I am trying to design a small control unit to control LED strips, but for the moment it does not seem to work correctly.

enter image description here

This is the current wiring diagram. Why does the MOSFET output only 8 V at room temperature of 20°C (68°F)?

Why do I have 10.5 V at the output when the MOSFET heats up to 95°C (203°F)?

Why does it heat up so much? Isn't the IRF540N a 33 A MOSFET? Yet it must supply a current of only 1.92 A.

  • 1
    \$\begingroup\$ I think the problem is with the gate drive. The MOSFET needs 10 volts to turn fully on, but you're giving it at most 4.4 V. And depending on the PWM frequency, R2 may be too large. \$\endgroup\$
    – Oskar Skog
    Commented Oct 4, 2022 at 16:31
  • \$\begingroup\$ How are you measuring the voltage on the drain? Is it average DC voltage across the transistor, or across the load? \$\endgroup\$
    – Oskar Skog
    Commented Oct 4, 2022 at 16:33
  • \$\begingroup\$ You are right @Oskar Skog ! I just did a test and powering the mosfet correctly and the problem is solved. But then why does the BC368 not supply 12Volt to the mosfet? \$\endgroup\$
    – Pinguto
    Commented Oct 4, 2022 at 16:48
  • \$\begingroup\$ The load was measured by powering it directly without the mosfet \$\endgroup\$
    – Pinguto
    Commented Oct 4, 2022 at 16:55
  • 2
    \$\begingroup\$ T1 is configured as an emitter follower - the voltage on the emitter is much the same as on the base. Thus the gate will be lucky to see at least 5V to turn on. You need to add a pnp transistor or choose a mosfet with a lower gate voltage requirement. \$\endgroup\$
    – Kartman
    Commented Oct 4, 2022 at 17:30

2 Answers 2


Your NPN transistor is a voltage follower, it can never put out a higher voltage than the input.

Since you are not fully turning on the MOSFET, it is operating in linear mode. You want a very low voltage drop from drain to source when the MOSFET is ON.

To get a higher gate drive, ground the emitter and connect the MOSFET gate to the NPN collector. Note that this circuit will invert the logic, so you need to invert the logic in the MCU output.


simulate this circuit – Schematic created using CircuitLab


With N channel FETs - ideally - the gate drive should be as high as the drain. If you want a mofet to dissipate little heat, it needs to be fully off, or fully on. If the gate voltage is in the middle, the Mosfet will be "half way" on and dissipate a lot of heat. Make the gate voltage higher on the IRF540 and you'll get better results. Another good thing would be to model your setup in spice or run hand calculations on it to check the power dissipation.


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