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12v fan controller circuit diagram

Using this circuit diagram can be made a 12v fan controller, for 12v fan whose consumption does not exceed 200mA.
The electronic circuit allows adjustment of fan operation, depending on temperature.
To ensure the satisfactory functioning of the fan in any case, the supply must not fall below the starting voltage. Its value is equal to 12 V supply voltage minus the voltage, Zener's T3-R6-R7. With values given in the scheme,minimum voltage for fan will be 7 V. If the fan does not start at 25 ° C, temporarily replaced the temperature sensor with a resistor of 1.8 k and reduce the value of R7. If fan speed is too high, slightly increase the value of R7.
When the temperature reaches about 35 ° C, the circuit must be activated, as indicated by increasing fan speed or voltage of supply. If this happens, change the value of R2 or adjust with a potentiometer placed in its place. At increasing temperature, fan speed will increase.

Circuit Diagram: 
12v fan controller circuit diagram


how does the 12v fan controller circuit work??? what are the roles of the BJTs, thermistor and capacitor??

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