I've attached two files.

The one with the three diagrams shows the simulation working as intended. The one with only one diagram shows a result I don't understand.

I'd like this circuit to do three things:
- Only turn on when a master device is turned on as well and turn off when the master is turned off. I'm doing this with a MOSFET
- Protect for overcurrent (5A). I'm doing this with a fuse (represented by a switch in this simulation)
- Have a status LED. off when off, green when on and the fuse isn't blown, red when on and the fuse is blown

If i used a normally open relay to turn on and off according to the voltage given by the master device I know this would work, but I'd like to do it with a MOSFET to keep the build small as I have several of these to fit in the enclosure.

Is there something I don't understand about MOSFETs used as switches? Could this be a glitch of the simulation?

If so, what characteristic / parameter do I have to look for to select the proper FET for this application once I'm ready to go to the breadboard?

 

Looking at the first of three diagrams, the gate of the MOSFET is floating so operation is undefined. Connect a resistor, say 10k, from gate to source.
In the second diagram the opamp inputs are connected together so the output will be the gain times the input offset voltage. Put a resistor, say 10k, in series with the non-inverting input and a resistor, say 100k, from the non-inverting input to ground

 

Awesome thanks. Unless I misunderstood some of your directions, I tinkered a bit with the values and the simulation works as intended now. Connecting the gate to ground and the non inverting input to ground, is that what's referred to as pull-down or pull-up resistors?