i've got a arduino board controlling a GLCD, a few relay and somme push buttons. Everything is working fine when the board is powered with a usb (so i presume the assembly need less than the maximum 500mA of the usb). I have a 12v 5A adapter laying around and i was wondering if it would fry my board.
I know the voltage is fine but what about the current ? My guess is, since my assembly is using at top 500mA, the board would only take the current it need.
Correct, the board and other components seem to draw less then the 500ma limit that the USB power provides so your 12vdc 5A adapter will work fine. The current demand is determined by the load(s) resistance, a voltage source does not 'force' a current to be drawn from it other then what the load resistance determines. Check with Dr. Ohm and his laws if you are in question about this. Now your on-board +5vdc voltage regulator may run too hot for your comfort as the regulator has to dissipate the difference between it's voltage minus 12v - 5v = 7 X whatever the current is in amps, so 500ma would generate 3.5 watts of heat and that's a lot of heat. So the lower you can find an external DC voltage source down to say 7-8 volts the better from a heat standpoint. No need for a power module rated for more then 1 amp as the limit is a on board diode and the +5vdc voltage regulator anyway.
Thank you !