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I have permission to access a (borrowed) Arduino Uno, and the documentation says it can be powered with 3.3 Volts from the 3V3 pin:

3V3. A 3.3 volt supply generated by the on-board regulator. Maximum current draw is 50 mA.

http://arduino.cc/en/Main/arduinoBoardUno

However, I would like to use a 24V DC power source.

The safe range is supposed to be 6V-20V.

If I could get a transformer in place to change the 24V DC to 8V DC, could I apply that 8V to the 3V3 input (without frying the board)?

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    \$\begingroup\$ STOP. Read the answers. A dead borrowed Arduino awaits if you do what you propose. \$\endgroup\$ – Russell McMahon Dec 18 '13 at 10:03
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Connecting anything much above 3.3V (as in say 3.6V) to that rail would fry the Arduino. A transformer only operates with AC voltage not DC, so the best way for what you want to achieve is a step-down regulator also known as a buck converter. Variable ones that can accept 24V DC and output 3.3V (be careful to adjust it before connecting) are available on e-bay. Here's the first example I found:

DC Adjustable Voltage Buck Converter 4.5-25.5V to 3-24V Step-down Power Module

But important: I'd taken at face value that your statement was correct that it was OK to power the board from 3.3V over that line. As Peter Bennet correctly states some parts are actually 5V based and the 3.3V rail is the output of a regulator. Without removing the regulator and knowing the exact consequences that isn't a good idea.

Instead you should consider setting the regulator for say 8V and connect it to the 6V-20V input socket on the Arduino where you'd normally apply an external power supply / wall wart etc. That gives you a bit of extra safety if the adjustment on the regulator ever gets changed or drifts a bit.

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  • \$\begingroup\$ Thank you for pointing me to the buck converter; I had suspected that some such component existed, but I didn't know where to find it. \$\endgroup\$ – Mercutio Dec 19 '13 at 2:22
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According to the Adruino info page, and to the schematic linked there, the 3.3 volt terminal is an output from an internal regulator, and cannot be used as an input to power the board. The schematic clearly shows the AVR powered from 5 volts.

According to that page, the board can be powered by 5 volts from USB, or by an external 6 - 20 volt DC supply. There is a warning that the on-board regulator may overheat if the external supply is above 12 volts, and may be unstable if the external supply drops below 7 volts (so it should really be spec'd for 7 - 12 volts, not 6 - 20, in my opinion.).

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  • \$\begingroup\$ I misread the schematic. Thanks for the correction. \$\endgroup\$ – Mercutio Dec 19 '13 at 2:22
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The board has a power supply input named X1 , the voltage that is connected there goes through a NCP1117 regulator to generate the 5v supply that feeds the 5v parts

enter image description here

and the 5v supply is then fed to another regulator LP2985-33 that outputs the 3.3v in order to supply the 3.3v parts

enter image description here

Apart from that, there is an available input that is intended to supply the board from the a USB cable (5v)

enter image description here

According to the board specs for the supply input X1

Input Voltage (recommended) 7-12V
Input Voltage (limits) 6-20V

The high input voltage limit of 20v is forced by the maximal input voltage of the NCP1117, a higher voltage would damage the regulator.

Basically you can only connect voltages to two points, one is the USBVCC which i intended for regulated 5v supply (from USB port or 5v wall supply) and the other one named X1 where you should connect a supply that is between 6v-20v and which is where you should connect the 8v or 12v (referring always to DC voltages) you talk about.
There is no 3.3v input.

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  • \$\begingroup\$ I need a lot more practice in reading schematics, I think. Thanks for your explanations. \$\endgroup\$ – Mercutio Dec 19 '13 at 2:23

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