Stepper Motors

5 wire unipolar motorThere are two main types of stepper motor commonly found printers and scanners:

The circuits that drive these motors are quite different. In the case of a bipolar motor two H-bridge circuits are used. Unipolar motors require a simple switching circuit for each phase, four being the most common.

Identifying coils and wires

Using an ohmmeter, the individual coils of a bipolar motor can easily be identified.

When dealing with unipolar motors the first task is to isolate the common wire. Using an ohmmeter, note the resistance between pairs of wires. As can be seen in the above diagram, the common wire will have half the resistance of the other wires across the coils.

Once the common wire has been found attach it to a supply voltage (5v to 12v in the case of printer motors)

 phase 1 Select one of the remaining wires and ground it. Assume this to be phase 1 and note the colour of the wire.

 phase 2 With phase one still grounded select another wire and ground that. If the shaft rotates clockwise it is phase 2.

 phase 4 If the shaft rotates anticlockwise it is phase 4 (i.e. before phase 1)

 phase 3 If the shaft does not rotate it will be phase 3 (i.e. opposite phase 1)

As can be seen, the individual numbering of steps is not important as the order of the sequence. Reversing the order will reverse the direction of rotation.

The total number of steps required for one 360 degree rotation is determined by how the motor is constructed and will therefore may different for each motor.