Automatic fan switch. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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To cool transistors and microcircuits in electronic equipment, small-sized fans - "coolers" are widely used. They can be controlled using various automata, one of which was described in [1]. It can be simplified if small-sized thermal contacts are used as a temperature sensor, which are used, for example, in rechargeable batteries of portable radio-video equipment. There they serve to protect the batteries from overheating when they are quickly charged - they open when a certain temperature is reached. A marking is usually applied to the body of a thermal contact - the temperature value at which it operates (opens).

A diagram of an automaton based on such thermal contacts is shown in fig. 1. Until the temperature of the sensor has reached the response temperature, its contacts are closed, the transistor is closed, the fan motor M1 is de-energized. When the temperature reaches this value, the contacts will open, the transistor will open, the fan will start to work. The controlled object (powerful transistors or microcircuits) is cooled.

Due to some difference between the opening and closing temperatures of the thermal sensor, the fan will operate until the object cools down by 10...20°. As soon as this happens, the contacts close and the transistor closes.

To reduce interference from a running fan and eliminate voltage surges on the transistor, a capacitor C1 is installed in parallel with the electric motor. If a low-power fan is used (with a current consumption of up to 0,15 A), in addition to that indicated in the diagram, any of the KT503A-KT503E, KT3117A transistors can be used in the device. In the case of using a more powerful fan, or several low-power ones connected in parallel, any of the KT815A-KT815G, KT817A-KT817G transistors must be used, and the resistance of the resistor R1 is determined by the formula

R1 \u21d hXNUMXEUp / lp,

where h21E is the static current transfer coefficient of the base of the transistor; Up - supply voltage; lp - current consumed by fans. In this case, the capacitance of the capacitor is also desirable to increase proportionally. Resistor R1 can be of types MLT, C2-33, P1-4, capacitor - K50.

Thermal contact must be placed directly on the object, the temperature of which is controlled. A resistor and a transistor can be mounted on it by surface mounting, and a capacitor can be installed on the fan terminals, so there is no need to manufacture a printed circuit board. When mounting the device, it should be taken into account that the air flow from the fan must not fall directly on the thermocontact.

One of the disadvantages of this device is that the temperature at which the fan turns on is determined by the parameters of the thermal contact. It is possible to make a simple design of an automaton based on a thermistor, with a continuously adjustable response temperature, if we use a microcircuit of the integrated voltage regulator KR142EN19A [2, 3]. It must be used as a voltage comparator (Fig. 2).

Thermistor RK1 serves as a temperature sensor. As long as the voltage at the control input of the microcircuit is less than 2,5 V, the current through it is less than 1 mA, so the fan does not work.

As the temperature increases, the voltage at the control input increases. When it reaches a value of approximately 2,5 V, the comparator will work. Current will flow through the microcircuit, the voltage at the anode will decrease to 2 V, and the fan will start to work. Due to the fact that the gain of the microcircuit for the control input is large, the fan turns on quickly enough. The temperature at which the fan turns on is set by the tuning resistor R1.

Due to the fact that the maximum current of the microcircuit is 100 mA, only low-power fans can be used in the machine. In addition, the fan will have a voltage lower than the supply voltage by 2 V, so it will not work at full capacity.

In the case of using more powerful fans, it is necessary to introduce a powerful transistor into the machine (Fig. 3). In this case, it will open when the comparator on the microcircuit is triggered and the supply voltage is supplied to the fan.

In this machine, you can use a thermistor with a negative TKS - KMT, MMT, ST1, STZ. Its resistance may be different from that indicated in the diagram, but it must be 4.. .5 times greater than the resistance of the resistor R1. Transistor - KT814A-KT814G, KT816A-KT816G. Trimmer resistor - SP3, SP4, constants - MLT, S2-33, P1 -4, capacitor - K50 or similar.

With a fan consumption current of up to 1 A, the transistor can be used without a heatsink. The thermistor must be reliably isolated from the current-carrying parts of the device and placed with glue on the object, the temperature of which is controlled.

Literature

1. Nechaev I. Automatic blower fan. - Radio, 2001, No. 6, p.60.
2. Yanushenko E. Chip KR142EN19. - Radio, 1994, No. 4, p.47.
3. Nechaev I. Voltage stabilizers with a KR142EN19A microcircuit. - Radio, 2000, No. 6, p. 57, 58.

Author: I. Nechaev, Kursk

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