I know this is an old post/question, but the last entry is still asking for an answer... Your initial question/statement indicates that you have a heated bed, powered by a SEPARATE power supply... I have a similar situation: MY heated bed runs off of 120V (AC). I think MY solution to the issue, would work for you, as well... I have the D8 (or is it D10) switched voltage (12V off the RAMPS boaby wardmd - RAMPS Electronics