Just wanted to share my experience on 12V heated beds on 24V. I've got the following setup: - Prusa i3 printer - 12V 5A PSU for RAMPS 1.4 and arduino - 24V 15A for heated bed cheap chinese PSU - RAMPS board is a clone that supports 24V (fuse2 rated at 30V, 9A, markings: 30 FU900) - MOSFETS on the board are IRF3205, Vds = 55V, Rdson = 8mOhm, Id = 110A. This MOSFET is a lot better than the originby imqqmi - RAMPS Electronics