Turning a digital indicator into a digital thermometer. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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In addition to microcircuits popular among fans of microcontrollers - digital thermometers of the DSXXX series, which give out the temperature value in a digital code, there are a large number of sensors whose parameters change with temperature.
In the temperature range of -40 +120 degrees Celsius, semiconductor ones are of greatest interest, since they are capable of producing a signal that does not require additional amplification.
The most common type: LM135 - LM335 series and their domestic counterpart - K1019EM1.
At its core, this is a microcircuit that enhances the Earley effect in a silicon PN junction, and therefore the permissible temperature range ideally ranges from -55 to +150 degrees Celsius. The output signal is a temperature-dependent voltage, so the sensor must be powered from a current source. This may be a resistor of several kOhm, but it is better to use a simple two-transistor circuit that does not require the selection of elements. The field-effect transistor current generator circuit looks simpler, but requires the selection of both a transistor and a resistor.
(click to enlarge)
The main characteristics of the sensor are the output signal increment + 10mV / deg, the supply current is 0,5-5 mA (the recommended current is 1 mA at which the characteristics were taken), the output voltage corresponds to the Kelvin temperature scale, i.e. the absolute temperature, where our usual 0 deg. Celsius = 273,16 deg. Kelvin, and there is no concept of negative temperature. We will not change our habits and introduce a negative bias into the power supply circuit of the sensor used, as a result we will get a familiar temperature scale. The "tuning" pin is not used - it is intended to shift the readings in the Kelvin scale, and we use our own adjustable voltage source.
The linearity of the scale is determined by the type of sensor used, for K1019EM1 it reaches 2 degrees at the ends of the scale. The simplest way to calibrate a thermometer in amateur conditions is to measure your own body temperature with it - does everyone remember what it should be? If the thermometer is used in a liquid medium, then the design of the sensor is of paramount importance.
It is better to solder the metal case to the thin-walled tube from the disassembled telescopic antenna with fusible solder and be sure to fill the inside with sealant / transformer oil / silicone fluid to prevent condensation inside the probe. The length of the connecting wires can be up to 5 meters. For particularly accurate measurements, it is necessary to ensure good contact between the sensor and the measurement object (filling with heat-conducting paste) and take into account that heat is removed from the object through the metal body of the probe.
So, we got a digital thermometer with a resolution of 0,1 degrees and a measurement range of -55 +150 degrees. Celsius when using a digital indicator with a measurement limit of 1,999 V.
Author: Sir Murr; Publication: radiokot.ru
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