RAMPS + DRV8825

I have ordered this Arduino Mega 2560 + RAMPS + 5 * DRV8825 + LCD 12864 set, loaded it with Repetier firmware and found out that it works nicely, hooray!

Things to keep in mind: DRV8825 boards were configured for Vref = 1.6-1.7 V, which is way too high. This translates into output current of 3.2-3.4 A, as the formula for DRV8825 is

    I_TripMax = Vref / (5 * Rs)

Or, as Rs is 0.1 Ohm (resistors are labeled as "R100") -

    I_TripMax = Vref / 0.5

which is the same as

    I_TripMax = 2 * Vref

See these links - 
http://reprap.org/wiki/A4988_vs_DRV8825_Chinese_Stepper_Driver_Boards
https://www.pololu.com/product/2133

This current can damage your motors, so before connecting the motors to RAMPS you must adjust Vref. Thankfully it's not that hard: plug in 12 V power supply into RAMPS (Arduino is powered from RAMPS board), and measure Vref between the ground pin of DRV8825 board and top of adjustment potentiometer, as shown:

I have soldered a wire to the ground pin and wound it onto the ground probe of my multimeter because I only have two hands. With the red probe touching my screwdriver that I used to turn the potentiometer I was able to adjust Vref and check its value on the multimeter.

The first link recommends setting Vref to 0.5 V (which translates into 1 A current) and using that as the starting point to further tweak Vref. However when using puny CD / DVD stepper motors, I think 1 A is waay too high, so I started with ~0.1 V, and went up to 0.16 V. With this Vref my motor works just fine and doesn't seem to overheat. I may end up increasing Vref when I add mechanical load to the motor though.