Heated bed works great for hours, then overheats

[jhoarty]

Hi, I have the Mendel Prusa i3 built with Arduino Mega 2560 and a RAMPS. It has the heated bed and it works great with one exception. The heated bed will get to temp and stay steady for a while ... sometimes three layers and sometimes hours ... but ineviably at some point the bed goes haywire and starts heating up way beyond the set temperature and my priter quits with the MAX TEMP error. How should I go about troubleshooting this? Any thoughts. Thanks!

[Mark]

seems like the mosfet that controls the heating is not stable, when you manually turn off the heating, does the LED lights of the mosfet still on ?

[jhoarty]

Yes! Exactly. It stays on even after I turn it off manually. Thank you. I have another RAMPS. I will switch it out.

[Mark]

I see, so you have to replace those mosfets, they are dead. Or check if they are shorted out or not.

[jhoarty]

OK, thank you very much!