Geeetech i3 pro b: Extruder heating without command and exceeds target temperature

[GeoBen]

Hi,

I built my printer succesfully and it worked well for the first 3 prints or so, without problem. However, during the last print it stopped early due to the extruder overheating. I tried another print and the hotend no longer heated. I replaced the hotend and it now heats up - unfotunately it begins heating as soon as the mains power supply is turned on (despite the extruder control being 'off' in repetier host), and then will not stop heating until the power supply is turned off.

The extruder thermistor is working, it still displays the hotend temperature. I have checked the wiring multiple times and the hotend heater and thermistor are wired into the correct slots. The red LED below the port for the hotend heater stays on constantly whenever power is supplied and the voltage across it is 12V.

The mosfet is constanly at 12V, regardless of whether the hotend is switched on or off from repetier host - which I assume is the problem, as this post (http://www.geeetech.com/forum/viewtopic ... 19&t=17039) suggests it should be 0V when on, and 12V when off.

I have also successfully re-flashed the firmware, but this has made no difference.
My board is GT2560 Rev A+ using the Marlin v1 firmware.

What is the solution, do I need a new mosfet, a new board, or could there be an alternative explanation?

Thanks in advance,
Ben

[_kaktus_]

Hi.


Your diagnosis is correct.
It is very likely that the MOSFET transistor activating the heater is defective.

[GeoBen]

Thanks for the reply - I'll go ahead and replace that then! Very new to all of this so it's great to have some help.

Cheers
Ben