Hi,
can anyone plz tell which transistor to choose and how to design stepper motor driver circuit.
stepper motor features as follows.

Type- Unipolar
Dc coil resistance -30 ohms and above
circuit works at- 8-18V dc
driver ckt should have max current of 600 mA.

Please help.

 

http://hades.mech.northwestern.edu/wiki/index.php/Unipolar_Stepper_Motor_Driver_Circuit

They mention a DS2003 IC on that page; it is only rated for 350mA, has only seven channels, and is obsolete.

Instead, you could use an ULN2803 which has eight Darlington pairs, rated 500mA per output. Use two channels per motor winding, but separate them. For example, for the first winding you could use channels 1 and 5, for the 2nd, use channels 2 and 6, etc. This will help in heat dissipation.

For a great many other options, use this search:
http://www.google.com/search?hl=en&q=Unipolar+stepper+driver+schematic

 

sgtwookie has provided a couple of good references. Do you have a specific question about stepper motors?

As has been mentioned there are many sources of information on stepper motors on the Internet.

hgmjr