Alberto :

Thank you for your reply. Yes I am aware of the possibility of burning out the L298 in this manner.I have seen the LEDs glowing dimly when I rotate the motor by hand.
This kind of damage is more likely to happen when you have a gear down assembly, so that moving the slide manually will result in very high rpm for a considerable time at the stepper.In my case it is a direct drive.
For the self locking I have found that keeping Enable high, without clocking pulses gives you a large holding torque. However this makes the L298 get quite hot.Is this a valid way to lock the stepper motor ? Or do you use a electromechanical brake of some sort ?
Regarding dual power supplies I am using two bridge rectifier ciruits one for 18V of the stepper and another for the 5V generation. Both however are input from the same step down transformer and the ground is common. Is this what you meant or you have better ideas ?

Thanks for your time

Thomas Fernando