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

[Donald Delmar Davis]

And again I refer to the datasheet sections 7 and sections 26
(Cause you know there only 2 kinds of people...).
If you reduce the clock speed to 8 mhz  an active mega168  maxes out  
at 12ma.
Running on the internal oscillator at 2v (v parts) drops this to .55ma.
Putting the processor to sleep can get this to micro-amps.


http://www.atmel.com/dyn/resources/prod_documents/doc2545.pdf

On Apr 1, 2009, at 2:39 AM, Paul Stoffregen wrote:

>
>> 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.
>
>
>
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