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.

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.

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

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.

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."

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.