I am attempting to replicate the circuit below on a through-hole breadboard and am not getting the expected results. The "Bed-Heat" input on the far right of the schematic is a digital signal from a logic board of 0V or 5V to turn the transistor on. The two 'VBED' inputs up top correspond to my +24V in from a DC power supply, and the GND node contains both the ground from the Bed_heat input (logic board) and the ground from the power supply. The output is on the left (X11J and X11I), and should yield +24V when the input signal is on and 0 when the signal is 0.

As of now I can get the output to show +24V when on, but it's never at 0V when I turn off the input signal; instead I get a value between +14 and 16V. I've experimented with a couple different mosfets and diodes, and tried a 10 Ohm in the place of the 100 Ohm resistor going to the gate.

The transistor in question I'm using is PSMN2R2-40PS, and the Schottky diode is SB3100E-G. I'm not using the LED shown so that part is an open circuit in my case.

The circuit drives a heated bed for a 3D printer powered by a Rambo board (arduino). I have a power supply unit just for the heated bed and the electronics fans, separate from the power supply for the logic board.

Appreciate any pointers!

Edit: see two pictures at the top of this post for the breadboard I'm trying to make this circuit on. Red wire at top right is VBED (+24V in), thin red wire at the bottom is Bed_heat +5V input signal. The grounds of both inputs (white wire at top, black wire at the bottom) are shorted to each other via the orange wire on the board. The terminal at the bottom right will have the outputs X11J and X11I respectively left and right.


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    \$\begingroup\$ Why is D10 needed? \$\endgroup\$
    – User323693
    Feb 7, 2020 at 16:14
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    \$\begingroup\$ "As of now I can get the output to show +24V when on, but it's never at 0V ..." - How are you measuring this? Across X11J and X11I? Is there also a load present on these terminals? If not, what happens if you connect a load and re-measure? \$\endgroup\$
    – marcelm
    Feb 7, 2020 at 17:57
  • \$\begingroup\$ @marcelm is correct in asking about load. If there is no load there is a current path through the LED which will provide voltatge to a high-impedance voltmeter. What you want to do is put a load on the output and measure the voltage across the load. \$\endgroup\$ Jun 6, 2023 at 15:24

1 Answer 1


For the N-FET to work correctly you need to make sure to tie the GND of both power systems together. In other words the GND of the Rambo (Arduino) board needs to connect to the GND of the bed heater power/switching system.


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