Stepper Motor Driver
A stepper motor driver is a circuit which take low voltage/current control signals for what the motor should do, and converts them to the high voltage/current needed to send to the motors.
There are generally two parts which are tightly integrated: a motor controller which converts gcode to motor signals, and a series of motor drivers which convert those signals to the levels needed for the motors. In the RAMPS system, the AM stands for arduino mega which runs a firmware which is the controller. The PS stands for pololu shield which is a few motor drivers connected to the arduino.
RAMPS is part of the main stepper driver for the RepRap project. It stands for RepRap Arduino Mega Pololu Shield.
It consists of an arduino shield which plugs into an arduino. Then you plug a few small pololu boards into it, each containing the actual motor driver chip for one motor.
CoolDRV is a RAMPS-compatible open source stepper motor driver. It's the small boards which would plug into the main RAMPS shield.
Kliment @Reprap IRC said, "I still think if you are only using 2.5A steppers on RAMPS then you should rather go with a drv8825-based design. That would be a drop-in for RAMPS. The ones I designed are available at https://github.com/kliment/cooldrv, GPLv3 licensed, and can be purchased at
They can definitely manage a 2.5A stepper with microstepping. Since their BOM cost is in the single-digit dollars, I definitely recommend you try them first. Their current control is at 0.25V/A, so you want to set them to 2.5A*sqrt(2)/2*0.25V/A = 0.44V. Remember, for a 2.5A stepper with microstepping, you only need sqrt(2)/2 of its max current in each coil, so 1.77A for a 2.5A stepper. The ones from Pololu are closed source and have slightly worse thermal design, but they should be able to keep up with your steppers too."
Pololu is a stepper motor driver. These are the small boards that would plug into a RAMPS board.
Versions of Pololu:
- The DRV8825 stepper motor driver carrier is a breakout board for TI’s DRV8825 microstepping bipolar stepper motor driver. The module has a pinout and interface that are nearly identical to those of our A4988 stepper motor driver carriers, so it can be used as a higher-performance drop-in replacement for those boards in many applications. The DRV8825 features adjustable current limiting, overcurrent and overtemperature protection, and six microstep resolutions (down to 1/32-step). It operates from 8.2 – 45 V and can deliver up to approximately 1.5 A per phase without a heat sink or forced air flow (rated for up to 2.2 A per coil with sufficient additional cooling). - 
Open source RAMPS-compatible driver. In progress.