Couple of things:
How are you powering the Pico?
If you connect just the 12V and ground to the fan, then most will run at full speed.
Assuming this is so, what is the voltage between:
Tach and ground?
PWM and ground?
Most fans have an open collector Tach output so the voltage on Tach would be 0, if it is you could probably get away with using the internal pull up on the pico input pin. If not an external pullup to 3.3v.
PWM is potentially more tricky. Some fans have an internal pullup to their supply voltage. You pull that pin down to 0 for half the time to achieve the pwm signal. If the voltage you measure at the pwm pin with nothing connected is above 3v then you need a transistor in between the pico and PWM pin. An open collector arrangement should be fine. You could use an optoisolator, or a mosfet but subject to the paragraphs below.
Mosfets can be tricky with things like a Pico because its hard to find ones where the low VGS voltages available fully turn them on. You should test the mosfet you are using to turn on the fans by turning it on with the fans connected, and with no PWM connection BRIEFLY, Just long enough to measure the voltage across the MOSFET Drain to Source.
With the fans in parrallel the MOSFET will be passing a current of ~2.6A. The MOSFET datasheet doesn't specify max power with no heatsink but its a fairly chunky device so you probably wouldn't blow it up, but any voltage across the MOSFET is stealing power from the fans. I would not want more than 1V, even then its stealing 2.6W. Non FET Transistors avoid that problem so are worth considering.
If you have already worked through the above then sorry for taking your time to read this! Good luck with your project.
How are you powering the Pico?
If you connect just the 12V and ground to the fan, then most will run at full speed.
Assuming this is so, what is the voltage between:
Tach and ground?
PWM and ground?
Most fans have an open collector Tach output so the voltage on Tach would be 0, if it is you could probably get away with using the internal pull up on the pico input pin. If not an external pullup to 3.3v.
PWM is potentially more tricky. Some fans have an internal pullup to their supply voltage. You pull that pin down to 0 for half the time to achieve the pwm signal. If the voltage you measure at the pwm pin with nothing connected is above 3v then you need a transistor in between the pico and PWM pin. An open collector arrangement should be fine. You could use an optoisolator, or a mosfet but subject to the paragraphs below.
Mosfets can be tricky with things like a Pico because its hard to find ones where the low VGS voltages available fully turn them on. You should test the mosfet you are using to turn on the fans by turning it on with the fans connected, and with no PWM connection BRIEFLY, Just long enough to measure the voltage across the MOSFET Drain to Source.
With the fans in parrallel the MOSFET will be passing a current of ~2.6A. The MOSFET datasheet doesn't specify max power with no heatsink but its a fairly chunky device so you probably wouldn't blow it up, but any voltage across the MOSFET is stealing power from the fans. I would not want more than 1V, even then its stealing 2.6W. Non FET Transistors avoid that problem so are worth considering.
If you have already worked through the above then sorry for taking your time to read this! Good luck with your project.
Statistics: Posted by Pi5_User — Wed Feb 05, 2025 11:53 pm