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What would go wrong otherwise?

  • In short, the power regulator (or 5V USB) may/can not supply enough current. Overcurrent can cause damage to devices or undefined behaviour. – Paul Mar 14 '19 at 11:25
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On an Uno, the barrel jack feeds a reverse polarity diode that can support 1A max, which then feeds a 5V regulator rated for 800mA max (800mA cannot be reached due to lack of cooling - but it does pretty well if barrel jack voltage is kept to 7.5V) which then feeds a 3.3V regulator rated for 150mA.

With too high a voltage or too much current, the 5V regulator either goes into thermal shutdown, or often just severely overheats and fails (sometimes spectacularly).

The 5V can also come from the USB source, which has 500mA PTC resettable fuse, so even less current is available.

So power your motor from another source, and use the Arduino output pins to control a transistor to turn the motor on and off.

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