Re: Best capacitor for back-up applications

On Aug 5, 5:54 pm, Chris Jones <lugnut...@xxxxxxxxxxxxxxxx> wrote:
Eeyore wrote:

Chris Jones wrote:

Eeyore wrote:
AJ wrote:
"Eeyore" wrote
AJ wrote:

I was just wondering what the best type of capacitor I should use
for a
back-up application (> 5uA). I am looking for something surface
mount around the 100-330uF range with a low profile and as cheap as
possible. Any
suggestions?

A coin cell ?

I would rather use a cap as I only need an hour or so of back-up time.

Well..... 330uF won't do it.

dV/dt = I/C = 5uA/330uF = 15mV/sec

In even 100 seconds, the voltage on the cap will have dropped by 1.5V.
So to get a reasonable voltage drop after an hour, you're probably
looking at ~ 33000uF. In other words nothing small.

Use a coin cell.

Or one of those super-capacitors that they put in VCRs for running the
clock
during power cuts. You can get them in values between 0.1 Farad and a
couple of Farads. They are usually cylindrical about 20mm or 3/4 inch
diameter and about 12mm high (half inch).

http://www.pollin.de/shop/shop.php?cf=detail.php&pg=OA==&a=NTc0OTg3OT...



That's hardly small is it ?

They're expensive too. A coin cell can be had for ~ 30c.

Graham

Well if you don't need a whole Farad, then it would be smaller. Whether it
is worthwhile probably depends on whether the power drain is such that
being rechargeable would be an advantage.

Chris

Recharging shouldn't be necessary for the battery, though it would
need replacement after it's 5-10 year shelf life has expired.
He's only drawing ">5 uA" for "an hour or so" at a time. Let's use 10
uA and 2 hours for a conservative estimate. That's 0.02 mA-hours.
A 2032 coin cell has about 200 mA-hr of charge, which equates to 10
thousand outages-worth. It will self drain, sitting there over the
course of it's shelf life, rather than become discharged from the
outages.

As for the cap, roughly 100,000 uF would be required (ballpark
calculation, based on 3V, 10 uA, and dropping by 25% of the 3V over 2
hours). Digikey has them for 7 to 10 $US, not too cheap, and not too
small at over an inch in size:

Digikey # P10002-ND
100,000 uF, 16V, 35mm dia. x 80mm long, 10.11 $US
http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=P10002-ND
http://www.panasonic.com/industrial/components/pdf/PIC_T-UP_SERIES_DNE.pdf

Digikey # 565-2588-ND
100,000 uF, 6.3V, 35mm dia x 45 mm, 7.15 $US
http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=565-2588-ND
http://rocky.digikey.com/WebLib/United%20Chemi-Con/Web%20Data/SMH.pdf

As Graham said, coin cells are around 0.30 $US:
http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=P189-ND

Regards,

Mark


.

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