I used the same circuit for my project as shown in the web site below:

http://instruct1.cit.cornell.edu/courses/e...2hard.html (Figure 3).

But my stepper motor had very very very low resistance and the rated current should be 1.5A (the specification didn't have rated voltage). That's mean I can use any voltage with current = 1.5A, right? I tried to use +5V directly connected to the stepper motor. My TIP31C got very very very hot. What is the meaning of rated current? Can I use smaller current to reduce the overheat problem but still can make my stepper motor to rotate? Also, should I need to add four heat sinks for faster heat dissipation (6 leads with 2 leads common gnd)?