High torque Hybrid bipolar stepper motor Nema 23 with 178 oz ... chip, and are then connected to an NPN transistor. The transistor used, TIP120, is actually a NPN Darlington ( ... a standard 1N4001 diode is usually placed across each transistor as shown in the figure, providing a safe ... way of dispersing the reverse current without damaging the transistor. Sometimes called a snubbing diode. The TIP120 ...