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'[EE]: Tantalums cap for power supply regulation'
2005\09\04@131111 by Daniel Chia

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Hi,
       SMD tantalums seem to offer a great advantage in terms of size
over normal electrolytic caps in stabilising a power supply. However I'm
concerned with voltage spikes killing the cap.

       Basically my application is a small robot with 2 motors
(Minimotor 1724 6volts version) with a current draw of around 300mA
each, through a MOSFET H-bridge. Power is provided by a LiPo pack
providing 7.4V nominal. What I'm concerned about is whether a 47uf 20V
SMD tantalum will help stabilise the supply adequately for my 5V LM2940
reg as well as whether motor spikes might kill the cap.

       Comments anyone?

Thanks a lot in advance,

------------------------------------------------------------------------
Daniel Chia

"Genius is one percent inspiration and ninety-nine percent
perspiration."

    - Thomas Edison

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2005\09\04@133716 by David Van Horn

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>        SMD tantalums seem to offer a great advantage in terms of size
> over normal electrolytic caps in stabilising a power supply. However
I'm
> concerned with voltage spikes killing the cap.

Tants hate large inrush currents, and this sounds like an app that would
have a fair bit of them.

Check the specs, and look at what the real inrush current could be,
figuring real impedances and lead inductances.



2005\09\04@144550 by Daniel Chia

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Hi,
       Sorry for my ignorance, but could you please elaborate on what
you mean by inrush currents and how would one go about figuring them
out?

------------------------------------------------------------------------
Daniel Chia

"Genius is one percent inspiration and ninety-nine percent
perspiration."

    - Thomas Edison

E-mail: danielcjhspamKILLspamyahoo.com.sg
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> {Original Message removed}

2005\09\04@151607 by Juan Cubillo

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I have always used Ceramic Disc 0.1uF caps as much as I can all over my
projects and they seem to work great. Adding an extra 47uF electrolitic on
the ouput of voltage regulators is also a big help.
Juan Cubillo


2005\09\04@155242 by Spehro Pefhany

picon face
At 02:52 AM 9/5/2005 +0800, you wrote:
>Hi,
>         Sorry for my ignorance, but could you please elaborate on what
>you mean by inrush currents and how would one go about figuring them
>out?

What's the peak current that can flow through the capacitor (eg.
suppose it supplies *all* the peak motor current), or what is the
peak current that will charge the capacitor. Is it limited, and if so,
to what maximum current? Think about all possible conditions.

Generally, tantalums are most *un* suitable for power supply applications.
They are quite prone to failure, sometimes in spectacular fashion. A
low-Z aluminum is a much safer bet.

If you *must* use them, then heavily derate the voltage (use a capacitor
rated for 3 times as high as the working voltage), and follow all data
book recommendations to the letter.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2005\09\04@163430 by Russell McMahon

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> Generally, tantalums are most *un* suitable for power supply
> applications.
> They are quite prone to failure, sometimes in spectacular fashion. A
> low-Z aluminum is a much safer bet.
>
> If you *must* use them, then heavily derate the voltage (use a
> capacitor
> rated for 3 times as high as the working voltage), and follow all
> data
> book recommendations to the letter.

Yes. This is a warning which must be noted. Tantalum capacitors are
superb when they work. They offer very high capacitance per size and
excellent ESR. But in an amateur or experimental situation where
conditions are not well behaved or well known they can fail
catastrophically.

A tantalum cap that experiences a voltage spike higher than its rated
voltage, even a very brief one, will often fail very spectacularly.
They often smell bad, make interesting sounds, smoke, catch fire or
explode in various combinations. I've seen  one do all of these in
that order :-).

Manufacturers spec sheets often specify minimum supply impedances they
should be used with and even suggest adding some ohms of resistance in
series.

A **solid** aluminium capacitor has the advantages of tantalum without
the catastrophic failure mode.



           Russell McMahon

2005\09\04@215810 by Daniel Chia

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face
Hi,
       First off, think I wasn't very clear in my post, but my motors
run off the unregulated side of the power supply. The regulated side is
for digital circuitry and such.

       Right now what I'm gathering is that the tant prolly isn't up to
the task of regulating the battery supply. Instead I probably could
consider a solid aluminium electrolyte. However I think that for the 5V
regulated side a 16V tant should do the job pretty well.

       Thoughts anyone?

------------------------------------------------------------------------
Daniel Chia

"Genius is one percent inspiration and ninety-nine percent
perspiration."

    - Thomas Edison

E-mail: danielcjhspamspam_OUTyahoo.com.sg
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ICQ: 37878331
------------------------------------------------------------------------


> {Original Message removed}

2005\09\04@222234 by Bob Axtell

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Juan Cubillo wrote:

>I have always used Ceramic Disc 0.1uF caps as much as I can all over my
>projects and they seem to work great. Adding an extra 47uF electrolitic on
>the ouput of voltage regulators is also a big help.
>Juan Cubillo
>
>
>  
>
That's been my experience, too. Using ceramic caps then adding tantalum
electrolytics
in parallel seems to utilize the best of both technologies, i.e. no
problems with failures
of tantalums due to spiking.

--Bob



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2005\09\06@092926 by alan smith

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Might also look at using Os-Con type caps....better than electrolytics at least

Russell McMahon <RemoveMEapptechTakeThisOuTspamparadise.net.nz> wrote:> Generally, tantalums are most *un* suitable for power supply
> applications.
> They are quite prone to failure, sometimes in spectacular fashion. A
> low-Z aluminum is a much safer bet.
>
> If you *must* use them, then heavily derate the voltage (use a
> capacitor
> rated for 3 times as high as the working voltage), and follow all
> data
> book recommendations to the letter.

Yes. This is a warning which must be noted. Tantalum capacitors are
superb when they work. They offer very high capacitance per size and
excellent ESR. But in an amateur or experimental situation where
conditions are not well behaved or well known they can fail
catastrophically.

A tantalum cap that experiences a voltage spike higher than its rated
voltage, even a very brief one, will often fail very spectacularly.
They often smell bad, make interesting sounds, smoke, catch fire or
explode in various combinations. I've seen one do all of these in
that order :-).

Manufacturers spec sheets often specify minimum supply impedances they
should be used with and even suggest adding some ohms of resistance in
series.

A **solid** aluminium capacitor has the advantages of tantalum without
the catastrophic failure mode.



Russell McMahon

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