I have been seeing the 7805 power regulator on several design made out of the Arduino, and it makes me curious. As i understand it, the Arduino already have this regulator implented (?) so why do people use it in their designs later on, like on their breadboard prototypes etc. ?
One possible reason is that the SMT low-dropout regulator used on decimila has somewhat limited power dissipation capabilities. If your input voltage is much above 9V, or you have additional chips that consume much more current than the arduino itself, it may be necessary to provide an additional regulator, possibly with better heat-sink connectivity, to provide additional (or all) of the power…
Aha i see, that pretty much answered my qestion.
Another factor, most LDO’s have limited input voltage range. In the case of the MC33269T the maximum input is 20V, the 7805 family are rated for 30V.