With the rapid development of modern computer technology, the main frequency and integration level of CPU and LC are constantly improving, so a large amount of heat is generated to raise the internal temperature of the chassis, and fans are usually used to ventilate and dissipate heat. At present, most personal computers and computer servers are equipped with hardware monitoring circuits to monitor and control the computer's operating conditions in real time, such as CPU temperature, computer motherboard temperature, fan speed, voltage, etc.Pulse width modulation ( PWM ) and automatic control technology are widely used in the cooling fans in these monitoring circuits.During computer operation, the monitoring circuits will adjust the fan speed according to the temperature of the tested components to cool the CPU and computer components. The computer monitoring program or application program works with the hardware.When the component temperature drops, the fan speed automatically drops, and when the component temperature rises, the fan speed automatically rises.The purpose is to reduce fan noise, save power consumption, and prolong the fan life and the effective operation of the system.

          In personal computers and computer servers, the size and electrical parameters of the fan socket on the computer motherboard are the factual standards that all manufacturers abide by, although the brand, manufacturer and CPU frequency are different and the shape and speed of the computer cooling fan are different.The definition and parameters of the 3-lead fan socket are shown in Table L.

Table 1

Sequence number	Name  Call it	Reference  Number
1	Go to    Speed	Pulse signal
2.	Wind    Fan	12V DC
3.	Ground	0

        More than two fan sockets are usually provided on the computer main board.Plug 2 in Table 1 is the positive pole of the power supply and provides + 12V working power for the DC wind flat.This terminal voltage can be adjusted when the fan speed is controlled by pulse width modulation output ( it can also be set to a constant value, and the computer defaults to 12V ). The plug 3 is a common ground; Plug L is the feedback signal terminal for detecting the speed of fan rotation. 2 The lead fan socket has fewer speed feedback terminals than Table 1. However, only one fan can be connected to each socket, nor can it be connected to other electrical devices.

      Most computer mainboard hardware monitoring circuits have PWM fan speed regulation function.The computer mainboard provides PWM fan socket as an interface.The circuit principle when PWM output controls fan speed is shown in Figure 1.The circuit includes PWM control signal input, filtering, isolated amplification and output driving.The output current IO=Looomamax can directly drive the relay. The hardware monitoring circuit mostly adopts highly integrated application-specific integrated circuits such as W 83782 D, ADM 1031, etc.Take W 83637 HF as an example to discuss and illustrate that W 83637 HF has three paths of PWM fan speed control, and the fan speed control method is used to adjust the fan working voltage.The PWM duty ratio is used to adjust the fan working voltage, and the PWM output is converted into analog voltage after smooth filtering.The PWM duty ratio is adjusted so that the swing ( adjustment range ) of the output voltage is 0-12V DC, which is essentially a digital-to-analog converter. The duty cycle of pulse width can be controlled by program.The default duty cycle is 100 %, corresponding to 12V ( VCC ) output voltage.The fan operates in this case when the speed is not adjusted. Speed regulation:circuit output voltage VO=VCC * Duty Cycle, Vorriiri=VCC * 0 %=0 V, VX=VCC * LOO %=VCC=12V.There are also DC 5V, 24V and 48V working power supplies in computer and intelligent instrument circuits, which also apply to the above analysis.