user19384: I am thinking about buying Arduino or Raspberry Pi.  I generally know Arduino advantages (fast start-up, low consumption, ADC converters) and disadvantages (low memory, low connectivity, hard programming), but I simply cannot figure out maybe the most important issue: current output for controlling devices.
One of the common cited advantages of Arduino system compared to Raspberry Pi system is that power per I/O pin is much larger.  Arduino Uno allegedly has up to 20 mA @ 5 V per pin and 50 mA @ 3.3 V per pin making quite substantial power of 0.15 W, compared to say typical 10 mA @ 5 V per pin for Raspberry Pi (although there is no official number available).
On the other hand

this power is far too low to control any motors and you still have to
provide shields,
new version Arduino 101 reduced that current to 20 mA and even less for some other Arduino models.

So, what is really the importance (if any) of these higher currents for home projects, compared between different Arduinos themselves and compared to Raspberry Pi?
There is also a question of total current for all pins together, which is also limited, but it is soooooo difficult to get official limits (for both Arduino and Raspberry Pi).
Please give me some insight into this issue, so I can decide.

user4143: The simple answer is: It doesn't matter.
Both Arduino and the Raspberry Pi, and every other common microcontroller board, are designed to control things, not provide power to things.
Digital IO pins aren't designed to drive heavy loads. Yes, some (like the Arduino) can provide slightly more current, but that is really unimportant for most projects.
Yes, some boards have more powerful power supply circuits on them so you can draw more current from their power supplies, but again that is largely irrelevant.
The one thing that you seem to be forgetting (or haven't realised) is that the microcontroller board does not have to supply all the power.
The Arduino, Pi, Teensy, chipKIT, whatever, is merely a controller. If your system needs more current it is up to you to provide that current. The Arduino etc merely controls the circuit or device that it is connected to.
It is very common to have multiple power supplies in a large project - each providing different voltages, or being used for different specific tasks within the project.
Just one thing you need to remember: Connect the grounds.

user313: Bottom line is: the controllers are just not meant for supplying power. 
They do have the ability to do so, within limits - fairly stringent limits for a controller chip itself, and slightly more generous limits for the on-board power supply. If you really need an all-in-one, you might consider making building your own controller board with a more capable on-board power supply and a number of transistor switches to provide power for the hungrier devices, that can be switched by the MCU chip. 
Note that this will not improve the chip's ability to supply power - you can't, by any practical means - but the board can, through the switching transistors. 
Figure out your project's power budget, or the budget you'd like to have for your future projects, and design an on-board power supply to exceed that by a healthy margin-of-safety. Likewise, for the individual switched circuits, in order to choose a sufficiently robust switching transistor.
If you prefer an off-the-shelf solution, one of the available motor-driver boards is probably a good choice but it will need to have an independent power supply if there isn't one on the driver board.

user10794: I bought a Pi a while back, and found it so tedious to use I put it in my parts drawer and haven't used it since. A Pi is like a small Unix PC - you have to install an operating system, boot it up (which takes time) and then install device drivers etc.
The Arduino (particularly the Uno) boots in under a second, and "just works". It is much easier to use for small to medium projects.
As for the current, as others have replied, you aren't supposed to be powering things, like motors, from the output pins. 20 mA is a nice amount of current to flash an LED, or send data to another device. The output pins are for data not for driving things like motors. If you need more than 20 mA you should be looking at driver circuits, like transistors or MOSFETs.

... hard programming ...

I wouldn't say that. You program in C++. What are you going to program the Pi in? There are many libraries for the Arduino to simplify programming, but you can get down to the "bare metal" quite simply.

19384: <p>I am thinking about buying Arduino or Raspberry Pi.  I generally know Arduino advantages (fast start-up, low consumption, ADC converters) and disadvantages (low memory, low connectivity, hard programming), but I simply cannot figure out maybe the most important issue: current output for controlling devices.</p>

<p>One of the common cited advantages of Arduino system compared to Raspberry Pi system is that power per I/O pin is much larger.  Arduino Uno allegedly has up to 20 mA @ 5 V per pin and 50 mA @ 3.3 V per pin making quite substantial power of 0.15 W, compared to say typical 10 mA @ 5 V per pin for Raspberry Pi (although there is no official number available).</p>

<p>On the other hand</p>

<ul>
<li>this power is far too low to control any motors and you still have to
provide shields,</li>
<li>new version Arduino 101 reduced that current to 20 mA and even less for some other Arduino models.</li>
</ul>

<p>So, what is really the importance (if any) of these higher currents for home projects, compared between different Arduinos themselves and compared to Raspberry Pi?</p>

<p>There is also a question of total current for all pins together, which is also limited, but it is soooooo difficult to get official limits (for both Arduino and Raspberry Pi).</p>

<p>Please give me some insight into this issue, so I can decide.</p>
 4143: <p>The simple answer is: <strong>It doesn't matter.</strong></p>

<p>Both Arduino and the Raspberry Pi, and every other common microcontroller board, are designed to control things, not provide power to things.</p>

<p>Digital IO pins aren't designed to drive heavy loads. Yes, some (like the Arduino) can provide slightly more current, but that is really unimportant for most projects.</p>

<p>Yes, some boards have more powerful power supply circuits on them so you can draw more current from their power supplies, but again that is largely irrelevant.</p>

<p>The one thing that you seem to be forgetting (or haven't realised) is that <em>the microcontroller board does not have to supply <strong>all</strong> the power.</em></p>

<p>The Arduino, Pi, Teensy, chipKIT, whatever, is merely a controller. If your system needs more current it is up to you to provide that current. The Arduino etc merely controls the circuit or device that it is connected to.</p>

<p>It is very common to have multiple power supplies in a large project - each providing different voltages, or being used for different specific tasks within the project.</p>

<p>Just one thing you need to remember: <strong>Connect the grounds.</strong></p>
 313: <p>Bottom line is: the controllers are just not meant for supplying power. </p>

<p>They do have the ability to do so, within limits - fairly stringent limits for a controller chip itself, and <em>slightly</em> more generous limits for the on-board power supply. If you really need an all-in-one, you might consider making building your own controller board with a more capable on-board power supply and a number of transistor switches to provide power for the hungrier devices, that can be switched by the MCU chip. </p>

<p>Note that this will not improve the chip's ability to supply power - you can't, by any practical means - but the board can, through the switching transistors. </p>

<p>Figure out your project's power budget, or the budget you'd like to have for your future projects, and design an on-board power supply to exceed that by a healthy margin-of-safety. Likewise, for the individual switched circuits, in order to choose a sufficiently robust switching transistor.</p>

<p>If you prefer an off-the-shelf solution, one of the available motor-driver boards is probably a good choice but it will need to have an independent power supply if there isn't one on the driver board.</p>
 10794: <p>I bought a Pi a while back, and found it so tedious to use I put it in my parts drawer and haven't used it since. A Pi is like a small Unix PC - you have to install an operating system, boot it up (which takes time) and then install device drivers etc.</p>

<p>The Arduino (particularly the Uno) boots in under a second, and "just works". It is much easier to use for small to medium projects.</p>

<p>As for the current, as others have replied, you aren't supposed to be powering things, like motors, from the output pins. 20 mA is a nice amount of current to flash an LED, or send data to another device. The output pins are for <strong>data</strong> not for driving things like motors. If you need more than 20 mA you should be looking at driver circuits, like transistors or MOSFETs.</p>

<blockquote>
  <p>... hard programming ...</p>
</blockquote>

<p>I wouldn't say that. You program in C++. What are you going to program the Pi in? There are many libraries for the Arduino to simplify programming, but you can get down to the "bare metal" quite simply.</p>
