1. Thermal Control
   The fan controls its fan speed according to the ambient/system temp. it senses and provides adequate airflow to cool the heat source. With this function, the fan will not always run at full speed, then the life of the fan will be longer and the noise is can be reduced.
   
   Case 1
   
   
   
   Case 2
   
   
   
   Case 3
   
   
   
   
2. Additional Features
   
   Delay Start
   The fan starts to run in X seconds after power on
   Benefit: Fan can be hot swap
   
   Ramp Start
   After power on, the fan runs X% of the full speed first, then ramps up to the full speed in X seconds
   Benefit: No inrush current will be seen
   
   Current Limitation
   It’s an overload protection
   
   Fan Speed Limitation
   The fan will run at constant speed, even if more voltage supplies to the fan
   
   Alarm
   Alarm signal will be sent out to the system when the fan is running below X rpm. People will know the fan is failing

Variable Speed

Pulse Width Modulation (PWM)
The PWM fan speed control method adds an extra 4th wire to the connector. The 4th wire is a Pulse Width Modulation (PWM) input that provides a duty cycle to the fan. For example a 50% PWM duty cycle is a perfect square wave where 50% of the signal is high and 50% of the signal is low. A 50% PWM duty cycle applied to the 4th wire of the fan will result in a fan speed of half the total maximum fan speed. That is, if the fan is rated for 4000 RPM max, a 50% PWM duty cycle will result in the fan running at 2000 RPM. An 80% PWM signal applied to the fan is a square wave were 80% of the signal is high and 20% is low resulting in a fan speed proportional to the duty cycle as referenced to the maximum speed of the fan or blower.

Micro Controlled
A micro controlled fan or blower is a digitally controlled air-moving device that is controlled by firmware contained within the microchip fan controller. This type of speed control is more efficient and can reduce current consumption. A micro controlled fan can also be PWM controlled, thermistor controlled, and voltage controlled.

Voltage Controlled
A voltage controlled air-moving device can be a 3 or 4-wire fan or blower. The voltage is reduced or increased to speed or slow the fan or blower.

Thermistor Controlled
A variable speed fan is automatically controlled by the fan circuit, which changes speed as the temperature changes to provide optimum air flow at all times. A thermistor speed controlled air-moving device can record and provide feed back to the fan or blower motor controller of what the ambient air temperature is near the fan. With this data, the fan speed can increase when circumstances are required for more cooling.