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The goal of the circuit below is to use a PMOS to turn an NMOS on and off which will blink the LED. A microcontroller is providing the square signal. Both MOSFETs are logic level and when I use them individually with the LED the behavior is what I expect. However in the circuit below when I send 5V to the gate of the PMOS the LED will just remain ON, but I expect it to turn off. Is it incorrect to wire the drain of a PMOS to the gate of an NMOS?

enter image description here

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    \$\begingroup\$ YOu need a drain resistor on the gate of M1. \$\endgroup\$
    – Trevor_G
    Mar 17, 2017 at 19:23
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    \$\begingroup\$ A 5V square wave from microcontroller can toggle the gate of a logic level MOSFET, if you connect it directly to M1. Why do you need the P-channel MOSFET? Is it there only for the [admirable] purpose of learning ins and outs of different kinds of MOSFETS? \$\endgroup\$
    – Nick Alexeev
    Mar 17, 2017 at 20:28
  • \$\begingroup\$ It was for the case that I have multiple LEDs and I want to control them with a single arduino pin. I know that MOSFETs are voltage controlled switches, but turning the NMOS on and off at a high speed would draw some current from the microcontroller(not sure exactly how much). I was afraid that this current(accumulated for multiple NMOSs) would be too much for a single arduino pin so I thought a high side PMOS switch attached to a separate 5v supply doing the controlling would be safer. \$\endgroup\$
    – tinker102
    Mar 17, 2017 at 20:44

1 Answer 1

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You need a drain resistor on the gate of M1. And by drain, I mean from the gate to ground.

The gate has a capacitance that needs to be drained off or the channel will remain open. Select a resistor size that will drain that in the order of magnitude of time that is reasonable for your application.

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    \$\begingroup\$ Thank you, that did solve the issue. I'm just curious, when I was using the NMOS only with LED and had the microcontroller connected to the gate, I didn't add a drain resistor and the LED was switching fine. Is it proper to still have a drain resistor in that configuration? \$\endgroup\$
    – tinker102
    Mar 17, 2017 at 20:23
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    \$\begingroup\$ @tinker102 It was fine before because the micro has a push-pull output so no bleed resistor was required. IN this version you basically have only push. \$\endgroup\$
    – Trevor_G
    Mar 17, 2017 at 20:45

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