PC fan control. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Computers

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To reduce the noise level, computers are equipped with fan speed control systems depending on the actual temperature of the radiators of the cooled devices (processors, power supply transistors, etc.) The article below is devoted to the control of the processor fan.

Almost all PC processor fan control devices regulate their speed (and therefore performance) by changing the supply voltage supplied to the motor. In some regulators, voltage reduction is achieved by reducing the potential at the positive output of the fan (in this case, its negative output is connected to a common wire), in others, by increasing the potential at the negative output (the positive is connected to the +12 V power wire).

Since the output of the fan tachometer is made on an npn transistor connected according to an open collector circuit (the emitter is connected to a common wire), in the first case there are no problems with transmitting its signal to the motherboard. In the second case, it is impossible to remove information from the tachometer, since the potential of the negative power output of the fan varies within 1 ... +7 V).

The situation can be changed by using the simplest signal level converter assembled on a transistor optocoupler (Fig. 1). Since the output voltage of the control device varies from 5 to 11 V, and the frequency of the tachometer signal does not exceed hundreds of hertz (at 12 min-000 it is 1x2 = 200 Hz - due to two magnets in the rotor), it is enough to ensure a clear operation of the optocoupler, and the signal from the tachometer will be transmitted to the system board.

The converter is included in the break of the tachometer signal wire between the fan and the system board. Resistor R1 is selected in such a way that the optocoupler reliably operates from the voltage applied to its LED (5 ... 11 V) and at the same time the current flowing through it does not exceed the permissible value. Instead of AOT123A, it is permissible to use any other transistor optocoupler (or a diode optocoupler with the addition of a transistor operating in the key mode).

In some cases, it is possible to modify the fan control unit, eliminating the need for a level converter. An example is the device described by L. Ridiko ( ). It was assembled on a comparator K554SAZ (KR554SAZ or LM311), operating in linear mode (like an op-amp). The fan is connected between the positive power wire and the output of the open collector comparator (pin 9), and the open emitter output (pin 2) is connected to the common wire. If the fan is equipped with a tachometer, then its signal level converter is required.

However, the K554SAZ comparator allows you to use an output with an open emitter to control the load. To do this, pin 9 is connected to the positive wire of the power source, the load is connected between pin 2 and the common wire, and the output of the tachometer is connected to the connector of the PC motherboard. In this case, the non-inverting input of the comparator becomes inverting, and the inverting input becomes non-inverting.

A schematic diagram of this version of the fan control device is shown in fig. 2 (in parentheses are the pin numbers of comparators in an eight-pin package). Due to the inverting of the comparator inputs, the R8C2 OOS circuit is connected between pins 3 and 2. The values ​​​​of resistors R4 and R6 have been changed, which is due to the use of a zener diode with a different stabilization voltage and a KT816 series transistor instead of KT814.

The device is mounted on a printed circuit board made in accordance with Fig. 3. It is designed for the use of fixed resistors MLT, tuned SPZ-38b, capacitors K52-1B (C1, C3), KM (C2) and a KS162A zener diode in a miniature glass case. Transistor VT1 is fixed on the processor heatsink. As an insulator, a thin mica plate was used, cut to the size of the transistor with a small margin (about 1 mm) at the edges. To reduce thermal resistance, it is lubricated on both sides with non-conductive thermal paste.

If it is intended to use a load with a significantly higher current consumption, it is connected to the output of the comparator through an additional transistor of the npn structure (series KT815 or KT817), connected according to the common collector circuit.

Resistor R5 is "responsible" for the displacement of the control characteristic, and R8 - for its slope. The resistance of the latter is 0,1 ... 1 MΩ (the larger it is, the lower the temperature of the radiator will reach the maximum fan speed). The temperature of the radiator, at which the rotational speed becomes maximum, should be 5...10°C lower than the critical temperature, when the stability of the processor and the system as a whole is already violated.

Resistors R4 and R6 are selected in such a way that at normal temperature (+25 ... 30 ° C) the voltage at the emitter of transistor VT1 is in the range of voltages at the upper and lower (according to the circuit) terminals of the tuned resistor R5. In conclusion, this resistor achieves fan rotation with a minimum frequency at a radiator temperature of +25 ... 30 ° C.

It should be noted that due to the features of the selected comparator switching circuit, the maximum voltage at its output does not exceed 9,8 V. Therefore, the fan should be taken with a margin of performance.

Author: M.Naumov, Moscow

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