Running unipolar stepper motor with TIP120

[irving2008]

Diodes are required with bipolar transistors like the TIP120/122. Do not run it without them, it will only end in tears...

In a motor driver configuration you often see two diodes, one from the collector to the +ve rail and one from the ground rail to the collector - the first protects against back EMF from the motor and prevents the collector voltage going above the +ve supply rail. The second protects against the reverse polarity spike and prevents the collector being pulled below the emitter which can also cause transistor failure.

The diagram below shows these diodes and how the transistors should be wired. Note the common ground point for all the high current wiring and the typical 8-diode protection arrangement. The diodes across the transistors are not needed for TIP120/122 as these have built in diodes, but you do need the ones across the motor coils.