PMOS_MasterSwitch_controlledByCheaperSwitch PUBLIC

The goal was to control a large current and voltage using a cheaper switch.
A solution is to use a PMOS, here M1, as master switch, itself controlled by the cheap switch.
When the cheaper switch is closed, Vg of M1 is less than its Vs. So M1 is conducting.
When the cheaper switch is raised open, Ir1 = 0, so Vg = Vs and thus, the PMOS does not conduct.

A PMOS (and an NMOS too by the way) does not conduct when the voltage at its source pin is equal to the voltage at its gate pin. From that state, to force the MOSFET to conduct, you have to decrease the voltage at its gate of a PMOS (or increase the voltage at its source, but that is far less commonly used). Here, that is done with a resistor between the source pin and the gate pin. Ans since de gate pin is directly connected to the ground, when the SmallSwitch is "ON", that leads to M1 to conduct too. From that state, if we raise the SmallSwitch ( "Off" ), the current through R1 becomes 0, so the voltage drop between the source pin and the gate pin become equals, closing the MOSFET and the circuit.