[dorkbotpdx-blabber] Re: solor powered Dorkboard, fact or fiction?

[Paul Stoffregen]

> Volts * Farads / amps = the amount of time it can put out that amount
> of amps.

This is the correct equation, but the "Volts" is not the voltage you've 
charge the cap up to, but rather the amount of voltage it will drop 
during that time.

The really bad news is might be a small voltage.  If you start at 3.0 
but your chip stops working near 2.7, then you can drop only 0.3 volts, 
so that 1F cap at 20 mA only lasts 15 seconds.  It will actually last a 
little longer if your current decreases as the voltage decreases.

You can increase the volts in the equation by charging the cap up to a 
higher voltage, and making your design run until the lowest possible 
voltage.  If you're using the 16 MHz resonator, the chip isn't specified 
to work below 4.5 volts.  Though it will "overclock" and run to lower 
voltages, if you can drop the clock to 8 MHz or less, you can safely run 
all the way down to 2.7 volts.  If you charge the cap up to 5 volts, 
then you've increased the "volts" to 2.3.

If the cap can handle higher voltage, you could try charging it up to 12 
volts and use a low power linear regulator like a LP2950 to get 3.0 
volts.  Then you can drop from 12 down close to 3, which is 9 volts.

Getting the current lower helps a lot, and Jared is right, using the 
powerdown mode really saves a lot.  The only other thing you can do is 
make the cap bigger.

Also, be careful charging the cap.  A solar cell often puts out much 
more than its rated voltage if the current is low.  You will probably 
need a zener diode or some other voltage limiting circuit in parallel 
with the capacitor if you're charging it up close to its maximum rated 
voltage.