The stepper drivers convert pulses into the necessary power drive to the stepper motor coils. They also support microstepping to get higher resolution – this is where each pulse does not shift the stepper shaft one step, but rather sends power to the coils so that the shaft is held positioned partway between steps. Microstepping increases resolution at the expense of torque.
The Pololu driver is actually soldered to a carrier breakout board to make it easier to plug into DIP sockets. The model I am using is the A4988.
It is important to note that some stepper motors are rated at 3V or 5V per coil, but the recommended input to the driver is 24V. Never attach a voltage higher than the rated amount directly to a stepper coil – the driver actually pulses the 24V so that the energy delivered is equivalent to that of continuous 3 or 5V, yet the higher voltage provides better motor control.
I bought some extra drivers which proved useful later when I busted one pressing down too hard with my custom modified CPU heatsink.
Note: You must adjust each driver for the amperage rating of your steppers to ensure proper current limiting. This is documented on the Pololu and Reactive Substance web sites.
Read on to find out how to choose a good stepper motor.