim for twice the rated
> supply voltage (+/-15V use 35V rated caps).
>    Anyway, back to value, 1uF is a little small in my book for a IC filter
> cap.  I'd aim for anything 10uF or higher (I use 47-100uF myself but then
> I'm a nut (according to Harry!)).  However if the supply lines are
> relatively clean, then it probably won't matter much what value you stick
> there!
>    Again, this is going under the assumption that those are being used as
> supply filters.  :)
>
>    Cheers,
>
>    Tony
>
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> "We wouldn't want to ship something that doesn't work"
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>      Excerpt from EE Times April 2, 2001

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