Overheating indicator. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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Most homemade devices with powerful transistors do not have thermal protection. Yes, and information about the thermal regime of electric motors, power transformers (especially welding ones) is also necessary. It will be given by the device, the scheme of which is shown in Fig. 1. The sensor on the germanium transistor VT1 controls the temperature. It is connected to a generator made on a transistor VT2. From the output of the generator, the signal goes to the amplifier (transistor VT3), loaded on the LED HL1.

As long as the temperature of the transistor-sensor case does not exceed 60 °C, the HL1 LED does not light up. At a temperature of about 70 ° C, the LED begins to flash at a frequency of several times per minute. If the temperature of the sensor rises to 85...105°C, the LED flashes at a frequency of 2...3 Hz. The frequency of flashes can be used to judge the degree of heating of the controlled object.

The choice of a germanium transistor as a temperature sensor is due to the fact that the reverse collector current increases sharply with increasing temperature at temperatures above 70 °C.

The device uses fixed resistors MLT, S2-23 indicated in the power diagram, the trimming resistor R2 can be SPZ-38a, RP1-63M or another small-sized one. Oxide capacitors - K50-16, K50-35. Zener diode VD1 - any low-power voltage of 7 ... 9 V, for example, except for that indicated on the diagram, KS170A, KS175ZH, KS191A, KS482A, D814A, D818B. LED - any of the series AL307, KIPD21, KIPD36, preferably red glow.

The transistor of the GT329 series was used as a sensor because it is one of the few germanium transistors that have a variant in a plastic case, which greatly facilitates its installation on a controlled surface. Instead, you can use transistors of the MP35-MP38 series, but then insulating gaskets will be needed. In place of VT2, it is permissible to use any unijunction transistor of the specified series. We will replace the KT315B transistor with any of the KT315, KT342 series.

The indicator can be powered with a voltage of 9 to 18 V. By selecting the resistor R1, the current consumed by the device is set to 8 ... 10 mA. At a supply voltage of 10 V, R1 should have a resistance of 220 ohms, at 24 V - 1,8 kOhm.

When setting up the device, the transistor VT1 is temporarily excluded and a 33 kΩ resistor is installed instead. By selecting the resistor R6, they achieve reliable extinction of the LED in the pauses between flashes. After that, the temperature sensor is connected. Trimmer resistor R2 adjust the sensitivity of the indicator. It is recommended to install it so that the LED starts flashing occasionally at a temperature of 65 ... 75 ° C.

The transistor of the GT329 series in a plastic case is fixed on the controlled surface with special springy petals used to install transistors of the KT818, KT819 series and the like on heat sinks.

If a germanium transistor in a metal case is used as a temperature sensor, then it is necessary to use thin insulating gaskets made of mica or fluoroplastic. It is also desirable to use heat-conducting paste KPT 8.

The lower limit of sensitivity of the indicator is about 50 °C. If, in rare cases, the sensitivity needs to be increased, then 1-1 of the same transistors should be connected in parallel with the transistor VT3.

Author: A.Butov, village of Kurba, Yaroslavl region

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