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I have an Arduino Uno, and from what I understand, the digital I/O pins on the side of the uno each have 5V and can be controlled from the Arduino software.

I am pretty sure from reading the documentation and testing it out with a 9V battery (Please correct me if I am wrong) that the VIM and ground pins on the opposite side of the uno are used for powering the uno from an external, non-USB power source.

What I find a bit hard to understand from the uno's documentation, is the purpose of the 5V and 3.3V pins. I tried hooking up a hobby motor to the 5V and ground pins, and it spun - so am I correct in deducing that the 5V and 3.3V pins are used to power external components that don't need to be controlled from the Arduino software?

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The 5V pin is the output of the on-board 5V regulator. Yes, you are correct that it can be used to power external components which use a 5V connection.

The 3V3 pin is the output of the on-board 3.3V regulator. Same as above as for powering components from it.

The VIN pin is slightly more complicated. If you are not powering it from USB but rather from an external power supply, that supply is directly available on VIN. However, the ATmega328 is still powered from 5V which is available on the 5V pin after being passed through the regulator. So the VIN pin is unregulated (unless your external supply is regulated) and should probably not be used to power external components.

Unfortunately, I believe all the pins on the arduinos are only rated for 40mA. So while your power supply might be able to provide more, if you take it from the power pins you should not draw more than that.

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  • \$\begingroup\$ ahh ok, that makes sense. So if I had a 9V battery rigged to a DC jack and used that to power the Arduino, the VIN would be unregulated 9V while the 5V would be regulated to just that? Also - are the Arduino expansion "shields" powered by those 5V and 3.3V pins? \$\endgroup\$ – snowflakekiller Aug 9 '11 at 16:29
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    \$\begingroup\$ Yep, you got it. As far as shields, it depends on how the designer made the shield. I have only used one as I like to make my stuff myself and just wire it over but it was powered by those 3.3V and 5V pins. \$\endgroup\$ – NickHalden Aug 9 '11 at 16:33
  • \$\begingroup\$ An example of the Vin pin: I've designed (and am building) a motor shield that draws the current for the motor via the Vin pin. As long as the arduino is powered off the DC jack, it's perfectly suitable. \$\endgroup\$ – Nick Johnson Aug 10 '11 at 4:31
  • \$\begingroup\$ As a minor point, the voltage at the VIN pin is about 0.6 V less than the external power supply voltage since there is a reverse current protection diode in between. \$\endgroup\$ – Kavka Aug 1 '13 at 13:56
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The VIN pin can also used to power the arduino. If you have a battery pack, instead of constructing a plug you can just wire it straight to VIN and ground.

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One use for the 5V pin is powering anything that draws more than 40mA. I have a motor that draws more than that. To control it safely using the Arduino, I have to use a digital pin to control a transistor, as a discrete transistor can switch much more current than the digital pins can handle.

The number of Vcc (aka 5V) and Gnd pins represent how much current the board can take, in increments of 200mA. So the Uno, with 1 Vcc and 2 Gnd, can push out 200mA total and is capable of safely sinking 400mA into Gnd.

It should also be noted that the 5V (and 3.3V) is coming out of a regulator. Whatever power goes in to the board gets regulated first, so those pins are guaranteed to be at their listed voltage. The Vin pin has 2 uses. If you supply voltage, 5V or more, to Vin and connect one of the Gnd pins, that will power the board. If you are using an external power source through the DC jack, then Vin provides direct, unregulated access to that source while Vcc give a regulated 5V.

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The Vin pin is connected directly to the DC jack on the board, which is connected to the input of the on board 5V regulator. The 5V pin is the common 5V volt rail shared by the USB input and the output of the regulator.

One use for the Vin pin is to power your Arduino using a 9V battery.

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