i am trying to get my head around p channel mosfets and bought some A03401 mosfets. As i understand the mosfet will not allow current to flow if Vgs == Vds and thats why the pull up resistor is there. I have not even attempted to connect a microcontroller yet because the led was on in this configuration. i read Vgs and Vds and they are both 0.6v and so i am not sure why the mosfet is allowing current to flow. any help would be much appreciated. thanks. ps. appolgies for poor picture quality, camera is broken.


  • 2
    \$\begingroup\$ It looks like your Source and Drain are swapped. You are talking about a PMOS device, correct? Remember how current flows in a PMOS and NMOS device :) \$\endgroup\$
    – user103380
    Jul 9, 2018 at 15:29
  • \$\begingroup\$ If you can't get the connections right at least try and get the part number correct (O not 0). \$\endgroup\$
    – Andy aka
    Jul 9, 2018 at 15:32
  • \$\begingroup\$ ah yes thats the problem thanks! i was still in the mindset of n channel mosfet, you'll have to forgive me i am just a hobbyist. thanks for the help. one last question. would it be safe to switch 4.2v with a 3.3v arduino using this configuration? \$\endgroup\$ Jul 9, 2018 at 15:43

1 Answer 1


You have the P-MOSFET connected wrong. The Source (S) terminal should be toward the 5V and the Drain (D) should connect to the LED in your diagram.

The condition where the P-MOSFET will be off is if the voltage between the Gate (G) and the Source (S) is less than the VGSTHR.

The reason that the LED lit up in your drawn configuration is because the body diode was forward biased and letting current pass from the D terminal to the S terminal.


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