AFAICT, PWM-controlled +12 V fans expect +5 V logic levels, maybe +12 V if you're lucky, not PWM-ed +24 V from the RAMBo's supply: you can't directly use the board's MOSFET outputs.ksevcik wrote:Wiring the fan's PWM pins straight to the PWM pins on the RAMBO should work fine
Nearly all of the RAMBo's PWM-capable, logic-level outputs go to those MOSFETs or other on-board functions, with maybe something useful on X34, the "PWM Extension" header. The schematic isn't conclusive, but I think you must gimmick the firmware to redirect standard G/M-Codes fan control codes to the PWM hardware on a non-standard pin.
There's also a quibble about the PWM frequency; one PWM fan spec sheet suggests a 25-ish kHz PWM signal, which is way higher than the stock firmware PWM. The firmware already defines the hardware prescaler for the timers, so whatever timer setup you use must work around that.
I'd say it's not impossible to use PWM-controlled fans, but if that function isn't already baked into the firmware, prepare for some serious skull sweat...