I'm a newbie here. This seems to be a very helpful site though

I found this circuit on microchip's website. (Sorry for the obvious questions but I just can't seem to figure this out =(

Here is the link: http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1824&appnote=en025234

I want to build this circuit but before I do, I want to understand it. Particuarly, I'm concentrating on the voltage regulator circuit to bring AC voltage to 5V to power the MCU. (.25A Fuse, R2, R3, D1, D2, C2). However, they post an explanation but it's very diluted.

I just need to be able to understand the calculations so that I can configure the circuit to how I want it. But I'm not sure how they make the calculations to choose the particular values of each component. Any help would certainly be appreciated.

Thank you in advance!

Erin