Electric thermometer 20-45 °C. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power regulators, thermometers, heat stabilizers

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Everyone had to measure their temperature with a mercury thermometer during illness. This procedure usually takes 5-7 minutes. If adults hold the thermometer calmly, then children have to be watched so that they do not accidentally break it.

The proposed device allows for 3 seconds to measure the temperature of a body or object (for example microchips) in the range from 20 to 45°C with an accuracy of no worse than 0,1°C. This range, if desired, can be easily expanded or shifted during manufacture.

Compared to a mercury thermometer, an electric thermometer is more convenient and safer, especially when you have to take the temperature of small children or animals.

Rice. 1.12. Electrical diagram of the thermometer

The bridge converter is taken as the basis for constructing the circuit (Fig. 1.12). A change in the resistance value of the temperature sensor R8 leads to an unbalance of the bridge and the appearance of a current proportional to the temperature on the pointer indicator PA1.

A feature of this device is the use of a thermistor type STZ-19 10 kOhm as a temperature sensor, which has a very small mass, due to which it is possible to obtain a high measurement speed. It is convenient to fix this sensor at the end of a plastic tube from a ballpoint pen and connect wires 1 ... 0.6 m long through the X1 connector to the measuring unit. A jumper is installed on the connector from the sensor between pins 1 and 2, which will not allow turning on the device circuit if a temperature sensor is not connected, which protects the PA1 measuring device from damage. The circuit is powered by any two accumulators or batteries with a total voltage of 2 ... 3 V and consumes a current of no more than 5 mA from the source.

Transistors VT1 and VT2 are used as low-voltage zener diodes and can be replaced by KT3102A, B, C, G.

Variable resistors, for ease of adjustment, it is better to use multi-turn, such as SP5-2 or similar.

The dimensions of the device are determined by the dimensions of the PA1 pointer indicator, and when using the M4205 microammeter for a current of 0 ... 50 μA, they do not exceed 85x65x60 mm (see Fig. 1.13).

The topology of the printed circuit board and the placement of elements on it are shown in fig. 1.14.

Setting up the device begins with measuring the resistance of the resistor R8 (preferably with high accuracy) at a fixed temperature of 20°C. For these purposes, it is convenient to use an industrial heat chamber with automatic maintenance of the set temperature, where the temperature sensor is placed. There are other ways to obtain a temperature of 20 ° C, but it must be borne in mind that the measurement accuracy of the device depends on the accuracy of measuring the resistance of the temperature sensor at this temperature.

Rice. 1.13. Appearance of the structure

Fig. 1.14

After measuring R8 from two resistors R6 + R7, we select the same resistance value and solder them into the circuit.

After that, setting the sliders of resistors R2 and R3 to the middle position, turn on the circuit with toggle switch S1 and perform the following operations in sequence:

a) set switch 82 to the CALIBRATION position and use resistor R2 to bring the pointer of the measuring device to the zero position on the scale;

b) place the temperature sensor in a place with a known, constant temperature (within the desired measuring range);

c) set the switch S2 to the MEASUREMENT position and with the resistor R3 set the instrument pointer to the scale value that will correspond to the measured value;

Operations a), b) and c) must be repeated successively several times, after which the setting can be considered complete.

In conclusion, I would like to note that in a tuned device, the measurement range can be shifted by resistor R2 when switching to the CALIBRATION mode and setting the arrow (its position will correspond to a value of 20 ° C) to any scale value. After that, when switching the device to the MEASUREMENT mode, the scale will be correspondingly shifted relative to the position of the arrow in the CALIBRATION mode.

The device has a large margin of sensitivity, which increases with decreasing resistance R3 (during the initial setting). You can make the device capture the temperature of breathing or the change in temperature during air circulation.

Publication: cxem.net

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