Digital 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 device is designed for accurate measurement of the temperature of various objects over a wide range and can be recommended for use both in everyday life and in technology. Unlike previously published similar devices, this thermometer uses a K572 series LSI, so it contains a relatively small number of elements. The thermometer is ready for use immediately after the power is turned on. But, unfortunately, the lack of serial sensors with low temperature inertia leads to a significant duration of the measurement process (about five minutes), which somewhat limits the scope of the thermometer.

Main Specifications

The block diagram of the digital thermometer is shown in Fig.1. A change in the temperature of the object in which the temperature sensor is located causes a change in the resistance of the sensor, which in block E1 is converted into a corresponding change in voltage. Converter U1 is powered by current stabilizer G1. The output signal of the E1 block is amplified by the amplifier A1 and fed to the analog-to-digital converter (ADC) U2, at the output of which the digital display unit H1 is switched on, which displays the current temperature of the controlled object.

Structural scheme

The switch SB1 (see schematic diagram) selects one of the temperature sensors RK1, RK2 installed on the object whose temperature is to be measured. The sensor is included in one of the arms of the DC measuring bridge, made on precision resistors R1 - R5. The accuracy and linearity of the indicator readings within the measured temperature is determined mainly by the stability of the current supplying the measuring bridge.

Schematic diagram (click to enlarge)

The bridge supply current stabilizer is based on the DA1.2 operational amplifier. Trimmer resistor R11 allows you to change the value of the output current within a small range, which makes it possible to change the steepness of the conversion of the resistance of the temperature sensor into voltage and provides the setting of the upper limit of the measured temperature. The lower limit is set with a tuning resistor R1.

The voltage from the diagonal of the measuring bridge, proportional to the temperature, is amplified by a differential amplifier made on the operational amplifier DA1.1, and from its output it is fed to the ADC input. Capacitors C1, C2, C4 serve to filter interference.

The ADC is implemented on the BIS K572PV2A and operates on the principle of double integration with auto-correction of "zero" and automatic detection of the polarity of the input signal. The signal carrying information about the current temperature of the selected object is presented at the output of the ADC in a form convenient for displaying with seven-element indicators. It enters the board, consisting of three LED indicators HG1 - HG3 and LED HL1.

The LED lights up at a negative temperature of the measured object. To separate integers and tenths of a degree, a comma is displayed on the HG2 indicator.

The thermometer is powered by an alternating current network with a voltage of 220 V through a transformer T1. To stabilize the bipolar supply voltage, parametric stabilizers VD1R18 and VD2R19 are provided. The reference voltage for the ADC and the current stabilizer is taken from the voltage divider on resistors R16, R17. It is additionally filtered by capacitor C12.

All elements of the digital thermometer are placed on two printed circuit boards (see Fig. 3 and Fig. 4), interconnected by corners.

Main board drawing

Additional board drawing

The device uses fixed resistors R2 - R5 - C2-29V-0,125: R18, R19 - MLT-0,5; trimmers - SPZ-38, the rest - MLT-0,125. Capacitors C1 - C5, C9 - K73-17-C7, C10, C11 - KT.1; C6, C8 - K10-7; C12-C 14 - K50-6.

To ensure the interchangeability of temperature sensors while maintaining the specified accuracy, commercially available resistance temperature converters ТСМ-6114 GOST 6651-72 with a nominal static characteristic gr.23 were used. In the absence of standard sensors, you can make them yourself. To do this, measure 619 cm of PETV wire with a diameter of 0,05 mm. wind it bifilarly on an insulating mandrel, solder one flexible lead to one end of the sensor wire, and two of the same leads to the second.

You can solder the sensor directly to the conductors of the supply cable. Each sensor will require three conductors in the cable. This connection allows you to compensate for the temperature error introduced by the cable conductors.

Next, a case is made that can work in the environment where the sensor will be installed, a mandrel with a winding is fixed in it and filled with epoxy resin. The resistance of the sensor at a temperature of 20 °C should be 57 Ohm.

The power transformer to reduce the size is made of four magnetic circuits PL6,5X12,5x16 (section about 3 cm.kv). Winding I contains 3000 turns of wire PEV-2 0,08, II - 2X130 turns of wire PEV-2 0,18, 111 - 70 turns of wire PEV-2 0,4. In the power transformer, it is possible to use a different magnetic circuit, however, the height of the thermometer case will have to be increased.

The K157UD2 microcircuit can be replaced by the K140UD20 with the corresponding correction circuits: K572PV2A - by KR572PV2A, but the pattern of the printed circuit board conductors will have to be changed, and with an increase in the permissible error to ± 0,3 ° C, you can also use K572PV2 with any letter index.

A thermometer accurately assembled from known good elements does not require adjustment, it is only necessary to establish the boundaries of the measured range. To do this, instead of the sensor, include its equivalent (a resistor store or an accurate resistor). First, a resistor with a resistance of 41,7 ohms is turned on, and a reading of minus 1 ° C is set on the display with resistor R50; then replace the resistor with another, with a nominal value of 75,59 ohms, and resistor R11 set the reading to plus 99,9 ° C. The calibration operation should be repeated twice.

If it is necessary to extend the measured temperature range up to 180°C, one more ALS324B digital indicator must be connected to the ADC. The remaining technical characteristics of the thermometer are preserved.

Authors: N.Khomenkov, A. Zverev, Orel; Publication: N. Bolshakov, rf.atnn.ru

See other articles Section Power regulators, thermometers, heat stabilizers.

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