I need to control 12v Fan with 3v3 uC. I need to stop and start it using uC gpio pins. I tried to use this schematic first: enter image description here But when I set uC gpio (14 in my case) to 0, fan starts to make a weird noice. I measured voltage on the fan and found about 6v on it. I replaced fet with the 2N7000 - the same result. So, I replaced fet with the npn then (removing r4) - the same. I thought it might be the problem with other pins, so I removed pins 3 and 4 in order to disconnect them - the same. Please help. I searched the internet and this site as well and found a lot of answers that it should work either with n-ch or npn transistor. What am I doing wrong? I didn't try to use 2 npns yet since a lot of sources says it shall work as it is.

I use tps62172 to power uC from the same 12v power supply as fan, so the ground plane is common.

  • \$\begingroup\$ I tried different 4wire fans (glkinst.com/cables/cable_pics/4_Wire_PWM_Spec.pdf) with the same result. For example thermalright tr-ty147a and scythe su1225fd12lr-rdp \$\endgroup\$
    – BUKTOP
    Sep 22, 2019 at 0:16
  • \$\begingroup\$ The ground is common, of course. The 12v power source is the only power source in the schematic, so there's no simple way to separate ground planes even if I'd like to. The fan definitely has a speed control pin, of course. It's the pin4 as it defined in the specs. It also has the sensor pin 3. But there's a 6v even when I remove these pins as I stated in the question. \$\endgroup\$
    – BUKTOP
    Sep 22, 2019 at 0:23
  • \$\begingroup\$ @jsotola lol it does not work this way. Please read the specs. Hint - I need to stop the fan completely ))) Anyway, it is not related to the question I ask \$\endgroup\$
    – BUKTOP
    Sep 22, 2019 at 0:35
  • \$\begingroup\$ @jsotola There's an explanation at the beginning. Ok, added some more details, didn't expect it's not clear from the schematic, thank you ))) \$\endgroup\$
    – BUKTOP
    Sep 22, 2019 at 0:48
  • \$\begingroup\$ you may need to use a high side switch ...... i read the datasheet again ... the fan connects to +12 V for power ... it has one open collector output that produces two pulses per revolution on pin 3 ... it has one speed control input on pin 4 ... this input has an internal pullup resistor to +12 V ... if left floating, the fan runs at full speed ... the fan speed is linearly proportional to the duty cycle of the control signal down to 15% of full speed ... speed below 15% is undetermined \$\endgroup\$
    – jsotola
    Sep 22, 2019 at 1:03


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