user32532: I am making my own arduino.  I am in the process of hooking up power to the Atmega328.
In both these articles they use the L7805 voltage regulator with two 10uF capacitors.
https://www.arduino.cc/en/Main/Standalone
http://www.instructables.com/id/How-to-Build-an-Arduino-Uno-on-a-BreadBoard/?ALLSTEPS
I am looking at the datasheet for the regulator at http://pdf.datasheetcatalog.com/datasheet_pdf/sgs-thomson-microelectronics/L7805CD2T_to_L7885CV.pdf
The data sheet provides a number of figures for how to correctly hook up the L7805 in different situations (pages 4, 16-23).  I think Figure 13 represents our situation (Fixed Output Regulator).  In the figure they use 0.33uF and 0.1uF
capacitors.  Could someone explain why the examples use 10uF and the data sheet specifies 0.33uF and 0.1uF?

user30005: 
Could someone explain why the examples use 10uF and the data sheet specifies 0.33uF and 0.1uF?

One obvious reason is that those two examples are non compliant with the datasheet. 
But generally speaking the ordinary 78xx regulators use emitter output and are quite tolerant to the output capacitance. 10if is not a bad number.
However, there are also variants of 78xxx that take their output from the collector. They are more sensitive to output capacitance, as most ldos are.
Generally it is a good idea to stay within the datasheet but that doesn't mean you shouldn't step out from time to time.

user4143: The answers are in the notes:


Although no output capacitor is need for stability, it does improve transient response.
Required if regulator is locate an appreciable distance from power supply filter.


So  the values aren't critical to the operation, but you should have them there. Bigger capacitors give more energy to the regulator and more energy to your circuit (input and output capacitors respectively) but at the cost of lower frequency response. The actual values you select depend on your circuit, not the regulator. 10µF is a good "ballpark" figure to use in most circumstances.
The input capacitor reduces the impedance of the power source by providing a local source of power. The output capacitor is like the "reservoir" capacitor you should have at the power entry to all boards anyway, and reduces the impedance to the rest of your circuit.
For the higher frequency responses you should have smaller value decoupling capacitors by all your active components anyway.
You can also add more smaller capacitors to the output of the regulator to improve high frequency responsiveness if needed - you're not just limited to the one capacitor.

32532: <p>I am making my own arduino.  I am in the process of hooking up power to the Atmega328.</p>

<p>In both these articles they use the L7805 voltage regulator with two 10uF capacitors.</p>

<p><a href="https://www.arduino.cc/en/Main/Standalone" rel="nofollow noreferrer">https://www.arduino.cc/en/Main/Standalone</a></p>

<p><a href="http://www.instructables.com/id/How-to-Build-an-Arduino-Uno-on-a-BreadBoard/?ALLSTEPS" rel="nofollow noreferrer">http://www.instructables.com/id/How-to-Build-an-Arduino-Uno-on-a-BreadBoard/?ALLSTEPS</a></p>

<p>I am looking at the datasheet for the regulator at <a href="http://pdf.datasheetcatalog.com/datasheet_pdf/sgs-thomson-microelectronics/L7805CD2T_to_L7885CV.pdf" rel="nofollow noreferrer">http://pdf.datasheetcatalog.com/datasheet_pdf/sgs-thomson-microelectronics/L7805CD2T_to_L7885CV.pdf</a></p>

<p>The data sheet provides a number of figures for how to correctly hook up the L7805 in different situations (pages 4, 16-23).  I think Figure 13 represents our situation (Fixed Output Regulator).  In the figure they use 0.33uF and 0.1uF
capacitors.  Could someone explain why the examples use 10uF and the data sheet specifies 0.33uF and 0.1uF?</p>
 30005: <blockquote>
  <p>Could someone explain why the examples use 10uF and the data sheet specifies 0.33uF and 0.1uF?</p>
</blockquote>

<p>One obvious reason is that those two examples are non compliant with the datasheet. </p>

<p>But generally speaking the ordinary 78xx regulators use emitter output and are quite tolerant to the output capacitance. 10if is not a bad number.</p>

<p>However, there are also variants of 78xxx that take their output from the collector. They are more sensitive to output capacitance, as most ldos are.</p>

<p>Generally it is a good idea to stay within the datasheet but that doesn't mean you shouldn't step out from time to time.</p>
 4143: <p>The answers are in the notes:</p>

<blockquote>
  <ol start="2">
  <li>Although no output capacitor is need for stability, it does improve transient response.</li>
  <li>Required if regulator is locate an appreciable distance from power supply filter.</li>
  </ol>
</blockquote>

<p>So  the values aren't critical to the operation, but you should have them there. Bigger capacitors give more energy to the regulator and more energy to your circuit (input and output capacitors respectively) but at the cost of lower frequency response. The actual values you select depend on your circuit, not the regulator. 10µF is a good "ballpark" figure to use in most circumstances.</p>

<p>The input capacitor reduces the impedance of the power source by providing a local source of power. The output capacitor is like the "reservoir" capacitor you should have at the power entry to all boards anyway, and reduces the impedance to the rest of your circuit.</p>

<p>For the higher frequency responses you should have smaller value decoupling capacitors by all your active components anyway.</p>

<p>You can also add more smaller capacitors to the output of the regulator to improve high frequency responsiveness if needed - you're not just limited to the one capacitor.</p>
