Most PCs are provided with a cooling fan to ensure that the internal temperature does not rise unduly. Unfortunately, in many PCs, the fan noise soon becomes an irritant. Since for a large part of the time, the fan cools the PC more than is necessary, it seems sensible to make the speed of the fan dependent on the ambient temperature. That is the purpose of the circuit shown. The circuit, designed with discrete component, is intended for the control of 12-V fans that do not draw a current exceeding 200 mA.
To ensure that the fan operates satisfactorily in all circumstances. the supply to it must not drop below its starting voltage. That voltage is equal to the 12 V – supply less the ‘zener’ voltage of T3-R€-R7. With values shown in the diagram, the supply to the fan will be at least 7 V. If the fan does not start at 25 00C. replace temperature sensor temporarily by a 1.8 IcS2 resistor and lower tie value of R. If the fan runs too fast, raise the value of R7.
Transistors T1 and T2 compare the fixed potential at junction R3-R4 with the temperature-dependent one at junction R1-R2. It may be found convenient initially to place 25 1(..Q potentiometer meter in the R2 position, adjust this till the fan runs correctly, measure the resistance and then replace it by a fixed resistor of that value. Place the temperature sensor in the warm air flow of the fan. When the computer is switched on, the speed of the fan, owing to Cl. will be fairly high, but will soon drop to a minimum. With a thermometer, measure the
temperature of the outflowing air Close to the sensor. When the temperature has reached a value of about 35 ∞C, the control circuit should come into action, indicated by an increase in tie speed of the fan or its supply voltage. if that does not happen, change the value of R2, or adjust the potentiometer in its place. When the temperature rises, the speed of the fan will increase. The maximum speed will be only slightly lower than that without the control circuit. This is thanks to the fact that T3 can be driven into hard conduction so that the drop across it is only some tenths of a volt.