On Fri, 5 Feb 2010 12:56:58 -0500, "Marc Nicholas" said:
> All this talk of cheap UPSes is timely...
> 
> I'd like to be able to provide a few seconds of +5vdc to a device so that
> when its main power line fails it gets to "finish what its doing" (which
> it
> will if it is no longer being fed data, but needs a second or two to do
> so).
> 
> Super or ultra caps come to mind, but I've never worked with them and
> some
> casual Googling hasn't turned up too much of interest.
> 
> Any suggestions on where to start or tips? Obviously I don't want to end
> up
> with blown caps and/or blown device!
> 
> TIA.
> 
> -marc
> 
> P.S: Oh, power draw for the device is 150mA nominal....so hoping I can
> just
> do something with a Schottky, resistors and cap!

I used a supercap in the propeller clock, so you could stop the motor
and the clock would keep running while you set it.

Here are a few things to consider. 

In any standby power situation, you need to deal with rising and falling
battery. You can't rely on the reset pin of a micro to actually reset
the micro, if the power supply rises or falls too slowly, or hangs out
at a volt for a while. Weird stuff happens in the micro and it won't
wake up until the power is brought all the way to zero again.

Supercaps do not charge instantly. So the first time power is applied,
then quickly removed, then applied again, puts you in a weird state.
This happens almost every time an AC adapter is plugged in, on-off-on a
few times.

So to make something bulletproof you want a circuit that actually turns
the power to the micro on and off. Not just the reset pin. And make sure
when the power is off it really goes all the way down before it can go
back on again.

For situations where less fussiness is required, you can get away with
less. But the propeller clock definitely was fussy sometimes, all
because of the supercap and simple diode setup.

Cheerful regards,

Bob


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