Heater Bed Issue

Heater Bed Issue - Ramps 1.4 Marlin
MK2B Dual Power PCB Heat Bed

For some reason my heatbed wont turn off - every time I turn the printer on the heat bed LED lights up and the bed heats away regardless of turning it off within the software. I have to physically turn the machine off to stop the heatbed. The display shows the temp rising so appears to be being reported correctly by the thermistor.

I have reflashed the firmware and checked the connections.

Any ideas ?

Check for bad solder joints on your heated bed mosfet.

Soldering looks spot on with good joints

Check the gate voltage when you switch heat on and off in software. If it swings normally (near zero to near 5V), the MOSFET is shorted and should be replaced.

Just to clarify - if the gate voltage goes from 0 to 5 when the software is calling for heat, that means the rest of the electronics are ok and the MOSFET is bad? I have the same problem with my hot end (likely caused by a short in the wiring, now fixed).

Do you have a link to how to measure the MOSFET gate voltage? I am going to search for that piece of info now.

Meter from the gate lead to ground . . . if that varies, and the output does not == dead MOSFET . . .

Onthe schematic/board, the gate is the only one of the three with no connection to the load or power.

- Tim

Edited 1 time(s). Last edit at 05/05/2015 12:27PM by tadawson.

Thanks!