Household digital thermometer. Scheme, description

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Household digital thermometer. Encyclopedia of radio electronics and electrical engineering

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

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The need for a temperature meter is due to many circumstances. In everyday life, for example, the need to quickly measure the temperature of a person’s body or water, for bathing a child, the temperature inside or outside the room, in a greenhouse or greenhouse, in the basement if vegetables are stored there, in the refrigerator or freezer chamber, water in an aquarium and many others objects.

Household thermometers are usually subject to such requirements as measurement accuracy - no worse than 0,5 C in the temperature range from -50 to +100 ° C (when measuring human body temperature - no worse than 0,1 ... 0,2 ° C), small size , cost-effectiveness, autonomy of power supply, low thermal inertia and hygienic safety. The relatively simple digital thermometer described here largely fulfills these requirements.

The sensitive element of the device is a temperature sensor, the principle of operation of which is based on the property of some materials to change their electrical resistance with a change in temperature. Temperature sensors may be different. In industry, for example, massive metal (copper or platinum) thermal converters are often used.

For household appliances, small-sized semiconductor thermistors MMT, KMT, ST1, ST3, TR-4 are most suitable. MMT-4, which, compared to metal transducers, have much less thermal inertia, have almost ten times higher temperature coefficient of resistance (TCR), higher electrical resistance, which makes it possible to completely neglect the resistance of the wires that connect the sensor to the device. The best characteristics are possessed by a miniature drop-shaped vitrified thermistor TR-4 with a reduced TCR. It has dimensions of 6x4x2,5 mm; flexible leads 80 mm long are made of wire with low thermal conductivity. Its mass is 0,3 g.

The main electrical characteristics of the TR-4 thermistor: nominal resistance - 1 kOhm ± 2% at a temperature of +25 ° C, TKS - approximately 2% / ° C, operating temperature range -60 ... + 200 "C, time constant - 3 s .

Household digital thermometer

The disadvantage of semiconductor thermistors is the non-linearity of the dependence of resistance on temperature and a significant spread in characteristics, which is the main reason that hinders their widespread use for temperature measurement. The graph illustrates the typical dependence of the resistance of TR-4 and MMT-4 semiconductor thermistors on temperature. However, the corresponding circuit design solutions for the linearization of the characteristic can largely eliminate these shortcomings.

The main technical characteristics of the thermometer using the TR-4 thermistor in it:

Schematic diagram of the thermometer is shown in fig. 1. The basis of the device is an integrating analog-to-digital converter (ADC) DA3, to the output of which a four-digit liquid crystal indicator HG1 is connected. Such an element base allowed to reduce power consumption and provide the device with small dimensions and weight.

Household digital thermometer
Fig. 1. Schematic diagram

Household digital thermometer
Rice. 2. PCB

The measuring circuit of the device is formed by a current-setting resistor R1, resistors R2 and R3, which form the reference voltage Uobr, thermistor R4, voltage Ut, on which it changes depending on temperature, and a compensating resistor, the function of which is performed by resistors R5, R6. To reduce the error from self-heating of the thermistor, the value of the current-setting resistor R1 is chosen so that the current in the measuring circuit is approximately 0,1 mA.

The device uses a direct measurement of the thermal resistance by the ratio method - the thermistor R4 and the reference resistor (R2 + R3) are connected in series and the same current flows through them. The voltage drop that occurs across the thermistor is fed to input pins 30 and 31, and the voltage drop across the reference resistor, which acts as a source of reference voltage Uobr, to pins 35 and 36 of the DA3 ADC.

With this method of measurement, the result of the ADC conversion does not depend on the current in the measuring circuit, which means that there is no need for traditionally used high-quality current and reference voltage sources, on which the accuracy characteristics of the meter largely depend.

For a device operating in the temperature measurement mode, a typical problem is to compensate for the initial value of thermal resistance at zero temperature. To do this, the resistance of the compensation resistor (R5 + R6) is chosen equal to the resistance of the thermistor R4 at zero temperature, and in order to compensate for the sum of the voltage values Ut + Uk supplied to the ADC pin 30, a voltage equal to 31 Uk is applied to its pin 2, which forms the operational amplifier DA2 with gain K=(1+R14/R13)=2. Then, taking into account the fact that the resistance of the thermistor decreases with increasing temperature, we have Uin ac = U+in - U-in = 2Uk-(Ut+Uk) = Uk-Ut.

The linearization of the non-linear dependence of the thermal resistance on temperature is implemented by shunting the thermistor R4 with the resistor R11 - roughly, and precisely by introducing the DA1 op-amp into the device. But the shunt resistor R11 only partially rectifies this non-linearity, somewhat expanding the operating temperature range.

The principle of accurate linearization is based on changes in the conversion coefficient of the ADC depending on the reference voltage Uobr. It changes due to feedback through the op-amp DA1. With such a connection, part of the input voltage Uin, determined by the gain of the op-amp DA1 V = [1+(R8+R9)/R7], is added to the voltage Uobr. The more the resistance of the thermistor increases with decreasing temperature, the faster the reference voltage grows, and this leads to a proportional decrease in the ADC conversion coefficient: -arr - voltages at pins 0 and 36 of the ADC, respectively.

If we take the division value of the least significant digit equal to 0,1 C, then in the final form the indication of the digital indicator NG1 will be determined by the expression N=100Uin/Uobr=100(Uk-Ut)/[(U0-B(Uk-Ut)]=100(R5 +R6-R4)/[(R2+R3)-B(R5+R6-R4)]

Other elements of the thermometer that ensure the operation of the ADC are typical. Transistor VT1, turned on by the inverter, serves to indicate the decimal point sign in the digital indicator HG1.

The details of the device are mounted on a printed circuit board made of foil fiberglass with a thickness of 1,5 mm. The DA3 chip is mounted on the side of the printed conductors. Sockets X1, X2 (from the 2PM connector) are soldered directly to the printed circuit boards. Printed pads are also provided for attaching the SA1 switch. Fixed resistors - C2-29V, tuning resistors - SP3-38a. Capacitors: C1 - K50-6, C3 and C7 - K22U, C5 - K73-17, C2 and C6 - K73-24. Switch SA1 - PD9-2, battery GB1 - "Korund". Indicator IZHKTS1-4/8 can be replaced by IZHTS-5.

The design of the sensor is arbitrary. For example, in a plastic rod with a diameter of 5 and a length of 65-70 mm, a through axial hole with a diameter of about 3 mm is drilled, and then a recess is drilled in one of its ends. Thin insulating tubes are put on the leads of the thermistor, the leads are passed into the hole in the rod, the thermistor is installed in the recess and sealed with BOV-1 glue named KO947 varnish. The ends of a two-wire flexible cable are soldered to the conclusions and a piece of thin-walled duralumin tube serving as a sensor handle is tightly put on the end of the rod opposite the thermistor. The length of the connecting cable is about 1,5 m.

Due to the significant variation in the parameters of semiconductor thermistors, three trimming resistors are introduced into the device: R5 - to set zero, R2 - to set the scale scale and R9 - to linearize the thermistor characteristics.

The simplest adjustment of the thermometer is conveniently performed according to three control temperature values: melt water (0 ° C), the human body (36,6 ° C) and boiling water (100 ° C). The first of these test points measures the temperature of the water in the ice, not the water with ice, which may be above 1 °C. At the second control point, a medical thermometer is used as a reference instrument. The boiling point of water must be corrected for atmospheric pressure. In Pyatigorsk, for example, located at an altitude of about 500 m above sea level, water boils at a temperature of 92,5 ° C.

The adjustment is started by placing the sensor in melt water. Trimmer resistor R5 set the indicator to zero. Then, by alternately adjusting the resistors R2 and R9, the indicator readings are obtained corresponding to the temperature values at the other two control points. Next, the sensor is again placed in the melt water and all control measurements are repeated.

More precise adjustment of the device can be performed using industrial mercury thermometers with a scale division of 0,2 ° C.

Instead of the TR-4 thermistor, other thermistors of wider application can be used in the sensor, but with the obligatory adjustment of the resistance of some device resistors. So, with MMT-4 with a nominal resistance of 1,3 kOhm, the resistance of the resistor R11 should be reduced to 3,3 kOhm, and with thermistor ST3-19 with a nominal resistance of 2,2 kOhm - up to 3 kOhm.

The operating modes of the ADC when using TR-4 and MMT-4 thermistors in the device are shown in the table. If there are not enough adjustment limits with trimming resistors, except for R11, then you may have to select resistors R3, R6, R8.

Author; V. Suetin, Pyatigorsk; Publication: cxem.net

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

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