I am having an issue with my heated bed not turning on or off automatically or with g code or acting automatically like a thermistat. Hotend is fine. I am running a separate PS through a FET board and so can easily monitor actual heater status via LED. I can turn bed on or off via console, that is push button control by setting PLA Bed Preheat, but it runs right past set temp. and will not turn off. Setting control higher or lower by console has no effect. Setting Cooldown on the console DOES turn it off - thus showing that it is under software control and 99% not a hardware issue. Commands from Repetier Host have no effect on the bed heater at all - either Repetier buttons or sending M140 commands manually, although they show up on the console LCD properly (that is - M140 50 shows 50 degrees is set on the LCD) But again, no action is taken when the actual bed temp runs right on past the set temp. I have changed everything I can think of in Marlin to no avail. I have set it to bang bang (I think) used different PIDs - Nothing. Any help will be appreciated.

Delta Mini-Kossel clone, Mega2560, Ramps 1.4

Edited 2 time(s). Last edit at 02/10/2020 11:08AM by BikerJim.

OK - Got it figured out. First off; I had the FET/relay for the H Bed connected to D9, which had power (12v and gnd) on when Hbed was turned on via control panel. Still don't know why that was, have to look into that later. It's the old engineer in me - gotta know. ANYWAY - all I had failed to do was to connect the extra 12v (12V2) to the power input. Everything works fine now and even the control panel control works, and on D8. Must have been a "sneak path" somewhere that gives bizarre conditions when hooked up wrong like I had it. Like I said; I'll research that later. Sorry I did not get any input from the forum but I guess it stumped others as it had stumped me until I finally looked up the Ramps schematic.

On your RAMPS, check to make sure that none of the metal tabs on the MOSFETs are touching each other. I had that problem with my RAMPS when I got it and I've seen at least one other report of that. If two MOSFETs are touching, turning on either of them will also cause the other to turn on. In my case, whenever I turned on the part cooling fan the heatbed would also turn on and go into thermal runaway.

The fix is just to bend the MOSFETs back until they are no longer touching.

Thank you MMclure. Yes - the tabs are connected internally to the MOSFET Drains and will cause the issue, I agree. In my case the tabs were not connecting and I used two different processor boards and Ramps 1.4 boards with the same indications. Looking again at the schematic for the ramps I am not seeing a sneak path right off. So I am still befuddled and we'll see if I have some time coming up to explore this more. As a design engineer I used to track down issues like this every day but I am out of practice and my best test equipment is currently down. Thanks again for the response and maybe this will help somebody else in the future.