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Topic: external 5V at +5V pin, and USB connection (Read 1 time) previous topic - next topic


I posted this on another subforum, but I got only one opinion, so I am hoping for better results here.

I designed a shield that powers Arduino Uno with 5V trough +5V pin.

For testing purposes I need to connect Uno to PC, and shield to 12V, but shield has a 5V regulator that powers Arduino Uno..

Will I damage anything if I connect Uno to PC with USB cable (that also has 5V supply)?

Here is the schematics, I want to controll idle speed on my car with a little steeper motor, and Daytime Running Lights:


in the schematics VCC is connected to the Vin pin, but I changed that, so it is now connected to +5V pin (as it was suggested by arduino member  retrolefty - thanks).
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My Arduino Diecimila has a movable jumper to select between USB power and the power jack.  Selecting the power jack will disconnect the USB power and prevent any possible conflicts.
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I don't think it will be a problem. If you have two 5V regulators both driving the same node, one of them will "win" (i.e., supply the majority of the current) and the other will mostly be off. It depends on which regulator is internally set to the higher voltage (e.g., one is trying to drive 5.001V and the other 4.999V, the one that tries to drive 5.001V will supply the majority of the current).

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Your shield regulator talk confuses me. But I have used (the Duemilanove) with 12V on the DC jack and on USB simultaneously, without problems.


Mar 01, 2011, 07:44 pm Last Edit: Mar 01, 2011, 07:45 pm by retrolefty Reason: 1

Many Arduino boards (Including Duemilanove and Uno) have automatic power switching.
Nothing like the official documentation for authoritative answers. Better than 100 opinions here.

The Arduino Uno can be powered via the USB connection or with an external power supply. The power source is selected automatically.

Yes, but that doesn't state which voltage source is select if both are avalible (external takes priority). And the OP had a setup where he is hard-wiring an external regulated +5vdc voltage source to the Arduino's +5vdc shield pin, so in his case there might be three different voltage sources and his question was relevant.

The Arduino automatic voltage switching circuit has added complexity from the prior manual jumper placement where one could 'force' voltage source from only USB or only external power source (or Vin pin) or neither, if one wished to wired an external regulated voltage source to the +5vdc pin.

What we are dealing with is the undesired, but workable in practice, of having two (or more?) active voltage sources 'hardwired' together, which is normally not a recommended good engineering practice from any voltage regulator datasheet I've read that talked about that issue.


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