I have a two amp 5VDC power rail. I have configured a motor shield to use external DC (by moving a jumper.) Is it ok to share the rail between the arduino's 5V and Gnd pins, and the motor shield?


the Vin pin on the Arduino is the source to the internal voltage regulator. It needs about 7 volts in order to work. If you try to feed 5V into the Vin then the Arduino won't get a full 5V.

Motors draw a lot of current, and cause transients on the power line as they switch on and off. (They will cause transients even in a regulated supply, because it takes the regulator time to respond to the rapidly changing load of the motor.) If you're going to feed both your motor shield and the Arduino's 5V from the same power supply, you will need a reservoir capacitor between 5V and ground as close to the Arduino 5V input as possible. The reservoir capacitor will even out drops and spikes in power to the Arduino.

You'd be much better off feeding 7-9V into the Arduino's Vin. That way it's power is regulated separately.

  • Thank you. How is it that the Arduino (Uno) wants 7-9VDC in, when it may be fed from 5VDC USB? Also, what size capacitor would you suggest?
    – HorseHair
    Sep 13 '14 at 13:12
  • 1
    You have 2 ways to power the Arudino: Through Vin, which feeds the built-in voltage regulator, or by feeding REGULATED 5V directly into the 5V line. When you feed 5V directly into the 5V line, you bypass the built-in voltage regulator. A linear voltage regulator like the one in an Arduino requires a voltage somewhat above the target (5V) voltage that it then regulates DOWN to the target voltage.
    – Duncan C
    Sep 13 '14 at 13:51
  • When you run an Arduino off of USB, it feeds the regulated 5V from the USB connector directly into the 5V pin.
    – Duncan C
    Sep 13 '14 at 13:51
  • When you have high-current loads, you should really regulate the load separately from your logic, or at least isolate it with a capacitor. As for the value of capacitor, I'm not sure. I suggest you post your question to the EE Stack Exchange.
    – Duncan C
    Sep 13 '14 at 13:52
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    Make sure your peak current load doesn't exceed the output of your power supply or your voltage will droop and reset/crash the Arduino even with a reservoir capacitor at the Arduino.
    – Duncan C
    Sep 13 '14 at 20:57

Yes, but I wouldn't try to do it through the 5V pin on the Arduino. Run a separate 18AWG or larger wire to bring power and ground to the shield.

  • What I mean is, I have a 5VDC wire and a ground, and those wires are going to two places: the arduino Vin and ground, and the shield external power. So they are sharing a source. The 5V pin on the Arduino I'm talking about is the Vin pin.
    – HorseHair
    Sep 12 '14 at 22:33
  • Then they're already on the same rail. Sep 12 '14 at 22:40
  • Yes, it works that way. I wonder if it's ok though; for example is there the possibility of ground loops or other negative consequences?
    – HorseHair
    Sep 12 '14 at 22:42

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