SD variable fan speed controllers regulate the speed of single-phase voltage controllable motors with a maximum current of 1,5 A. The output voltage towards the motor can be manually regulated via the central knob from maximum to minimum. The minimum speed can be adjusted via an internal trimmer. The enclosure allows inset mounting (IP44) or surface mounting (IP54). Their contemporary design is not the only difference with the good old MTX series. The new SD series are microprocessor controlled to guarantee accurate motor control and to minimise motor noise. Phase angle control (Triac technology) is used to vary the motor voltage and to regulate the motor speed.