Howto use SuperCaps (Hardware)

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#1
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Hi guyzz

I was wondering about how to interface a "SuperCap 5.5v 1F" to a circuit :?:
I mean if i just put it directly on my 7805 it will probably destroy it on power on. And what is the advantage/disadvantage vs. a lithium batt :?:

If i just want to "back up" a RTC + Ram , does ine use a diode to prevent it ro power the whole circuit , what about the voltage drop across the diode :?:

Does anyone have some tips and maybe a link to a schematic ???

Ohh is 1.25$ for a 1F Cap , a fair price ???

/Bingo

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Use a series resistor to limit current to/from the cap and a schottky diode between 7805 and ram/rtc power pins. Lithium batts have much higher capacity but are not rechargeable.
An additional note: be sure to use a gating circuit which sets the ram cs signal high during power failure - or the supercap will discharge really fast. Or use an integrated solution - MAX69x/ADM69x or similar that provide power fail detector, battery backup switch, CS gating and reset generator.

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$1.25 is good price.

8)

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You can connect the cap directly to a 7805 without damage. This is the great thing about a 7805. It will produce the maximum current. That's probably 500ma. Check the 7805 data sheet. If the 7805 gets too hot (125C junction temperature), it will shut down until it cools. The only question is will the cap charge fast enough. At 500ma, it will take 10 seconds to get to 5 volts.

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Surely, the place to look for this is on your SuperCap manufacturer's web site?!

Obviously, they want you to use their devices - so they are the ones to contact regarding the use of their devices!

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@awneil

As i am an electronics "beginner" it would be a lot easier for me to look at some existing schematics that shows a battery/SuperCap interfacing to Ram/RTC.

So i would prefer some hints , in that direction. And the caps that i haven't received yet .. are from e-bay , and i don't know the manufactor yet..

/Bingo

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There are any number of 8 pin RTC chips available that will keep time for years on a single coin cell. For instance, go to www.maxim-ic.com and search on DS1306. Also, there are any number of serial EEproms and serial data flash chips that require no backup power. Search the Atmel site for those yourself. You will have to figure out how to use the SPI port (gasp!), but your problem will be solved.

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Usually many watchdog chips used in the old days also have some battery backup switch function with CE disabling, like MAX791.

Guillem.

Guillem.
"Common sense is the least common of the senses" Anonymous.

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I believe an SMD inductor can also solve the inrush current problem without shutting down or saturating the 7805.
incal99

________________________________ We dream of a world where current does not need the voltage to flow.