By adjusting the knob, the PWM duty cycle can be manually adjusted from 100% to 0%. Speed limiter controller for 4-pin PWM fans. Alternatively, with separate power supply, the fan speed can be manually controlled via a knob. Can be used with a motherboard to limit the fan's maximum PWM duty cycle/maximum speed; within the limit range, the motherboard can still freely adjust the fan speed. Even with an external motherboard PWM signal, the fan speed can still be freely adjusted within this limit range to achieve speed limiting.