External Power and USB connected//Conflicts

A quick question. If I have my Arduino connected to a computer with a USB cable and also connect a 9v DC supply to the Arduino at the same time, what happens exactly? Will the Arduino draw its power from the external supply or the USB cable? Furthermore, will attaching an external supply interfere with the Serial Communication along the USB connection between the Arduino and the Computer?

I need that Serial Connection, but don`t want to draw power from the computer along the USB.....more than needed for the Serial communication I mean.

I am running (and receiving) a number of multiplexers and relays from the digital and analog ports and am thinking the safest way to deliver the steady power for all the outputs is from external power. It is connected to a mac mini that is getting rather hot and must stay on for an extended period of time during the installation. I want to tax the mac mini as little as possible...

Any suggestions, and information regarding how the external power supply and the USB will interact would be greatly appreciated.

Gratsi, arduiYES

It automatically switches between them as long as you are using the jack. If you power VCC directly it is not automatic. Then it is not best practice to have two power sources in parallel, but I did this quite a bit before I realized that it was happening and it did no harm.

There are essentially two power nodes on the Arduino. "Vin" and "5v". Vin is the Vin pin (duh) and the DC Barrel Jack. This node is connected to the on-board 5V regulator's input.

The "5v" node is connected to the 5v Pin, the output of the regulator, and USB.

As arbarnhart mentions, you do not want to put a 5V supply on the 5V pin AND be connected to USB at the same time. This will cause the node to have a voltage drop and is not good.

The auto-switch circuit will try to switch 5V or Vin, if the voltage on the Vin node is >7.5V. (Which is rather convenient since the regulator needs at least 7V in order to function properly anyway.)

Using a 9V Wall-Wart and USB at the same time, the Arduino will automatically use 9V.

By 9v wall wart do you mean to endorse the expansion-capitalistic wall fart corporation? the ones who put all the mom and pop harware shops out of business and continue to spread world dominance like a furious shoe odor? buy bread and wrenches at 3am...one stop shop til you drop. if i use another adaptor from another firm will my free market diodes glow a brighter and more heart warming green?

Seriously, thanks for the very knowledgeable and helpful solutions. will put a 9volt dc adaptor therein.

ban wall-mart. for one year, if all stop buying from the shop = in one year, wall-mart is out of business. a ghost shack parking lot. dreams of the future.

As stated the arduino auto-voltage switch will make external Vin voltage override USB power. Your concern and desire to understand this concept is important, because the larger question is what is your Arduino trying to drive external it's internal needs? The USB power is limited to 500ma max, so it's your total system current requirements that determine how you are going to have to set-up and wire your project. Also how to power off your system if plugged into both usb and external power connector should be understood and accounted for.


on second thought, perhaps wall-wart is your own personal business and you make your own power adaptors. if that is the case, more power to you. at the thrift store around these parts they sell em (power adaptors) for a buck. not a bad deal when its raining and would be far too messy for the typical high-tech dumpster dive.

retrolefty, thanks for the technical info. 500ma. that helps a great deal. also, thanks to jamesc4s and arbarnhart. i think all is clear now and i can go on to the next level. greatful.

Uh, chill out: