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Topic: powering Arduino from external 5Volt power source (Read 1 time) previous topic - next topic



what is the proper way to power an Arduino (Duemillanove in this case) from an external 5Volt power source?

I have a project where I have a 5Volt 8x64 LED display matrix that drawn more current than the 5V regulator on the Arduino can handle. So I have an external switch mode 5Volt 5Amp power supply and connected it directly to the 5Volt and GND pins on the Arduino and to the LED matrix.

It does work fine this way when it is running stand-alone. My concerns are when I connect the Arduino to the computer via USB to upload new sketches and diag prints on the serial port.

- When I power up the project with the external source and at a later time connect the USB cable to the computer the Arduino sometimes resets, sometimes hangs and generally does weird things.
- When I connect USB *before* the external power source, the Arduino is trying to power the LED matrix from the USB 5Volt
- I'm also concerned I might damage something on the USB port on the computer

Location: Berkshire, UK
My Astro and DIY projects website: http://yesyes.info/


My guess is that you are maxing-out the capacity of the output pins on the board. The digital pins on the Duemilanove only deliver about 40mA. Most designers will place a separate LED driver board between the Arduino and the LED array.

The page "Arduino: What Adapter?" may help answer your question.

James C4S

The problem is that the 5V pin/node is connected to the USB's 5V pin.  Your external supply and your USB supply are being connected together at the same time.  This is not a good idea.  You are right to be concerned about damaging something on the USB host.  There really isn't a good way to prevent this, other than don't connect the two at the same time.  Which makes debugging through serial difficult.

The ideal connection would be through the VIN/DC Barrel jack for the Arduino.  However, you need at least 7V there. 

One option would be to use a FTDI breakout board.  It can be powered by USB and connected to only the TX/RX lines of your Arduino.

My guess is that you are maxing-out the capacity of the output pins on the board.

There is nothing in the original post to suggest this is the case.
Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com


This sounds like what's been discussed:

I'm not knowledgeable enough to know whether it'd work, but I was wondering about cutting the USB power pin, and reconnecting it with a diode to prevent current flow. Probably there's a good reason to not do that.
... it is poor civic hygiene to install technologies that could someday
facilitate a police state. -- Bruce Schneier


Exactly justjed. I saw the same while developing my last project - had to disconnect external power to duemilanove prior to connecting USB cable to be able to download new sketch.
I also installed prominis in sockets so I could remove them from the project board for programming.
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