I'd just like to document here, how, for my RapRaps, I solved the known issues of the Sanguinololu 1.3a board with respect to the 5V conflict between the USB port and the 7805 regulator, and the possible backfeeding of the USB 5V into the 12V input line. I don't know if others have posted the same ideas before (I searched, but didn't find). So in case I missed something, please be gentle with me - it's my first post here ;-)
Instead of just cutting a trace or even completely removing the 7805 regulator (which are usually recommended), I simply added two diodes, one into USB Vcc input line, and the other into the 12V input line of the 7805 regulator, as is shown on the attached modified Sanguinololu schematics. These diodes ensure that the current is not flowing in the wrong direction.
For keeping the voltage drop as low as possible, I use a Schottky diode on the USV Vcc line. I found it easier to solder the anode of the diode onto the 5V pin of (usually unused) USB2TTL connector.
For avoiding the backfeeding of the USB 5V into the 12V input (and through it back into the switched-off power supply), I solder the diode directly to the input leg of the 7805 before soldering it to the PCB, as shown on the attached pictures.
I use BAT43 Schottky diodes that allow forward currents of 200mA. This is just fine as long as you do not connect heavy external 5V loads to the Sanguinololu. For higher currents, you might want to use 1N5819 or similar.
I have used these mods on at least 6 Sanguinulolu boards over the past year, and never again had any power problems, regardless whether I have power just from USB (e.g. to program the ATmega1824), just from 12V supply (when printing from SD-card) or have powered both USB and 12V supply (when printing via USB).
Instead of just cutting a trace or even completely removing the 7805 regulator (which are usually recommended), I simply added two diodes, one into USB Vcc input line, and the other into the 12V input line of the 7805 regulator, as is shown on the attached modified Sanguinololu schematics. These diodes ensure that the current is not flowing in the wrong direction.
For keeping the voltage drop as low as possible, I use a Schottky diode on the USV Vcc line. I found it easier to solder the anode of the diode onto the 5V pin of (usually unused) USB2TTL connector.
For avoiding the backfeeding of the USB 5V into the 12V input (and through it back into the switched-off power supply), I solder the diode directly to the input leg of the 7805 before soldering it to the PCB, as shown on the attached pictures.
I use BAT43 Schottky diodes that allow forward currents of 200mA. This is just fine as long as you do not connect heavy external 5V loads to the Sanguinololu. For higher currents, you might want to use 1N5819 or similar.
I have used these mods on at least 6 Sanguinulolu boards over the past year, and never again had any power problems, regardless whether I have power just from USB (e.g. to program the ATmega1824), just from 12V supply (when printing from SD-card) or have powered both USB and 12V supply (when printing via USB).