Capacitor Sizes for 7805 Regulator

First Starting?

Build it with either; it will work fine for you. The second option with the large capacitors and the extra small one is "more stable." Just build this, do not worry about why if you are just wanting to start your project. Let us know if something does not work.

100nF

Use the values that are suggested in the datasheet. If you add capacitors a factor of 10 smaller (called decade capacitors) it will help with higher frequency noise (RF, or radio band noise), as an effects of the impedance of a capacitor.

Feel free to add as many decade capacitors as you want, but you will not need them unless you start having FCC testing. They cannot hurt.

100uF

When someone increases this they are allowing their circuit to pull power from the power source for longer. If you have a long power line this will act as a small tank of power. If you are going to have long power lines, or very inductive power lines, add this.

The Fairchild datasheet says on page 23

2. C_I is required if regulator is located an appreciable distance from power supply filter.
3. C_O improves stability and transient response.

where C_I is the input capacitor and C_O is the output capacitor.

So depending on your circuit you have some freedom in picking C_I.