Thermometer with linear scale. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power regulators, thermometers, heat stabilizers
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The use of a semiconductor diode as a temperature sensor made it possible to obtain a linear temperature reading scale. The dependence of the voltage drop across the diode on temperature at a fixed forward bias current is linear and for silicon diodes in the temperature range of -60...+ 100°C is 2...2,5 mV/°C.
The thermometer allows you to measure the temperature in the range from 0 to +150°C with an error of no more than 0,3°C.
The voltage across diode V1 is set by the bias current determined by resistor R1. The change in voltage across the diode, proportional to the change in temperature, is measured by a DC voltmeter made on the A1 microcircuit according to a balanced circuit (input 1). A stable voltage of about 0,5V is supplied to the other input of the microcircuit, created by a stable current generator on a field-effect transistor V2 and a reference voltage source VS. The field-effect transistor V4 is also used to stabilize the current consumed by the voltmeter, due to which a decrease in the supply voltage to 7V has practically no effect on the readings of the device.
When the temperature changes, an unbalance voltage occurs, which is recorded by the pointer device RA. The arrow indicator in the thermometer - M265M. The device is calibrated by two temperature measurements. The first one (0¦С - the temperature of melting snow) corresponds to the zero mark of the scale. The thermal probe with diode V1 is immersed in melting snow and resistor R5 is set to zero by balancing the microcircuit. The second temperature measurement, for example 50¦С, is performed in heated water while monitoring its temperature with a calibration thermometer. In this case, the position of the indicator arrow is set by adjusting the resistor R3.
When measuring a negative temperature, the polarity of the indicator P1 should be changed. Resistor R7 creates a thermal stabilization mode of the field-effect transistor V2, in which the drain current does not depend on the ambient temperature. Unlike bipolar transistors, in which the collector current increases with increasing temperature, the drain current of field-effect transistors, depending on the voltage at the gate, can increase, decrease, or remain unchanged with temperature.
To find a thermally stable mode of a field-effect transistor, it is necessary to experimentally measure the drain-gate characteristics at different temperatures. To do this, a graphical dependence of the drain current on the gate voltage (with respect to the source) is built, first at room temperature, then at elevated temperature, heating the transistor by several tens of degrees with a lighting lamp. Then the gate voltage is determined, at which the drain-gate characteristics taken for different temperatures intersect on the graphs. The point of intersection of the characteristics is called thermally stable.
Instead of the gate voltage, you can determine the position of the R7 potentiometer slider, at which the drain-gate characteristics of the transistor do not depend on temperature.
See other articles Section Power regulators, thermometers, heat stabilizers.
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