A simple temperature-voltage converter. Scheme, description

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A simple temperature-voltage converter. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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For an amplifier with a balanced differential input (Fig. 1), under the condition RV = R1" = R1, the output voltage is determined by the expression [1]:

Uout \u2d -Uin (R3-R1) / (R3 + R1). (one)

In the negative feedback circuit, instead of the resistor R2, you can include a linear temperature sensor, the resistance of which has a temperature dependence

Rt = R0 + αt, (2)

where R0 is the resistance of the temperature sensor at XNUMX °C; a - coefficient of temperature sensitivity, Ohm/°C; t - temperature, °C.

Taking the resistance R3 equal to R0, we get

Uout \u1d -Uin αt / (R0 + R3). (XNUMX)

If we provide the condition

Uinα/(R1 +R0) = 1, (4)

then, as follows from expression (3), we obtain a voltage at the output of the op-amp, numerically equal to the temperature of the temperature sensor. The resistance R1 to ensure this condition will be determined from the relation

R1 =αUin-R0. (5)

To obtain good stability of the converter, it is necessary to use a high-quality operational amplifier, for example, K140UD17A.

The possibility of using such a converter as an attachment to a digital multimeter based on the ADC KR572PV5 is shown in fig. 2. The set-top box is powered by the multimeter battery. Temperature measurement is carried out on a voltage scale at the limit of 200 mV.

(click to enlarge)

The resistive temperature sensor is a standard temperature sensor with parameters R0 = 50 Ohm, a = 0,214 Ohm/°C. For the converter, the voltage Uin corresponding to the reference voltage used for this purpose from the internal stabilizer of the ADC is 3 V [2]. Since voltage measurements are made in millivolts, from expression (5) we get

R1 - 30000,214 - 50 = 592 ohms.

The bridge is balanced by the resistor R5 when the resistor R = R3 = R0 is connected instead of the sensor.

To determine the value of resistors R1 and R2, it is recommended to measure the UBX value of each specific instance of the multimeter. Measurements are made by connecting the measuring input of the multimeter "V / Ω" to the positive terminal of the battery. The second ("blank") input of the multimeter remains unconnected.

It should be noted that for the one shown in Fig. 2 connection diagrams of the set-top box on the multimeter, the "-" sign is reflected at positive temperature values, and at negative, on the contrary, the "+" sign. You can eliminate this inconvenience by using an external bipolar stabilized power supply for the set-top box. In this case, the resistor R3 of the set-top box is connected to the midpoint (common wire) of the source. If high measurement accuracy is not required, then to correctly reflect the sign of the Celsius scale, it is enough to change the connection points of the elements R4 (Rt) and R3 (Fig. 2). The systematic measurement error in this case will be about 1°C at 50°C and slightly more than 3°C at 100°C.

In the absence of a standard sensor, it can be made independently according to the recommendations [3], specifying the resistances of the resistors R1, R2, taking into account the temperature coefficient of sensitivity of the material. In the converter, it is advisable to use precision resistors C2-29 (or analogues) of the closest ratings from the E96 or E192 series. In extreme cases, using the same multimeter, you can select instances from metal-dielectric resistors of close denominations with resistances close to the calculated values.

Literature

Alekseenko A. G., Kolombet E. A., Starodub G. I. Application of precision analog microcircuits. 2nd ed. - M.: Radio and communication, 1985, p. 75, 76.
Biryukov S. Application of ADC KR572PV5. - Radio, 1998, No. 8, p. 62-65.
Homenkov N. Zverev A. Digital thermometer. - Radio, 1985, No. 1, p. 47-49.

Author: B. Porokhnyavy, Krasnoyarsk

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