I built this circuit and it seems to work if i use a the same power supply to the motor that is powering the IC. I am using an LED in place of the motor for testing. But the circuit does not work if i use a separate power supply for the motor(LED). I am using a 12 volt power supply for the IC and a 9 volt battery for the motor(LED). I am trying to control a 90 volt DC motor and plan to rectify 110VAC to power the motor. My question is should I be able to power the motor with a separate supply? Does the ground for the MOSFET need to be the same ground that the IC uses? I am using a IRF630 for the MOSFET.