How to use diodes and transistors to measure temperature

To measure temperature, semiconductor diodes and transistors can be used as thermal transducers. This is due to the fact that at a constant value of the current flowing in the forward direction, for example, through the diode junction, the voltage across the junction changes almost linearly with temperature.

For the current value to be constant, it is sufficient to include a large active resistance in series with the diode. In this case, the current passing through the diode should not cause it to heat up.

It is possible to construct the calibration characteristic of such a temperature sensor using two points — at the beginning and at the end of the measured temperature range. Figure 1a shows a circuit for measuring temperature using a diode Vd... The battery can be used as a power source.

A circuit for measuring temperature using a diode (a) and transistors (b, c). Bridge connectors allow you to increase the relative sensitivity of the device, compensating for the initial resistance value of the sensor

Rice. 1. Temperature measurement scheme using a diode (a) and transistors (b, c). Bridge circuits allow to increase the relative sensitivity of the device, compensating the initial resistance value of the sensor.

Temperature has a similar effect on the emitter-base resistance of transistors. In this case, the transistor can simultaneously act both as a temperature sensor and as an amplifier of its own signal. Therefore, the use of transistors as temperature sensors has an advantage over diodes.

Figure 1b shows a schematic of a thermometer in which a transistor (germanium or silicon) is used as a temperature transducer.

In the manufacture of thermometers, both diodes and transistors, it is necessary to build a calibration characteristic, while a mercury thermometer can be used as an example measuring instrument.

The inertia of diode and transistor thermometers is small: diode — 30 s, transistor — 60 s.

Of practical interest is a bridge circuit with a transistor in one of the arms (Fig. 1, c). In this circuit, the emitter junction is connected to one of the arms of the R4 bridge, a small blocking voltage is applied to the collector.

How to use diodes and transistors to measure temperature

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