Hi, I made a circuit to control heated bed with external mosfet assembly.

Hi, I made a circuit to control heated bed with external mosfet assembly. I checked it on breadboard, it works just fine.
But when I connect heated bed wire and control connector from ramps it powers up and begins heating bed, even though I have not set it to heat up.
Why could this happen?

Inverted logic in the heat bed signal perhaps?

Inverted heat bed power supply?

It seems right, but I will recheck it once more.

Most heatbed and extruder heater outputs are switching the ground side and +dc is always on. So the HB output will probably float high when the HB is off due to an led or something on the motherboard but be driven low when the board turns on the HB.

@James_Armstrong ​ You were right. Just measured switched off voltage and it was 6 volts. Now I need to find how to cut voltage below 10 volts.

Is it hooked to a Ramps board?


Yeah @James_Armstrong is right. Can you swap it for a logic level P-channel Mosfet?

@Gerald_Giarmo ​ I will try to do it.


Ok, 3 mistakes here that can be fixed. First R2 and R3 are acting like a voltage divider. Forcing the gate to a constant 6V, reduce R3 to something like 10 - 500R. Second, you need to cut the GND path between R3 and Q1 source. It completes the circuit and causes Q1 to always be on. The 3D printer drivers mostly leave VCC connected and put Nmos on the GND line. So if you connect the GND pin to a return you defeat the control circuit on the 3D printer board. Third, when you make the 2 above changes you will have one last issue. When the Driver board tries to turn the bed off it will actually be turning it on and Vic versa. You can probably change the code to compensate for this(not sure how hard it would be).

I posted a Optoisolated circuit that I think does what your trying to do, should have by protoboards in a few days to see if it will stay cool enough to work the way I made it.