Unipolar Stepper Driver with 20A Handling Capability

The project illustrates a unipolar stepper motor driver that is able to handle 20A current.

When driving a stepper motor, the voltage provides the speed while the current provides the torque. For the current to rise, the trick is to get the voltage into the windings fast enough. The stepper can turn up to a limit when the initial voltage is higher while limiting the current. The result is always a compromise through the challenges created by raising the voltage.

The popular L297 driver IC is the basis of the driver since it is widely available and very well documented. From the circuit found on the site, a monostable U4 with time constant of approximately 10s is triggered by the incoming pulses so the output is normally high and Q7 OFF. The output goes low and Q7 starts to conduct after 10s of inactivity which keeps the motors and driver cooler.

The L297 enable line will be pulled low because the comparator will flip when the temperature rises above 65°C that is determined by a simple thermostat. This will also signal ESTOP to the CNC software and to an indicator light.

Rest of the project