PIC Current Limiter
Adding Current Limiters to the PIC/Generation 1 Electronics
The PIC/Generation 1 electronics do not have any form of current limitting. They rely on the stepper motors resistance to keep the current below 2 Amps at 12 Volts.
V = IR (V = voltage, I = current, R = resistance)
R = V/I
If V = 12 and I = 2 then R = 6 Ohms
Stepper motors with a resistance of 6 Ohms and up will work with the PIC/Generation 1 electronics with no modifications.
Unfortunatly the more commonly available Steppers of the size used in RepRap have much lower resistances. e.g. The current BitsFromBytes supplied Steppers have a resistance of XXXX.
Current limiting can be added to the PIC/Generation 1 electronics via a few modifications to the existing board and the creation of a daughter board.
Creating the daughter board
Bellow is a diagram of a peice of strip board layed out to provide current limiting to 4 coils, 2 motors.
This may look complex, but that comes partly from showing both sides of the board at once.
Modifying the main board
Below is the schematic for a single coil, each stepper motor has two coils. As each coil requires one comparator an LM393 (dual comparator) could be used. It is almost certainly cheaper though to use an LM339 (quad comparator) and build the circuit to handle 4 coils / 2 motors.