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Can you power the Arduino off a USB cable alone, without using the regular power connection? If so, what's the point of the additional power connection: to provide extra power? Also Why not just have two usb ports one for power one for connecting to the computer, or a micro usb.

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As Hyperbola pointed out, the USB port function is primarily to allow for programming the board. Now, since the USB port has the power cables too, it is convenient to power the device from the USB port.

The main drawback of USB port powering, however, is that it can supply a limited current: usually the USB2 ports can deliver up to 500mA before shutting down. In order for the arduino board to be compliant with this, consequently, a 500mA fuse was put on the board just after the 5V usb input.

Now, what if your final circuit requires more than 500mA? Will you have to change board? Of course not! you can just use the external power input to power the arduino board and all the other circuitry without worrying about the 500mA limitation. Of course the onboard regulator has limits too, but you have more flexibility.

Moreover portable applications run on batteries. No battery can supply exactly 5V (usually alkaline ones deliver 1.5V per battery, while lithium have usually 3.7V/cell, but this varies over time). Consequently they put an onboard regulator for providing 5V from any 6.5V+ source.

Summarizing: the USB power is convenient, since you will usually power it from the PC when programming. An external connector, however, is very important because it gives you more flexibility when choosing the power supply. You don't have to use both: you can just choose the most suitable for your needs.

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    thanks this answers my question in addition to the comments and pictures by Robert Cole. Mar 10 '16 at 17:54
  • So, what's the MAX voltage I can provide to Arduino ? Nov 2 '20 at 23:36
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    @HenriquedeSousa this highly depends on your arduino board and project. For instance, on the UNO there is an NCP1117 after a silicon diode. Let's say your application draws 75mA total (so including also the arduino board). Let's suppose the board will stay at max 30°C; this means that the junction can rise by 120°C. The calculations are: Rth = 160°C/W -> MaxPower = 120°C/160°C/W = 0.75W -> MaxDeltaV = 0.75W/75mA = 10V; Max input voltage = 5V + 10V + 0.7V = 15.7V. The input voltage of the regulator is max 20V, so the power one is the max one.
    – frarugi87
    Nov 3 '20 at 11:17
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    @HenriquedeSousa please consider that the calculations provided are just one example. For instance, if the current is 250mA instead of 75 the max voltage gets down to 8,7V. If, on the other hand, you change the board this will also change: for instance if you use a pro mini 5v instead of uno, at 75mA you will be able to use max 10,7V (since the regulator is different)
    – frarugi87
    Nov 3 '20 at 11:31
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Can you power the Arduino off a USB cable alone, without using the regular power connection?

You can use the USB cable alone to power the board.

If so, what's the point of the additional power connection: to provide extra power?

You can use either to power the board. By using just the power plug you can make your Arduino portable by using a battery box or power it via a wall plug.

Also Why not just have two usb ports one for power one for connecting to the computer, or a micro usb.

Only ONE USB is needed to power and program.

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Arduino, in its very esence, is an embedded system and can be used as a standalone device. The USB port is provided for:

  1. To power up the device.
  2. To program it through a computer i.e. dump the sketch on it which one can program in the Arduino IDE.

Once you have dumped the program on it, it will stay on it forever unless you change/erase it and hence you can use the Arduino anywhere without actually bringing a computer (USB port) along.

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