Although the motor fan has high reliability, it is still a mechanical device. When it is used for a long time, its speed may drop or even stop. Therefore, it is best to monitor the running status of the fan in real time to find problems in time. The alarm sensor can give an alarm signal when the fan speed is lower than a certain threshold value, and the speed signal output can realize the real-time monitoring of the fan speed.
The alarm signal output from the fan circuit has two states: "high level" and "low level". The meaning represented by the two levels is generally in accordance with the positive logic system. High level means "fault" and "low level" means "normal". The speed signal output from the fan circuit is usually in the form of pulse. Each wave head indicates that the fan has turned one circle. Such a signal can be directly provided to the host through the data bus for display. The output speed signal of some fans is not the true speed of the fan, but a multiple of the speed. For example, 2, 4 or 6 pulses are generated per revolution, which must be processed to form a signal reflecting the true speed of the fan. If you want to distinguish whether the fan speed is the real speed or a multiple, you can use the tachometer to measure the actual speed, and then compare it with the displayed data.
The fan speed measurement signal is generally output from the three lead plug. The yellow and black of the three leads are +12v power supply and grounding respectively, and the other color is the speed signal output line. It should be noted that the third lead of some three lead fans is not the speed measurement signal output line, but the speed control signal line, through which the speed control signal is input to the fan motor.
