Thermometer. Encyclopedia of radio electronics and electrical engineering

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

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The operation of the device is based on the dependence of the voltage drop across the pn junction of a silicon diode on temperature when a fixed direct current flows through it. It linearly decreases by 2...2,5 mV with each degree of temperature increase in the range of -60...+120 °C. The thermometer, the scheme of which is shown in fig. 1 is essentially a DC millivoltmeter. It has adopted a number of measures that reduce the effect of changes in the temperature of the elements (except for the sensor - diode VD1) on the readings.

The sensor current is stabilized by the transistor VT2, operating at a thermally stable point of the output characteristic (stabilization current is approximately 200 μA). Similarly, the transistor VT3 stabilized the current in the circuit for generating the exemplary voltage. Both transistors of the DA1 chip are located on the same semiconductor chip and have identical parameters that are equally dependent on temperature. As a result, the readings of the PA1 microammeter depend only on the temperature of the sensor.

On the transistor VT1 and the zener diode VD2, a thermometer supply voltage stabilizer is assembled. The drain current of the transistor VT1 remains approximately 3,5 mA when the supply voltage changes in the range of 8 ... 12 V. This further improves the stability of the output voltage of the stabilizer and the readings of the device.

The device is assembled by surface mounting on a small textolite board. It can be mounted directly on the screw-pins of the microammeter RA1 - M42304 with a zero mark in the middle of the scale. It is convenient to choose a microammeter in such a way that the total deflection current of its needle in microamperes corresponds to the required temperature range in degrees Celsius. Then, without changing the numbers on the scale, it is enough to correct the unit of measurement indicated there.

You can also use a conventional microammeter (with zero at the beginning of the scale), connecting it according to the circuit shown in Fig. 2. But with a change in the sign of the measured temperature, you will have to switch the SA2 switch to the appropriate position each time.

Transistors KP103L can be replaced with KP103Zh. If possible, as VT2 and VT3, transistors selected at the factory with similar parameters should be used. The index P (KP103ZhR, KP103LR) is added to the designation of such transistors and they are supplied in pairs in a common package. The KR159NT1 microcircuit can be replaced by an integrated switch K101KT1A containing two transistors with a common collector, or its imported analogue KS809. In extreme cases, you can use two separate transistors, for example, KT3102 with any letter index, but it is unlikely that you will be able to achieve high device stability. Nevertheless, such a solution is quite acceptable if the measuring part of the device is constantly located in a room with a relatively stable temperature. In this situation, you can go for even more simplification by replacing the VT2R1 and VT3R7 circuits with the same fixed resistors with a nominal value of 100 kOhm.

Diode VD1 is placed where it is necessary to control the temperature. The length of the shielded twisted pair of wires connecting the sensor to the device can be up to five meters or more. To eliminate interference caused by the detection of high-frequency signals from nearby radio and television stations, it is useful to shunt the sensor diode with a ceramic capacitor with a capacity of at least 0,1 μF. In addition to that indicated in the KD102A diagram, other small-sized silicon diodes are also suitable as a sensor. Experience shows that the rate of reaction to a change in temperature is higher, the smaller the size of the diode and the thinner its conclusions.

Starting to establish a thermometer, first of all, you should find the thermostable operating points of transistors VT2 and VT3. Please note that careless performance of these operations will result in a completely incorrect operation of the device. To adjust the current stabilizer on the transistor VT2, a microammeter is switched on in series with the VD1 diode or instead of it (any of the widely used digital multimeters is suitable) and a current of approximately 1 μA is set here with a trimming resistor R200. Alternately heating the transistor with a soldering iron and cooling it with cotton wool moistened with acetone, they select such a position of the resistor R1 engine at which the current through the sensor does not depend on the temperature of the transistor. Similarly, by including a microammeter in the open circuit R5R6, they find the thermostable operating point of the transistor VT3 by adjusting the current with the trimming resistor R7.

Before proceeding with the calibration of the scale of the device, it is necessary to protect the diode-sensor VD1 and the place of soldering of connecting wires to it from moisture. Protected areas are covered with some kind of acid-free sealant. Acid-based compounds (they are distinguished by the characteristic smell of vinegar) are unsuitable in this case, as they corrode the thin terminals of the diode and have a noticeable electrical conductivity. Accurate sealing will protect the sensor from harmful effects during operation, only slightly increasing its thermal inertia.

For calibration, you will need a vessel with melting ice and a heating device with boiling, preferably distilled water. The sensor is lowered into the melting ice, trying to place it as close as possible to the water-ice boundary. Trimmer resistor R5 achieve zero readings of the microammeter RA1. The sensor is transferred to boiling water and the microammeter pointer is set to +3 °C with a trimming resistor R100. It is useful to repeat these operations several times, adjusting, if necessary, the positions of the trimmer resistors. An additional control point can be the temperature of the human body (+36,6 °C), which, if necessary, can be easily clarified with a medical thermometer.

Author: S.Gants, Gubkinsky, Yamalo-Nenets Autonomous Okrug

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