I am trying to finish my new printer and the bed is reverse. Idle the power is flowing and bed it heating, signal it to heat and it turns off?
How the heck can this happen? Unplugged the bed, same. Tried to reverse the bed power polarity, nothing works. The part numbers between two boards on the mosfet are showing the same, but they do look different. Is there anyway this circuit can be reversed (like if the LED is backwards) or is this some sort of knock of mossfet that is backwards? No stray solder blobs or bridges, the Heater zero is working so I assume the tc4427e is okay.
Of course it is the only skr I have here on hand, sold out of the rest.
Are you using any kind of SSR or mosfet after the skr, before the heated bed?
When BED PWM is off, the pin 1 (HEAT_BED, (-) on the bed connector) should be 12V (or 24V). When BED_PWM is on, that (-) should be 0V. Both of those are measured from the power supply ground.
If the LED was backwards, then the led wouldn’t work. But I don’t think that would reverse the logic.
This is a low side drive mosfet. An N-channel. I can’t imagine it working in any backwards or P-channel mosfet configuration. But maybe.
Are you sure the bed power in is the right way? I imagine if that was backwards you would have bigger problems.
I double check and then even reversed the polarity. What if I swap them…
Physically It is plugged in just like the other 4, I have checked that a few times.
I checked continuity vs another board and the mosfet reads the same in all combinations of the three pins.
Bed temp set to 0, mosfet led is on, on at boot. Negative PS to bed - =0, ps neg to bed+=12v
Bed set to 50 led goes off, ps neg to bed negative 10.5v, ps neg to bed+= 12v
Just a bad mosfet? Why 10.5v grrrrrrrrrrr.
I am going to swap the LR board out for this one I guess. I do not think I will be able to resolder that mossfet it is sitting on a bunch of VIAs. even using hot air, I think that would be tough.
The 10.5V makes some sense. It is floating, but tied to 12V through an LED. If the voltage drop across the LED was 1.5V, then I think that makes sense. The voltage would climb until it can’t light the LED anymore.
Are you sure the firmware is right? It sound too perfectly reversed to be electronics.
Before blaming the mosfet take a quick look at U7, it is supposed to be a TC4427E which is dual non-inverting… make sure it is not a TC4428, which has 1 inverting and one non-inverting o/p…maybe probe the pins to check i/p vs. o/p
