Thanks Bob.

Just to be clear:

- I want to put the super cap inline with a regulated 5vdc power supply and
my device so that if the mains power fails my device gets a few seconds of
life
- I don't need to "tell" the device the power is failing, it'll stop
receiving data and clear its cache at that point


-marc

On Fri, Feb 5, 2010 at 1:23 PM, Bob Blick <bobblick@ftml.net> wrote:

>
> 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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