Best capacitor for back-up applications

A coin cell ?

Graham

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

Regards

AJ

AND

Something is puzzling about your numbers. If I've done my math correctly:

At a steady 5 uA draw, the voltage of a 200 uF cap would drop by 90V in 1 hour. I guess that would be reliable enough if it was starting out at several hundred volts or more, but somehow I doubt that is the case (you didn't object to a lithium coin cell having too low a voltage ...)

Perhaps you could give more details about your application to help get a useful answer.

Mark

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.

Graham

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).

Chris

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

I don't think they come significantly smaller.

Graham

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

Digikey # 565-2588-ND

As Graham said, coin cells are around 0.30 $US:

Regards,

Mark

Yes, from what the OP has written, it looks like the coin cell would be better for his application. Still, he asked for a type of capacitor, and if (for some reason that I do not know) he really did need a capacitor, then in answer to his question I think that those goldcap things would be the most practical kind of capacitor.

Chris

They do, I have very occasionally seen them inside appliances.

Chris

True enough. Hopefully he now has enough information to decide which would work better for his application.

That's possible, though it seems he was a few orders of magnitude off on the capacitance value he thought he needed. Never did tell us what the voltage was, though, so we can't even be sure the Lithium coin cell would work for him.

Went back and checked out your link. Hey, those look pretty good too. Searched at Digikey for "gold cap" and came up with a 0.1F version for 2.33 $US, size is 0.53" dia. x 0.3" thick:

Regards,

Mark

Thanks for the replies guys..... I will go with the coin cell.

Best regards,

AJ