120degC MOSFET, huge voltage drop on MK2 heatbed

I'm testing the electronics on my newly constructed Prusa, and I have two major issues: my heatbed MOSFET reaches over 120degC, and my heatbed will not go over 65degC.

I am using an ATX power supply that is rated to provide 16A at 12V. The wiring between the power supply, RAMPS 1.4, and MK2 heatbed is 18AWG speaker wire. The heatbed has a resistance of 1.5 ohms. The heatbed thermistor is kapton taped to the underside (silkscreen on top) with a drop of thermal paste for accuracy. The RAMPS 1.4 was ordered from www.reprapdiscount.com.

If I turn the heatbed on, but disconnect the heatbed wiring, I measure 11.67V from the power supply and across the MOSFET pins (D8). However, if I reconnect the wiring and turn it on, I get 11.13V from the power supply, and only 9.34V across the MOSFET. The MOSFET also becomes extremely hot (measured 120degC with a thermocouple). The heatbed thermistor never records higher than 63degC.

Will you please offer your advice?

Edited 1 time(s). Last edit at 01/23/2013 08:45PM by russd.

I would try wiring the heat bed directly to the power supply and see what temp and voltage you get.

Might help you rule out either the heat bed or the ramps as the fault.

Also, make sure of your connections - both the screw terminals at the ramps board AND the solder joint on the bed itself. I had a similar issue and the problem was the soldering.

Also, did you wire in an led, and if so what resistor value did you use.

Check for shorts her where you soldered. I'm not sure what the correct value should be, but 1.5 ohms sounds low.