jaunedeau wrote:-Why plan a pic18f2250 version ? The PIC18F25K50 is almost compatible, have same ram / flash / eeprom / instruction set, has cap touch, does NOT required external crystal for usb, and is half the price of the 2550. It drops one of the 16 bits timers for a 8 bit, and is not (yet ?) avaiable in QFN.
cause Ian wanted usb and 18F2550 was smallest USB enabled pic I had in a drawer. Also IIRC when I started this PIC18F25K50 was not readily available (at least not for me locally) while 18F2550 was available on every corner. Another reason is that I know 2550 pretty well while new PIC's have bunch of bugs and you really need to study both datasheet and errata before you design anything.
jaunedeau wrote:-Why is the line after the diode bridge labelled 35V ? If I remember correctly (from 17 years ago :) ), 24V AC (rms) after the diodes and capacity should give 24V DC ?
You do not remember correctly. 24VAC (RMS) will have peak of 24*SQRT(2) so 34V peak that will drop only by diode voltage drop after you rectify it and add capacitor so depending on the type of diodes and the current you will have 33-34V on the output.
jaunedeau wrote:-Could'nt you make a simpler / cheaper PSU by using just a half bridge rectifier with a bigger capacitor (then getting a not perfect 12V DC), and then with this lower voltage and only 600mA needed just use a 5V regulator ?
I tried, it was heating up too much, and no - you do not get 12V using half bridge only, try it out :D, if you use big enough cap you will have same voltage as with full bridge, you just get with smaller average voltage with smaller capacitor but you still have peaks at same 34V as with full bridge so it wil not really solve the heating problem .. it's way simpler to go with DCDC konverter