Normally Closed MOSFET Switch


I am looking for a way to use a MOSFET as a normally closed switch such that when a battery charger is attached it opens the switch to disconnect the battery from the rest of the circuit. I want the load to be between the MOSFET source and ground, instead of between the battery and the MOSFET drain.

EDIT: I want the load of the battery to be disconnected. Thus the battery is only connected to the charger. The battery is a single cell LiPo and the charger will be built into the board so just charging power is needed to charge the battery.

Best Answer

Look at depletion-mode FET's. They're on by default and require some gate voltage to turn off.

Where an enhancement N-channel requires positive voltage to turn on, a depletion N-channel requires negative voltage to turn off. Similar relation between P-channels.

I'll leave the construction details to the student.