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[on:]

Hi,

I have been reading the tutorial http://tronixstuff.wordpress.com/2011/01/19/tutorial-arduino-and-gsm-cellular-part-one/ on Sparkfun's Cellular shield (http://www.sparkfun.com/products/9607).

They say

Secondly, the current draw. Sending an SMS (a tiny burst of data) will generally draw around less than 400 mA, which is within our normal Arduino specification. If you are using your shield to make or receive actual telephone calls, the shield can draw up to two amps of current. [...]

Therefore if communicating in this way you cannot run your Arduino and this shield from the USB port or the DC socket. And don’t be lazy by trying it anyway.

Instead, power your project with a high-capacity 5V 2 amp power supply to the 5V and GND pins on your shield.

It is clear that we shoudn't use the USB port, since it can give less than 500 mA. But why couldn't use the DC socket with a 5V-12V (using the board's power regulator) adapter rated at 2 A or more ?

(I'm thinking about a Duemilanove board)

[Reply #1 on:]

But why couldn't use the DC socket with a 5V-12V (using the board's power regulator) adapter rated at 2 mA or more ?

I think you meant 2 A not 2ma. And you can't use that method because that power just feeds the on-board +5vdc voltage regulator that is only rated for about 800mA, so even though your adapter might have a 2 amp capacity only 800ma of it can be converted to +5vdc to power the board and shield.

[Reply #2 on:]

This is because of the power dissipation (heating) in the regulator chip. THis in turn is dependent on the input voltage, the lower the voltage the less power is burnt, until you reach a point when you don't have enough voltage to drive the regulator.
See this for a worked example:-
http://www.thebox.myzen.co.uk/Tutorial/Power_Examples.html