Re: [avr-chat] AVR power supply current

[Robert L Cochran]

John Rehwinkel wrote:
> Bob Cochran asks:
>
>> I was planning to use a lithium polymer battery that
>> pumps out 12v at 1300 mA. I connected that to a power supply board which
>> will give me 5v output and 1100 mA current. That is way too much current
>> for the Atmega644, isn't it?
>
> That's just how much current the power supply can deliver. It doesn't
> force that much current into its load.
> If you give the AVR the correct voltage, it will draw as much current
> as it needs (assuming the power supply
> can deliver enough). In your case, the power supply just has plenty of
> extra capacity, and will have no
> problem powering the AVR. However, if you hook it up wrong or have a
> short circuit, that beefy power supply
> will be entirely capable of burning up components.
>
>> Of the "wall wart"
>> power supplies I am using, I have one that puts out 5v and I've measured
>> 350 mA current. So I need to cut down the current for the AtMega 644
>> device, right?
>
> Nope. As long as the voltage is correct and the supply is clean
> (minimal spikes or ripple), everything
> will work fine. Problems come when you need more current than the
> supply can deliver. The other
> way around is actually how things usually are. Most of my supplies can
> provide 1000mA at 5V. One
> of my projects draws maybe 1.2mA. Works just fine.
>
>> I can reduce the current with resistors?
>
> You could, but it's not really what you want to do. If you put
> resistors in series, they'll have a voltage
> drop proportional to the current through them, times the resistance
> (Ohm's law), and your load won't
> get the same voltage the power supply is delivering. This is not what
> you want at all.
>
> The other approach is to connect both a resistor and your load to the
> power supply in parallel. This
> way, the resistor "eats" current, leaving less available for your
> load. However, this just makes the
> resistor and your power supply hot. It doesn't accomplish anything
> useful, it just wastes energy.
>
>> I will Google to
>> learn how to do this.
>
> This is basic electronic theory. Luckily, you don't need to get in to
> it. Both your power supplies
> (battery based and wall wart) should be entirely adequate to power
> your AVR (as long as they're
> clean -- a decoupling capacitor mounted near your AVR is always a good
> idea).
>
> -- John
>
John,

Thank you very much for taking the time to explain this to me. The way
you write this made the light bulb at last "turn on" for me and I'm
beginning to understand more about power supplies.

Thanks a lot!

Bob Cochran
Greenbelt, Maryland, USA