How can I apply a ZERO when J4 is open?
Note that J4 represents that CARD-1 is out of the backplane. Also, J3 represents that CARD-2 is out of the backplane.
So, how do I detect that J4 is open?
Also, keep in mind that the MOSFET is useded to prevent a reverse polarity of the power supply. Replacing that MOSFET with a diode I can solve the problem. But, the problem is that the circuit needs about 20A and that means a big power loss in the diode.
(Note: R3 and R4 are 2 ohm and not 100 ohm)