Has any one made use of the Ramps circuit for a bed heater, D8 is defined as the Bed Heater circuit and derives its 12V supply from the second pair of power supply connectors on the 4 pin connector so can use a second power supply. The D8 circuit appears to have the same power capability as the Extruder heater, the Marlin software has the settings to use either Bang/Bang or PWM control of the bed heater. The temperature can be measured with a thermistor or use a thermocouple via an AD595 again built into the software.