Question: 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)?

Answer: 
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.).
The board has a power supply input named X1 , the voltage that is connected there goes through a [NCP1117](http://www.onsemi.com/pub/Collateral/NCP1117-D.PDF) regulator to generate the 5v supply that feeds the 5v parts

![enter image description here](https://i.stack.imgur.com/bNsoC.jpg)

and the 5v supply is then fed to another regulator [LP2985-33](http://www.ti.com/litv/slvs522n) that outputs the 3.3v in order to supply the 3.3v parts

![enter image description here](https://i.stack.imgur.com/wWIH0.jpg)

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](https://i.stack.imgur.com/8P5tM.jpg)

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.