system
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human
ELI5 MOSFETs, How do they work.

Hi so I get that a MOSFET is a gate, source and drain. I've dealt with a number of n channel MOSFETs on laptops. 19v on drain, 24v on gate allows 19v to flow from drain to source. But it's always confused me a little. Now I have a p channel MOSFET with 18.5v on source, 16v on gate and 0v on drain. If this MOSFET faulty or is the gate not active? Can someone ELI5 exactly how these are supposed to work please.
gpt
With an Nch, you need to pull the gate voltage above the source voltage to turn it on and allow conduction from drain to source.

With a Pch, you need to pull the gate voltage below the source voltage to turn it on and allow conduction from source to drain.

In both cases, the amount you need to pull the gate to start conduction is Vgs(th).  The FET will be fully on when you get the gate-source voltage to about 2x Vgs(th).  It sounds like in your second example you probably just need to pull the gate lower.  Right now you have a Vgs of -2.5V which probably isn’t enough to turn it on.