user-1: I would like to power my Raspberry Pi 3B using an Arduino Uno.
I have been able to do this by connecting the 5v on the Arduino to the 5 V on the Pi and connecting the GND to GND, and the Pi turns on. But I get a constant under voltage sign.
How can I get the Arduino to output the right amount of power (5 V and 2.5 A)?

user35089: If you are using PC USB, your total current cap is <500mA. Arduino + Pi + accesories is exceed this cap.
Use an adapter for Arduino. Ardiuno will suck up some current and rest can flow to Pi or whatever else is there.
Arduino has a linear voltage regulator. Try not to use >5v adapters. Voltage regulator will generate more heat as more current pass.

user50469: If you reverse which board gets the power adapter you should be able successfully power them both from a single 5V 2.5A power supply.
Connect the power adapter to the Raspberry Pi3 and power the Arduino from the Pi's 5V pin.  The Uno should only draw about 200mA @ 5V from the Pi (depending on everything else you have connected).

user30002: You can't.
The on-board regulator of the Uno, the NCP1117, is only able to supply up to 1A before starting to dropout voltage (hence your undervoltage warning).
Even worse, because the NCP1117 lacks a proper heatsink, it will probably start dropping out voltage before reaching 1A, due to its internal thermal shutdown protection.

-1: <p>I would like to power my Raspberry Pi 3B using an Arduino Uno.</p>

<p>I have been able to do this by connecting the 5v on the Arduino to the 5&nbsp;V on the Pi and connecting the GND to GND, and the Pi turns on. But I get a constant under voltage sign.</p>

<p>How can I get the Arduino to output the right amount of power (5&nbsp;V and 2.5&nbsp;A)?</p>
 35089: <p>If you are using PC USB, your total current cap is &lt;500mA. Arduino + Pi + accesories is exceed this cap.</p>

<p>Use an adapter for Arduino. Ardiuno will suck up some current and rest can flow to Pi or whatever else is there.</p>

<p>Arduino has a linear voltage regulator. Try not to use >5v adapters. Voltage regulator will generate more heat as more current pass.</p>
 50469: <p>If you reverse which board gets the power adapter you should be able successfully power them both from a single 5V 2.5A power supply.</p>

<p>Connect the power adapter to the Raspberry Pi3 and power the Arduino from the Pi's 5V pin.  The Uno should only draw about 200mA @ 5V from the Pi (depending on everything else you have connected).</p>
 30002: <p>You can't.</p>

<p>The on-board regulator of the Uno, the NCP1117, is only able to supply up to 1A before starting to dropout voltage (hence your undervoltage warning).</p>

<p>Even worse, because the NCP1117 lacks a proper heatsink, it will probably start dropping out voltage before reaching 1A, due to its internal thermal shutdown protection.</p>
