All the values for the settings are what works for me with a GDSTIME 24V Dual Ball Bearing Fan. You might need to adjust these for your setup.
Copy the included aux_fan.cfg from the firmware folder to your klipper config folder and add [include aux_fan.cfg] to your printer.cfg.
Edit the aux_fan.cfg and replace the PIN in the [fan_generic Aux_Fan] section with the pin you have your fan connected to.
Klipper does not support controlling additional fans with M106 and M107 by default, so I added some replacement Macros which include a selection parameter P kinda like Marlin does. Index 2 gets matched to the Auxiliary cooling fan, because that is how BambuStudio and its forks (e.g. OrcaSlicer) control the auxiliary cooling fan.
Be sure to add a M107 or M106 P2 S0 to either your PRINT_END-macro or in the end-gcode of your slicer to make sure that the auxiliary fan and partcooling fan stops. Often there is only a M106 S0 executed at the end, which only turns off the partcooling fan but not the Auxiliary fan.
The aux_fan.cfg also adds Controls for the auxiliary fan to the Display in the Control Menu, so you can also control the fan with an V0-Display.
These new controls should appear right beneath the partcooling fan inputs, if they don't, you might need to adjust the index value in aux_fan.cfg.
...
[fan_generic Aux_Fan]
pin: PB7
...
Config-Example: Fan controlled with MOSFET connected to GPIO13 on your Raspberry Pi and your Pi is configured in klipper with a [mcu host] section:
...
[fan_generic Aux_Fan]
pin: host:gpiochip0/gpio13
...
After you have added the configuration to your klipper instance and reloaded klipper you should be able to control the fan using SET_FAN_SPEED FAN=Aux_Fan SPEED=XX where XX is a (decimal-) number between 0 and 1 (1 being 100%, 0.5 = 50% and so on).
If you are using Mainsail or Fluidd you should see a Aux Fan Slider right under the normal Partcooling-fan control, with which you can also set the fan speed.
There is no widespread support for auxiliary cooling fans in slicers.
The only one I know of is BambuStudio and its Forks (e.g. OrcaSlicer). The way BambuStudio controls the auxiliary cooling fan is by sending a M106 P2 Sxxx, where xxx is the speed on a scale of 0 to 255. That is why there is an included Macro for Klipper that allows this G-Code to function with klipper.
In the Filament settings window in the Cooling section set the auxiliary Cooling fan speed to what you want. This will enable the fan to the set speed after the first layers where the partcooling fan is disabled.
In the Printer settings under Basic inormation enable the Auxilliary part cooling fan setting. Then in the Filament settings set the fan speed to your wanted speed (like with standard BambuStudio).
