Refinement of the AT power supply. Scheme, description

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Refinement of the AT power supply. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Computers

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As you know, the fan in the power supplies of AT format computers rotates at a constant frequency, regardless of the temperature of the high-voltage transistor cases. However, the power supply does not always deliver maximum power to the load. The peak of power consumption occurs at the time of turning on the computer, and the next maximum - during the period of intensive disk exchange. If we also take into account the fact that the power of the power supply is usually selected with a margin even for maximum power consumption, it is easy to conclude that most of the time it is underloaded and the forced cooling of the heat sink of high-voltage transistors is excessive. In other words, the fan pumps cubic meters of air in vain, while creating quite a lot of noise and sucking dust into the case.

To reduce fan wear and reduce the overall noise level generated by the computer, you can use an automatic fan speed controller, the diagram of which is shown in fig. 1.

Refinement of the AT power supply

The temperature sensor is germanium diodes VD1 - VD4. included in the opposite direction in the base circuit of the composite transistor VT1VT2. The choice of diodes as a sensor is due to the fact that the dependence of the reverse current on temperature is more pronounced than the similar dependence of the resistance of thermistors. In addition, the glass case of these diodes makes it possible to do without any dielectric spacers when installing power supply transistors on the heat sink.

Resistor R1 eliminates the possibility of failure of transistors VT1, VT2 in the event of thermal breakdown of the diodes (for example, when the fan motor is jammed). Its resistance is chosen based on the maximum permissible value of the base current VT1. Resistor R2 determines the threshold for the regulator.

It should be noted that the number of temperature sensor diodes depends on the static current transfer coefficient of the composite transistor VT1VT2. If, with the resistance of the resistor R2 indicated on the diagram, room temperature and the power on, the fan impeller is stationary, the number of diodes should be increased. It is necessary to ensure that after applying the supply voltage, it confidently begins to rotate at a low frequency. Naturally, if with four sensor diodes the rotational speed turns out to be significantly higher than required, the number of diodes should be reduced.

The device is mounted in the power supply housing. The outputs of the same name of the diodes VD1 - VD4 are soldered together, placing their cases in the same plane close to each other. The resulting block is glued with BF-2 glue (or any other heat-resistant, for example, epoxy) to the heat sink of high-voltage transistors on the reverse side. Transistor VT2 with resistors R1, R2 soldered to its terminals and transistor VT1 (Fig. 2) are installed with the emitter output into the "cooler" hole of the power supply board.

Refinement of the AT power supply

Establishing the device is reduced to the selection of the resistor R2. Temporarily replacing it with a variable one (100-150 kOhm), they select such a resistance of the introduced part so that at a nominal load (the heat sinks of the power supply transistors are warm to the touch), the fan rotates at a low frequency.

Attention! To avoid electric shock (heat sinks are under high voltage!) You can "measure" the temperature by touch only by turning off the computer.

With a properly debugged device, the fan should not start immediately after turning on the computer, but after 2 ... 3 minutes after the power supply transistors have warmed up.

Author: V.Portunov, Bryansk

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