| Author |
Memory backup capicitor as timing element
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| gfretwell@aol.com 2007-01-20, 3:25 am |
| 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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| phil-news-nospam@ipal.net 2007-01-20, 1:25 pm |
| On Sat, 20 Jan 2007 01:29:28 -0500 gfretwell@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.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2007-01-20-1053@ipal.net |
|------------------------------------/-------------------------------------|
| |
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| gfretwell@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.
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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| gfretwell@aol.com 2007-01-22, 3:25 am |
| On Sat, 20 Jan 2007 22:25:23 GMT, ehsjr <ehsjr@bellatlantic.net>
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?
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.
| |
| John Gilmer 2007-01-22, 9:25 am |
| 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."
<gfretwell@aol.com> wrote in message
news:rqc3r2pdr71bpos4vsd4jjmnmi5roecb6m@4ax.com...
> 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.
| |
| Palindr☻me 2007-01-22, 9:25 am |
| John Gilmer wrote:
> 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.
>
> <gfretwell@aol.com> wrote in message
> news:rqc3r2pdr71bpos4vsd4jjmnmi5roecb6m@4ax.com...
>
>
>
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