Hello Bruce,

Overheating of the TIP31 is a straight forward issue - you are exceeding its dissipation limits. But the hero of your problem is that stepper guy.

I think before you want to drive a stepper you need to decide on a few things :

- Current drive or voltage drive.
- Type of clocking - with or without acceleration control.
- Is it Bipolar or unipolar.
- Drive electronics - discrete or Integrated.

I think in your case it is a voltage drive, unipolar with discrete electronics. And if your transistor is heating then maybe you forgot the limiting resistor in series with the winding . 5V / ( Dropping resistor + Winding resistance ) = rated current of each winding.

I am not sure if you can switch over to packaged ICs at this stage. If yes then log on to ST / Allegro - they have excellent bipolar drive ICs that come packaged with everything you want - like the L297+L298 control & drive pair from ST. With it you can get stabilized chopper current drive+ half or full step mode+ full over load protection. And the 297 is a breeze to interface with MCU's

And for stepper applications whatever the final drive stage, dont forget to use freewheel diodes of the FAST RECOVERY TYPE. A common IN4007 wouldnt do the job.

Good luck

Raghu