A Simple PWM Temperature Control Fan Circuit Design
In order to effectively solve the heat dissipation problem and minimize the unnecessary operation of the cooling fan, this paper designs a temperature control circuit based on the PWM speed regulation principle. The circuit uses a NE555 time-base integrated circuit to generate a triangular modulated wave, and the NTC negative temperature coefficient thermistor generates a modulated voltage signal that varies with temperature. The triangular wave and the modulating voltage generate a pulse width modulated PWM pulse width modulated wave through the comparator. The MOS power tube is then driven to control the speed of the fan to achieve continuous adjustment of the fan speed. The principle is shown in Figure 1.
In the figure, a triangular wave generating circuit is composed of U1 (NE555), D2, R2, R3, and C2. The frequency of the triangular wave is determined by the values of R2, R3 and C2. According to the parameters in the figure, the oscillation frequency is about 20KHz. The waveform measured at test point T1 is shown in Figure 2.
The PWM pulse width modulation wave is generated by the single-chip comparator Figure 1 PWM temperature control fan circuit schematic U2 (LM311). The triangular wave signal enters the positive input pin of the comparator, and the temperature-converted modulation voltage signal enters the negative input pin of the comparator, so that the PWM square wave is obtained at the 7th pin of U2 as shown in FIG. According to the voltage value of the top angle and the bottom angle of the triangle wave, the voltage dividing circuit of the temperature resistor can be designed to complete the relationship between the temperature and the PWM duty ratio (corresponding to the fan speed). In the figure, the potentiometer W1 is used to adjust the resistance division ratio to change the relationship between temperature and speed.
In this paper, several simple components are used to control the speed of the cooling fan, which avoids unnecessary rotation of the fan at low temperature and effectively reduces unnecessary running noise. The circuit has low cost and simple structure and has been successfully applied.