Patent ID: 7579794

Claim:
A fan system with hysteresis character, the fan system comprising: a fan; a hysteresis circuit comprising: a temperature sensor; a first operational amplifier having a first negative input end coupled to the temperature sensor and a first output end; and a second operational amplifier having a second negative input end coupled to the first output end and having a second positive end coupled to a first power supply for outputting a first voltage when the temperature sensed by the temperature sensor is greater than a first temperature, and for outputting a second voltage when the temperature sensed by the temperature sensor is lower than a second temperature and a second output end; and a fan switch comprising a metal-oxide semiconductor (MOS) coupled to the hysteresis circuit and the fan for controlling a rotational speed according to the first voltage or the second voltage outputted by the hysteresis circuit; wherein the first temperature is greater than the second temperature.