Fan speed controllers can be divided into five key types.
Conventional models rely on electrical resistance. They contain a wired controller circuit attached to an adjustable dial or toggle for changing the speed of the attached fans. These devices contain wire spools - conductors - with varying amounts of resistance to electrical current. Adjusting the dial on the fan speed control switch aligns a conductor with a particular fan - a process called placing it in series - and the new level of electrical resistance then changes the power reaching the fan, and thereby its speed. Higher levels of resistance will reduce speed or switch off the fan altogether.
However, this electrical resistance produces heat and so uses energy which could potentially match the energy saved by reducing the speed of the fan. Capacitor-based models address this issue by increasing the speed of the fan when the voltage drops - and decreasing it when the power increases. This means that no heat and energy are lost through electrical resistance. A capacitor is a device which stores energy as an electrical charge. The amount it can store is called its capacitance.
Capacitor-based models are typically smaller and provide fine, linear control of speed.
Meanwhile, thermostatic models respond directly to temperature. Sensors monitor the temperature within the chassis and switch the fan on or off according to predefined settings. This reduces fan noise when usage levels are low.
Time switch controllers make use of a timer switch. This is an electrical timer controlling a set of switches. Users can pre-set fans to run at certain times and speeds.
Pulse width modulation (PWM) is the default choice for laptop and PC fan controllers on the circuit boards of computers from many leading manufacturers. PWM provides a way for digital, binary devices like the microcontroller units (MCUs) on circuit boards to control non-digital devices like fans. PWM technology produces short, modulated pulses of voltage to simulate an analogue-style control output. Once in motion, inertia ensures that the fan continues to revolve even during the brief interruptions to power caused by the modulation. The microcontroller, in conjunction with the fan speed software, can make swift, on-the-fly adjustments to cooling fans.