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I'm designing a shield for Arduino which drives two stepper motors. They are small and require 5V but their max combined current is around 1A, way too much for the USB supply. Therefore I will have external 5V power supply connected to my shield.

The entire device will usually operate being connected to a PC through USB, but still I'd like to have a possibility to run it separately.

The question is: how to power Arduino itself?

  1. From my external PSU through the pins: Sounds as the simplest solution but I'm bit worried about the onboard regulator of Arduino while reverse powered.
  2. Separate power circuits for electronics and motor: My L293D chips allow for separate power and even different voltage for the motors. It could even be an advantage for my shield, as I could use other motors and different power for them in the future. I will, however, lose the possibility of running the device without USB connection.
  3. Same as above with additional barrel jack power connection to supply Arduino when USB is disconnected. This, however, could fail with 5V supply, because of the voltage drop of the regulator which handles power coming from barrel jack.
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Option 1 is by far the simplest. Just have one 5V power supply for everything. Make sure you have enough capacitance on there to absorb the noise of the motors - maybe separate the digital side from the drive side with an inductor.

You can add a diode connected backwards between the 5V and VIN pins to bypass the regulator when reverse powered. That is, anode to 5V and cathode to VIN.

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What I am doing for similar is powering 7.2v to motorshield EXT_PWR and same to RAW pin of Arduino mini pro, where taking 5v out from vcc and input to motor shield 5v. It works for my car well.

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