Unipolar Stepper Motor Driver

A unipolar stepper motor driver is a circuit that is responsible for energising the coils of a unipolar type stepper motor. The driver part of the circuit is responsible only for energising the coils; however, it is not responsible for the sequence logic. A driver is used when the logic circuits such as a microcontroller or logic IC does not possess sufficient drive current to directly energise the coils. Usually microcontroller pins can supply only around 20 mA, which is insufficient if the motor requirement were to be 1 A. The driver part of the circuit is responsible for providing sufficient drive current and voltage to energise the coils of a stepper motor. It does this with the use of NPN bipolar junction transistors (BJT), or MOSFETs, to switch current to the coil loads.

Depending upon the manufacturer and model, some unipolar stepper motors do not provide the centre tap but instead four wires of the coil, which you have to join externally to make the centre tap. They do this to give the engineer more flexibility, and usually there is component documentation to show which wires to connect for the centre tap. Since the centre tap is a common connection, the implementation of the driver circuit is extremely simple.

This circuit will work for CMOS and TTL logic. In this circuit diagram, we use simple NPN BJT to energise the coils.

In this example, the manufacturer of the stepper motor does not provide the centre tap, and instead we have four wires of the coil, so the centre tap establishes externally. Therefore, box A in the circuit diagram shows phase 1 and phase 2 coils, which are on the same stator, and box B shows phase 3 and phase 4 coils on a different stator. Since their centre taps connect to the positive voltage rails, the circuit becomes very simple.

This circuit requires four identical, transistors, base resistors, and diodes. For the transistors, you could use NPN Darlingtons with high current handling capability. The protection diodes could be 1N4004, and the base resistors could be around 1 kΩ in value. A base resistor ensures the correct input current for proper saturation of the transistor.

Phase 1 and phase 2 coils have a centre tap to the positive voltage rail. Phase 3 and phase 4 coils also have a centre tap to the positive rail. The driver part of the circuit could be on a separate voltage rail from the logic part of the circuitry when the motor has greater voltage and current requirements.