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I've always was wondering why they chose 9v, well really 9-12v.. Why not 5v or 3.3v or 12v?

Surely there was some good thought put into this. Does anyone know or have sources?

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You can actually supply 6-20 min-max (they don't recommend it though) and Arduino's recommendation is 7-12v. One reason for this is because with the linear voltage regulator the arduino uses will have a voltage drop of around 2V and since the Arduino has been designed around 5V there is a ~7V minimum. The AVR micro controllers the Arduino uses can go below 5v, but not at the full clock speed. Some Arduino boards are 3.3v boards, and have different minimum and maximum voltages.

The reason the max recommended voltage is 12v is probably because any excess voltage will be let off as heat and the voltage regulator can only dissipate so much heat! You can operate the arduino from 5v, but you need to go around the voltage regulator. That's how the arduino is powered from USB, which has a 5v line.

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    \$\begingroup\$ Also a 9V alkaline battery's voltage starts decaying faster after about 7V. It seems a natural fit to a 7805 regulator. \$\endgroup\$ – eryksun May 4 '11 at 1:20
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A common supply voltage for the AVR chips used in the Arduino boards is 5v. The supply needs to be regulated, and regulator chips generally need to be supplied from at least 2V more than the output voltage. A 9V supply ensures that the regulator will always function properly, and is a very common value for the mains supplies that are available.

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As the others have said the input voltage to the Arduino needs to be greater than the 5V it runs off of to allow for the regulator to work, so why not 9V to allow it to run off of a 9V battery? And you can still run it off of a 12V battery as well as the others also pointed out. It's kind of a win-win.

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  • \$\begingroup\$ or it would be win-win except for the 10mA quiescent current drawn by the reg \$\endgroup\$ – geometrikal Apr 10 '12 at 12:44

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