Temperature Sensors
In the previous chapters, we have learned about the very basic building blocks for constructing an electronic circuit—resistors, diodes, and transistors. In this chapter, we will learn how these basic components can be used as transducers to sense physica
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Temperature Sensors
In the previous chapters, we have learned about the very basic building blocks for constructing an electronic circuit—resistors, diodes, and transistors. In this chapter, we will learn how these basic components can be used as transducers to sense physical variables (or physical sensors). Temperature sensors are probably the most studied and the most widely used type of physical sensors. Temperature sensors are commonly used together with biosensors, as temperature is a very important parameter for many biological systems. Especially for this type of application, temperature sensors must exhibit high sensitivity and fast response. The semiconductor temperature sensors, i.e., resistor-, diode-, and transistor-based, show much higher sensitivity compared to the others. These are typically operated in direct contact with media (usually water), and their response is very fast. Therefore, we will focus on the semiconductor temperature sensors in this chapter; namely, thermocouple, thermistor, diode, and transistor temperature sensors.
4.1
Thermocouple
A thermocouple is perhaps the oldest type of temperature sensor. It consists of two dissimilar metals joined together as shown in Figs. 4.1 and 4.2. The temperature where two metals joined together is referred to as the hot junction temperature, or Thot; the temperature at the open junction is the cold junction temperature, or Tcold. If there is temperature difference between Thot and Tcold, heat flows through these metals. In turn, this heat flow creates a flow of electrons (thus electric current) which is known as the Seebeck effect. Although metals are considered conductors, they do have a small degree of resistance, which should create voltage drops for each metal. As two different types of metals are used which have different resistances, the voltage drops should be also different from each other. Vout ¼ VAVB is measured at the cold junction, which is a function of the metal pair used and the temperature difference (ThotTcold). J.-Y. Yoon, Introduction to Biosensors: From Electric Circuits to Immunosensors, DOI 10.1007/978-1-4419-6022-1_4, # Springer Science+Business Media New York 2013
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4 Temperature Sensors
Fig. 4.1 A thermocouple (type K)
Fig. 4.2 Working principle of a thermocouple
Vout is linearly proportional to the temperature difference, and the slope is determined by the metal pair used. Essentially, the thermocouple is a resistor-type temperature sensor, where the pair of metals acts as resistors with extremely low resistance values. The typical voltage output of thermocouples is usually between 15 and 40 mV/ C. The actual slope of the Vout—temperature curve varies with the choice of metal pair. Several metal combinations are referred to as Type E, J, K, R, S, and T, as shown in Fig. 4.3. As shown in Fig. 4.3, a thermocouple can be used for a wide range of temperatures up to 1,500 C, and its temperature response is quite linear. Its sensitivity, however, is inferior to the other temperature sensors described in Sects.
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