Memory backup capicitor as timing element

I am looking for a 15 minute timed drop out with minimum parts count. I was thinking about one of those half farad or so "super caps" they
use for mermory backup. If the load was 2k ohms or so wouldn't this get me out in the 800-900 second range? The timing is not really critical, 10-15 minutes would be fine.
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On Sat, 20 Jan 2007 01:29:28 -0500 snipped-for-privacy@aol.com wrote:
| I am looking for a 15 minute timed drop out with minimum parts count. | I was thinking about one of those half farad or so "super caps" they | use for mermory backup. If the load was 2k ohms or so wouldn't this | get me out in the 800-900 second range? The timing is not really | critical, 10-15 minutes would be fine.
Do you really need to use a low resistor like that? You can get a long time with a higher resistor and a smaller capacitor. And I know you know that so there must be some reason you're aiming at this range of C and R.
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snipped-for-privacy@aol.com wrote:

Maybe. The tc is 1000 (r*c) at which point the voltage across the cap will have dropped to about 1.85 volts from the original 5 volts. The drop is not linear, so the question is what voltage does the 2K load need during the 10 - 15 minutes?
If you have power available, you can use a circuit to shut it off when a cap discharges to some specific level, sacrificing your minimal parts requirement. By the way, there's a 1F supercap available at Allelectronics for $3.50
Ed
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wrote:

Thanks for responding. It turned out the memory super cap didn't work that well so I added a darlington and a relay to my plan and got it to work with 3300 mfd and 330k. The relay drops out pretty close to my 15 minute goal. I still got there with 4 parts and none of them are particularly high tech. My wife keeps warning me not to make things that I am the only person on the planet who will understand if they break.
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You may likely find that it's less expensive to make a cheap RC regulated pulse generator (some gates in a ring with some RC delay) and a divider. You also get greater accuracy in the timing.
It's been years since I was actively involved in digital design but it seems to me that 2 or 3 catalog ICs and some small resistors and caps would be easier to "package" than some kind of "super cap."

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John Gilmer wrote:

Nothing that complicated.. A 4060 (1.26 GBP) or an inexpensive PIC (1.47GBP) will do this, as will an ICM7555IPA Low Power Timer (1.28GBP). All for less cost and with better accuracy than a big electrolytic.

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