Stepper Motor Driver (Circuit Diagram & Schematic)

What is a Stepper Motor Driver?

A stepper motor driver (or stepper motor drive) is the circuit that sequences and powers a stepper motor. It usually has a controller, a current-handling driver and the motor connections.

A microcontroller pin cannot supply winding current, so the driver IC or transistors sit between the controller and the motor.

Ready-made boards now take most of the wiring: connect the phases, apply motor power and send the step pattern.

Once the phases and supply are connected, the motor will step as soon as the controller outputs a valid sequence.

These circuits come in current and voltage ratings, so match the driver to the motor’s ratings.

Components of a Stepper Motor Drive

1. Controller (a microcontroller or a microprocessor)
2. A driver IC to handle the motor current
3. A power supply unit

Stepper Motor Controller

Pick the controller first. It needs at least four output pins for a four-phase stepper. Timers, an ADC or a serial port are extras that depend on the job, such as speed control from a pot.

Stepper Motor Driver

Most new boards use a driver IC instead of a handful of discrete transistors.

Those ICs cost little and cut the wiring and layout time.

Match the driver to the motor current and voltages. The ULN2003 family is a common low-side array for unipolar motors. That hobby stepper motor drive sinks phase current; a bipolar motor needs an H-bridge or a current-regulated chip instead.

Each Darlington pair in the ULN2003A is rated 500 mA collector current and 50 V. High duty or several channels at once still needs a heatsink and derating.

Power Supply for Stepper Motor Drive

Small unipolar hobby motors often run at 5 V to 12 V and 100 mA to 400 mA. Larger hybrid steppers need more current and a different driver. Use the supplier ratings and a regulated supply so torque does not sag as the windings heat.

Power Supply Unit

A 7812 voltage regulator is typically rated about 1 A, so the figure adds an outboard pass transistor for a higher current. That extra current is set by the transistor, not a fixed 5 A. Fit a heat sink sized for the total current draw.

The block diagram shows the flow and interconnections between the driver board’s components.

Miscellaneous Components

1. Switches, Potentiometers
2. Heat sink
3. Connecting wires

Comprehensive Stepper Motor Drive

The power stage does nothing useful until the microcontroller sends the phase sequence through the driver.

A stepper can run in full step, wave drive or half-step. A usable board lets the user pick the mode, change speed and start or stop the shaft.

Those functions take extra microcontroller pins. Two pins can encode the mode and the run/stop command.

One analogue pin reads a pot. The ADC value then sets the step delay, which sets speed.

Program Algorithm

1. Initialize the port pins in input/output modes.
2. Initialize the ADC module.
3. Create separate functions for half-stepping, full stepping, wave drive and delay.
4. Check two port pins for operating mode (00-stop, 01-wave drive,10-full step, 11-half stepping).
5. Go to the appropriate function.
6. Read the Potentiometer value via the ADC and accordingly set a delay value.
7. Complete one cycle of sequence.
8. Go to step 4.

Driver Board

If you lay out your own board in CAD software such as EAGLE, make the motor tracks wide enough that they do not overheat.

Motor windings are inductive, so keep those traces away from quiet signal paths and run ERC and DRC before you etch or order the board.

Note:
Building a Stepper motor driver is mainly a power-supply and driver-IC choice. The microcontroller is the easier part.

Many micro-controllers can output a step sequence. The voltages and currents at the windings are what usually limit the design.

A board that covers a useful voltage and current range can be reused on more than one motor, instead of a new layout for each project.