PWM Fan Control in Your Grow

In most DIY grow boxes the fan runs either at 100% or not at all. Sometimes in between, via a cheap dimmer controller that causes the motor to oscillate and produces a high-frequency whine that goes through walls.

PWM — Pulse Width Modulation — does it right: stepless speed control, silent, energy-efficient, with feedback on actual RPM. The Growix system controls three independent fan circuits with it.

What PWM is — the basics

PWM switches a motor's control signal very rapidly between 0 and 100%. The motor integrates these pulses due to its inertia into an average speed. The duty cycle — the percentage of time the signal is HIGH — determines the effective speed.

The frequency — why 25 kHz is non-negotiable

PWM at low frequency — 50 Hz, 200 Hz, even 1 kHz — is within human hearing range. The motor responds to each pulse as a mechanical event: coil engages, releases, engages. This creates a buzzing or whining noise.

25 kHz is the standard PWM frequency target value for PC fans (4-pin standard). The Growix uses this frequency on all three channels. The result: 48–50 dB sound level at 1 metre — barely perceptible in operation through the closed box.

3-pin vs. 4-pin fans

The Growix uses exclusively 4-pin fans. The tachometer signal typically outputs 2 pulses per revolution — the Pi reads these pulses and calculates the actual RPM. This means: the controller always knows if a fan is blocked or has failed.

Minimum duty cycle — the stall threshold

PWM is non-linear. Below a certain duty cycle, a fan no longer starts reliably. This stall threshold lies at 10–30% duty cycle depending on the fan. Growix OS calibrates this threshold automatically at first start for each channel:

# fan_calibration.py
def find_stall_threshold(pwm_channel, tach_pin):
    """Find the minimum duty cycle at which the fan runs stably."""
    for duty in range(5, 50, 5):
        set_pwm(pwm_channel, duty, freq_hz=25000)
        time.sleep(2)
        rpm = read_tach(tach_pin)
        if rpm > 100:
            return duty  # Stall threshold found
    return 30  # Fallback

The three fan circuits of the Growix

Growix OS keeps exhaust permanently slightly stronger than intake — this creates the desired negative pressure that prevents odour seepage. When VPD deviates by more than 0.15 kPa, the system responds with stepped ventilation increases — first exhaust, then intake — to dampen VPD fluctuations rather than amplify them.