Vik009:
I am trying to figure out where to place my USB Host shield. The voltage from the Arduino pin is to be supplied to the gate. Do you have any idea what would be the connection point for the ground and VCC of the USB Host shield. I am trying to understand the functionality of the switch. Once I apply gate voltage, current would start flowing from the battery towards the drain as the junction within MOSFET would become forward biased. Should we put Vout as the VCC of the USB host shield. Then where shall we join the ground of the USB Host ?
You have a good point. If you wire up the host shield like Nick's schematic shows, with the shield in place of the load, when the transistor is off then that is like pulling the ground from the shield. This is usually not a good thing. You usually want everything to share a common ground. The circuit works good for closing a relay or lighting a light, but does it work for powering a circuit board?
Perhaps what is needed is a different circuit, maybe using a P channel FET.
BTW: It's probably not important, but why do you want to be able to switch the shield OFF and ON?