Universal electronic 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 thermometer described here allows you to measure the temperature at individual points of the motor, transformer, transistor housing, diode, soldering iron tip and other devices. Ranges of measured temperatures - 0...100°С and 0...1000°С.

The temperature sensor of the thermometer is a chromel-alumel thermocouple welded from wires with a diameter of 0.2 mm. The value of the EMF created by the thermocouple is proportional, as is known, to the temperature difference between the "hot" and "cold" ends of it. The electronic thermometer in question has automatic compensation for the temperature of the cold ends of the thermocouple t. ("room") with that. so that the measuring device indicates the temperature of the object t. and not its difference: t - t.

Schematic diagram of the thermometer is shown in the figure.

It consists of a measuring bridge (VT1, VT2, RK1, R1-R5). voltage stabilizer for its supply (VT3, VT4, R6), thermocouple VK1. voltage amplifier (DA1, DA2, R7-R11, SA1), microammeter PA1, power switch SA2 and power supply GB1.

A copper thermistor RK1 and a resistor R3 are included in the lower arms of the measuring bridge, and the current stabilizers of these resistors on transistors VT1 and VT2 are included in the upper arms. and in its measuring diagonal - a thermocouple VK1 and non-inverting inputs of microcircuits DA1, DA2 of the voltage amplifier. Due to the very high input impedance of the amplifier, there is practically no current in the measuring diagonal, and its input voltage (Uw) is not affected by the voltage drop across the resistors R3. RK1 and thermocouple conductors. The cold junction of the thermocouple must be in the thermometer housing.

When the temperature t changes (at a constant t), the voltage on the thermistor RK1 (Urk1) and the EMF of the thermocouple E change in antiphase so that their sum always remains constant. In order for zero on the scale of the measuring device PA1 to correspond to a temperature of 0 ° C and the thermometer readings do not depend on the temperature tk, the voltage across the resistor R3 is set equal to URz \u10d UPC1 \u1d K / LRx. (1). where Urk0o - voltage on RK1 at t.=1°C; K - coefficient of thermoelectric power of thermocouple: LRK1 - temperature coefficient of resistance of resistor RK1. Dependence (3) is valid if the following inequality is observed: LRk2 "LR1 (3). This condition can be easily met if RK1 is wound with a copper wire, and the MLT resistor is used as R2. If the requirements (3) and (8) are met, the input voltage Uk = K t (0). The same voltage will be applied to the resistor R10 (in the range of measured temperatures 9 ... 0CGS) or to the resistor P1000 (in the range 1 ... 2X). Since the op-amp DA1 is connected according to the voltage follower circuit, and the op-amp DA10 - according to the scheme of a non-inverting amplifier. Therefore, the current in the feedback circuit RA8. R9 will be equal to: loc = Uin / R, where R is the resistance of the resistor R3 or R1. Taking into account equality (XNUMX) ╡os \uXNUMXd K t / R, i.e. the current through the microammeter PAXNUMX is directly proportional to the temperature of the object t.

As PA1, a 100 μA microammeter was used. Resistor RK1 is wound on a 20 mm thick textolite plate 10x1 mm with insulated copper wire 0.1 mm in diameter up to a resistance of 60 ... 100 ohms. Transistor VT3 is included as a voltage regulator of the measuring bridge. Its functions can be performed by any low-power silicon transistor with a breakdown voltage of the base-emitter junction below 7 V. Transistors VT1, VT2, VT4 - any low-power field-effect transistors with p-n junction Cut-off voltage VT1. VT2 - no more than 4 V. a VT4 - no more than 2 V. The sum of the cutoff voltage of the transistor VT4 and the stabilization voltage of the transistor VT3 must be less than the battery voltage GB1. and the smaller this amount, the deeper the battery discharge, the thermomef will remain operational.

Micropower op amps are used only for reasons of minimal power consumption. When powering the thermometer from the mains as DAI, DA2, it is desirable to use precision op amps. Trimmer resistors R2, R5, R8, R9 - multi-turn - SP5-2V or others like them. The remaining resistors are MLT-0.125.

The adjustment of the thermometer begins with the calculation of the voltage UR3. For the "chromel-alumel" thermocouple K = 4.065 10-2 mV/°C. For copper LRK1 = 4.3 10-3/°С. Using equality (1). we get URc =4.065 10-2/ 4.3' 10-3 = 9,453 mV. Next, by closing the switch SA2. a voltmeter (preferably digital) is connected in parallel with resistor R3 and the calculated voltage is set with resistor R5 with the highest possible accuracy. After that, switch SA1 is moved to the "100°" position. lower the thermocouple junction into a vessel with melting ice and resistor R2 set the arrow of the microammeter PA1 to 0. If the resistor R2 or P5 does not have enough control limits, then the resistor R1 or R4 should be replaced accordingly. Then the thermocouple junction is lowered into a vessel with boiling water and the resistor R8 sets the arrow PA1 to the last division of the scale - 100 μA. Further, without removing the thermocouple from boiling water, switch SA1 to the "1000 °" position and resistor R9 set the arrow PA1 to 10 μA. This completes the setup.

During the operation of the device, the PA1 arrow going off scale at the measurement limit of 100 ° C at room temperature indicates that the GB1 battery is discharged and needs to be replaced. The maximum supply voltage of the thermometer is determined by the allowable supply voltage of the OU (for K140UD12 microcircuits UMa.c = 15 V) or the allowable drain-gate voltage of the VT4 transistor plus the stabilization voltage of the base-emitter transition of the VT3 transistor. The minimum supply voltage is different from the sum of the stabilization voltage VT3 and the cut-off voltage of the transistor VT4 (the author's Umin was 7,5 V) The current consumed by the thermometer is 0,6 ... 0,9 mA.

When measuring negative temperatures, the ends of the thermocouple connection to the thermometer should be swapped.

The chromel-alumel thermocouple was used by the author because of its high operating temperature (up to 1300°C). If the limit of measured temperatures does not exceed 500 ° C, then you can take a Chromel-Kopel thermocouple or weld a thermocouple from another available pair of metals (alloys). It is obvious that the new pair will have a different value of the thermoEMF coefficient K and, accordingly, a different value of Ug. The value of the coefficient K can be calculated by taking from the handbook the thermoEMF values ​​of these metals paired with platinum and subtracting them from each other, or determining the value of K experimentally. To do this, the thermocouple should be connected to a digital millivoltmeter and placed first in a vessel with melting ice, and then in a vessel with boiling water, each time recording the voltmeter readings (taking into account the sign). Then you need to find the difference between the obtained values ​​​​and divide it by 100.

In conclusion, I would like to note the advantages of a thermocouple over other temperature sensors. Firstly, small dimensions (the diameter of the thermocouple solder ball welded from a wire with a diameter of 0,2 mm does not exceed 0,5 mm; if the wire is thinner, then the ball will be smaller). Secondly, interchangeability, i.e., the possibility of periodically connecting to one thermometer any number of thermocouples installed on different objects or at different points of one object. With semiconductor thermistors or diodes, this is not possible due to the spread of their parameters. Thirdly, the high operating temperature, which makes the thermocouple indispensable when measuring temperatures above 15°C. Fourth, negligible cost and ease of manufacture and repair. Fifth, in the overwhelming majority of cases, there is no need to isolate the thermocouple from the environment, even when measuring the temperature of electrolytes. Due to the small value of thermoEMF, the electrochemical process in the thermocouple is impossible, therefore, the electrolyte does not close it, of course, provided that the materials of the thermocouple itself do not chemically interact with this electrolyte.

Author: V.Burkov, Ivanovo

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