I have MacMini with broken PWM (pulse-width modulation) 4 wire FAN controller (FAN runs at full speed all the the time) and as MacMini schematic isn't available, I bought an external PWM FAN controller, but turns out that FAN socket +12V is constantly on, so I have idea to add simple switch, which get input from FAN socket PWM control signal + 3.3V and switch +12V, 0.5A output to the external FAN module.

Found this thread - Switching 12V with an active low 5V signal

So basically I need similar schematic, but with lower input voltage and with the contrary logic, so that the output voltage is +12V when the control signal is +3.3V, and 0V when the control signal is 0V.

Maybe adapt this one - NPN transistor to run 12V 0.5A from 3.3v 4mA

Does anybody have such schematic?

  • \$\begingroup\$ Can you leave the +12V connected and switch the ground side? \$\endgroup\$ Jul 7, 2017 at 7:37
  • \$\begingroup\$ Yes, switching the ground is also fine. \$\endgroup\$
    – ksuuk
    Jul 7, 2017 at 8:15
  • \$\begingroup\$ @Bruce Abbott would electronics.stackexchange.com/questions/30737/… solution 8 work for me? \$\endgroup\$
    – ksuuk
    Jul 7, 2017 at 8:59

1 Answer 1


A MOSFET based switching will be ideal solution for you. Basically you need a high switching speed MOSFET with gate control voltage of 3.3 volts and 0v at gate will anyway result in 0v drain voltage.

Digital Output configuration

I had chosen this MOSFET for my project and it worked well for me. The input is coming from a micro-controller GPIO pin having 3.3v voltage level. You can connect your 3v3 control signal directly at this point.

Further, the output is supposed to be connected to inductive loads such as motor or solenoid, in which case freewheeling diode is added on purpose. Since your load is FAN which might be having an inductive motor, adding such diode will be suitable for you.

VCC5 is mentioned for 5v compatible loads. You can directly connect this point to 12v voltage source as the MOSFET is compatible to sustain upto 60v. Read the datasheet for full details.

  • \$\begingroup\$ As external module ( aliexpress.com/item/… ) controls the fan, I think that the FAN type isn't important? Easiest way would be taken +5V from motherboard USB socket and use 5VDC relay to switch +12V to the external FAN module, but I'd just like it to keep as simple as possible and manage only between motherboard socket and external module. \$\endgroup\$
    – ksuuk
    Jul 7, 2017 at 8:23
  • \$\begingroup\$ Then a MOSFET as I suggested is good to go. It would be cheaper than relay and having a small form factor also/ \$\endgroup\$ Jul 7, 2017 at 9:12
  • \$\begingroup\$ Your's is dual, would electronics.stackexchange.com/questions/30737/… solution 8, singe work for me? What diode did You use and do I need use input gate resistor? \$\endgroup\$
    – ksuuk
    Jul 7, 2017 at 9:27
  • \$\begingroup\$ I used dual for my project, you can use single one. Diode is STPS3L40U schottky type. The gate resistor is used for GPIO protection and known voltage level, refer this electronics.stackexchange.com/a/13077/11813 \$\endgroup\$ Jul 7, 2017 at 9:43

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