Setting up the digital multimeter temperature meter. Scheme, description

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Setting up the digital multimeter temperature meter. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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A. Butov's note "Correction of the error of the M890C multimeter when measuring temperature" ("Radio", 2001, No. 11, p. 22) describes a method for adjusting the temperature meter by selecting the resistance of one of the resistors. However, it is easier to do otherwise, because the device has special tuned resistors for this purpose.

The figure shows a diagram of a temperature meter, which is formed when the multimeter operation type switch is set to the temperature measurement position (contacts SA1.1-SA1.3 are closed). This circuit is typical for almost all multimeters that have the function of measuring temperature, but in devices from different companies there may be minor differences, associated in most cases with a change in the resistance of resistors.

Setting up the DMM temperature meter

As you can see, the temperature meter is made according to the usual measuring bridge scheme, the left arm of which is formed by the resistor R2 (the numbering of all elements is conditional), the diode VD1 and the resistor R3, and the right arm is formed by resistors R4-R6. The diagonal of the bridge includes in series the temperature sensor VK1 (thermocouple connected to the socket XS1) and the differential inputs of the analog-to-digital converter (ADC). Thus, the EMF generated by the sensor is fed directly to the ADC inputs.

As is known, the EMF of a thermocouple is proportional to the temperature difference between the hot and cold junctions, therefore, with accurate measurements, the temperature of the cold junction is fixed, usually by immersing it in water with melting ice. In a simplified version, a thermo-dependent element is introduced into the meter (in this case, the diode VD1). If the temperature of the sensor, the device and the environment are the same, the EMF of the thermocouple is zero, in which case the diode works as an air temperature sensor.

The initial adjustment of the readings is performed by a trimming resistor R5 according to the ambient temperature. However, this is not enough - you also need to adjust the sensitivity of the meter. For the ICL7106 ADC, it is determined by the value of the reference voltage at pins 35 and 36. When measuring electrical quantities (except for resistance), the reference voltage is set by the voltage divider R7R12R14 and is equal to 100 mV. In the thermometer mode, an additional voltage is applied to input 36, taken from the divider R8-R10, and the required sensitivity is set with a tuning resistor R9. The latter is inversely proportional to the reference voltage - the smaller it is, the higher the sensitivity.

Thus, the setting of the "thermometer" is performed in two steps. First, the trimmer resistor R5 sets the readings of the device equal to the ambient temperature, then the temperature sensor is heated to a known temperature (for example, immersed in boiling water) and the trimmer resistor R9 achieves the appropriate readings. These adjustments are interdependent, since changing the sensitivity also affects the initial readings. Therefore, the tuning operations must be repeated several times until the desired results are obtained.

Since the numbering of the elements in the diagram, as noted, is conditional (it is unrealistic to give designations for all models of devices), the question arises: how to find the necessary tuning resistors in the multimeter? The easiest way is with the help of an ohmmeter ("continuity"). To do this, the type of operation switch is set to the temperature measurement position and, without turning on the device, trimming resistors are found connected to pins 30 and 36 of the ADC chip. It should be remembered that in order to avoid damage to the ADC chip, the supply voltage of the ohmmeter should be no more than 1,5 V.

Author: D.Turchinsky, Moscow

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