by cvoinescu » Mon Sep 16, 2013 10:50 pm
Can you turn the heater off at all, for example by setting the temperature setpoint to zero? If the MOSFET is shorted, you would not be able to turn it off at all.
Carefully measure the voltage on the gate of the MOSFET (vs. ground); it should be a few volts while the temperature is below the setpoint, then it should go close to zero; it may oscillate a few times around the setpoint. If that happens but the heater keeps going, look for a short, a bad MOSFET, or a wiring error. Note that the MOSFET switches the ground (negative pole) of the heater; the positive pole is tied to the supply voltage (via a fuse). If the gate voltage stays high, then the problem could be with the microcontroller, the firmware, or, again, a short-circuit somewhere on the Arduino or the RAMPS. Are you sure you're set to use the right MOSFET output? Are the thermistors wired to the correct inputs? Are you looking at the right temperature measurement on the display? The same problem can occur if you miswire thermistors and not realize you're looking at what the firmware thinks it's, say, the second hot end temperature, but it's actually the bed.