This has happened to me probably ten times with N-channel MOSFETs. I have a 10Kohm resistor from the gate pin to GND, I have source connected to GND and drain connected to cathode of LED. Anode of LED is connected to +3V3. I apply 5V to gate pin, the LED lights, I take the 5V off and it stays on, not for a few seconds but permanently. I check drain and source for a short, I see that they are connected with a resistance of about 400 ohms while gate shows about 1 millivolt with a MOSFET that has a gate threshold of 2V at minimum. (meaning the MOSFET is fried?). MOSFET has max drain-source voltage of 60V and it can carry 20A. Tried with different MOSFETS using a 9V battery and a lab bench power supply.

What's wrong here? Please help me, I am desperate. I just "fried" four SMD MOSFETs on my custom board. How can it break with such small voltages and currents?



  • 3
    \$\begingroup\$ "What's wrong here?" Some people may care, others not, but your schematic is "upside down". Traditionally, ground is toward the bottom of the schematic, and Vcc or whatever the supply voltage is called, is at the top. \$\endgroup\$ Commented Oct 28, 2023 at 15:09
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    – SamGibson
    Commented Oct 28, 2023 at 16:36
  • \$\begingroup\$ (The answer-comments have been deleted.) \$\endgroup\$
    – SamGibson
    Commented Oct 28, 2023 at 17:53

2 Answers 2


Try connect it this way instead (sorry for turning it upside down)

enter image description here

A little bit of explanation, the way the MOSFET is oriented in your schematics is that it will always conduct through the body diode. Why the FET is destroyed I don't know.

  • \$\begingroup\$ This what they describe in their question as being the wiring scheme already. \$\endgroup\$
    – tobalt
    Commented Oct 28, 2023 at 15:09
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    \$\begingroup\$ @tobalt It's not what they describe in their circuit though. \$\endgroup\$
    – marcelm
    Commented Oct 28, 2023 at 15:13
  • \$\begingroup\$ This schematic is better but there should be a resistor between the gate and the switch. Preferably a 10K resistor. Please correct the schematic. \$\endgroup\$
    – Fredled
    Commented Oct 28, 2023 at 16:08
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    \$\begingroup\$ Maybe "sorry for turning it upside down" should be "I turned it the conventional way up for you so that others can read it more easily"? \$\endgroup\$ Commented Oct 28, 2023 at 17:30
  • \$\begingroup\$ "Why the FET is destroyed I don't know." In the original circuit \$3.3V - V_{LED}\$ is short-circuited over the MOSFET (there is no current limiting resistor) \$\endgroup\$
    – Velvet
    Commented Oct 30, 2023 at 10:54

The way you have drawn the NMOS in your schematic is wrong i.e., like pointed out by MiNiMe, you have interchanged the source and drain.

Do you really have no current limiting resistor in series with your LED? If yes, that is also an issue.


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