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The arduino website states that the operating voltage of an arduino is 7-12V.

Why does the arduino work powered on just usb then? As usb only outputs 5V and 500ma.

Is it preferable to say run the arduino on an external 9V dc power source, or does it make no difference?

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The arduino website states that the operating voltage of an arduino is 7-12V.

That's when it goes through the voltage regulator. It's a linear regulator, meaning it wastes a lot of power as heat. The regulator outputs 5V that's given to the MCU. The USB voltage bypasses the regulator and goes to a fuse before going to the common 5V for the board. If less than 7V is given to the regulator, it will produce less than 5V.

Is it preferable to say run the arduino on an external 9V dc power source, or does it make no difference?

Usually, yes. A 9V supply would give you more than 500ma, giving you flexibility to add higher current components like motors and lots of LEDs. It will make no difference if you have a single LED connected.

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    Although you wouldn't want to run too much off the built-in regulator... – Ignacio Vazquez-Abrams Jul 30 '14 at 1:57
  • @Ignacio, true, but IIRC it's over an amp vs 500ma max... also if you use the 9V directly for sensors. – Anonymous Penguin Jul 30 '14 at 2:02
  • this may seem like a dumb question but will the following be a sound power supply for the arduino? thepihut.com/products/7-port-usb-hub-for-the-raspberry-pi I have a 9V 1A PSU also, not sure which is the best to power it off. Or are both sound bets? – c_breeez Jul 30 '14 at 11:00
  • @c_breeez I can't advise you because I don't know anything about your situation. – Anonymous Penguin Jul 30 '14 at 12:35
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    @c_breez as long as the RFID reader and the wifi dongle comply with USB 2.0 specifications (no reason why they shouldn't), then the USB hub should be plenty fine. I'd be surprised if the Arduinos used 1/10th of an amp... the only thing is I think the Pi might need to be powered on for power to be delivered to the devices... – Anonymous Penguin Aug 2 '14 at 0:08

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