Hi, I'm having a slight problem. My Arduino UNO gets warm at the part number 3. It only heats up when it is connected to a power supply but if it's not connected it works fine. I change to a new arduino board and that that part it gets warm like a laptop warm. I have to run the arduino for 1 month.
Your sending too much voltage to the Vin pin or plug! Use at a maximum of 9v volts, this will prolong the life of the voltage regulator.
Looks like the 5V regulator. It needs about 2.5 volts more then 5 volts to work. All the voltage above that point gets thrown away as heat.
Be careful when using the power plug or Vin. Powering through Vin or the power jack means that the Arduino and all peripherals that are on the 5V rail are powered by the onboard 5V regulator. The on board 5V regulator is not heat sinked so will supply limited current before it overheats and shuts down. The amount of current depends on the voltage input to Vin or the power jack. The higher the voltage the less current can by supplied. I would use a buck (DC DC) converter to drop the higher voltage to 5V and connect that to the 5V on the Arduino, bypassing the, weak, 5V regulator. Then the rated current of the DC DC converter is available on the 5V line.
Hi,
How are you powering the UNO when not on USB?
What have you got the 5V pin of the UNO powering?
Can you please post a circuit diagram of your project?
That is the linear regulator to provide 5V to power the UNO, when you use the external DC socket.
It is not used when you use USB power.
Tom... 
The current available when using the power plug or Vin is limited by the input voltage. At 12V to Vin or the power jack there is enough current available to power the Uno itself and not much more.
The current that is available when using USB is limited to 500mA by the polyfuse on the Uno board. If your project needs more current at 5V an external supply is required.
Hi,
Please draw with pen(cil) and paper YOUR circuit diagram, a Fritzy image does not do much for trouble shooting.
Include component/hardware labels and pin names.
Where is your external power supply in that image?
Which UNO gets hot?
Tom...
The RPi is the external power source.
Hi,
I will believe it when I hear it.
If the OP is using USB power, the regulator will not be in circuit so should not get even luke warm.
What other power supply?
Tom...
@safiraraffi, if this is about an Uno, why did you post it in Arduino Yún? Your topic has been moved to a more suitable location on the forum.
As explained, an Arduino is not a power supply.
Or to put it another way, you simply do not use "Vin" or the "barrel jack" to power a UNO(, Mega 2560, Leonardo, Nano, Pro Mini) if you are connecting anything else to the Arduino.
Why? Because the on-board regulator that you indicate, has almost no heatsink and is not capable of providing 5 V beyond the needs of the Arduino board alone with a few very minor exceptions. 
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