stepper motor torque

[JoeJoe]

hello all, just a question on increasing the torque from my stepper motor. i built a controller using this schematic:


i am using an old computer power supply to power the motors (rated at 12vdc 75 ohm's). i know you can increase the torque by increasing the voltage, but you have to limit the current but i have no idea how to do that. to give a baseline, lets say i wanted to triple the voltage to 36v, what would the schematic be for the current regulation?