A temperature sensor plays an important role in many applications. For example, maintaining a specific temperature is essential for equipment used to fabricate medical drugs, heat liquids, or clean other equipment. For applications like these, the responsiveness and accuracy of the detection circuit can be critical for quality control.
More frequently, however, temperature detection is part of preventative reliability. For example, while an appliance may not actually perform any high temperature activities, the system itself may be at risk to overheating. This risk arises from specific external factors such as a harsh operating environment or internal factors like self-heating of electronics. By detecting when overheating occurs, the system can take preventative action. In these cases, the temperature detection circuit must be reliable over the expected operating temperature range for the application.
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Temperature sensing is one of the most sensitive properties or parameters for industries like petrochemical, automotive, aerospace and defense, consumer electronics, and so on. These sensors are installed into devices with the purpose of measuring the temperature of a medium accurately and efficiently in a given set of requirements.
Designing a robust temperature detection circuit does not have to be expensive. Nor does a low cost detection circuit have to compromise on responsiveness and accuracy. This article looks at the different types of temperature detection technologies available and what each has to offer. It also explores the requirements of various applications and how engineers can design a temperature detection circuit optimized for their specific needs.
Temperature Sensor Types
Temperature detection is the foundation for all advanced forms of temperature control and compensation. The temperature detection circuit itself monitors ambient temperature. It can then notify the system either of the actual temperature or, if the detection circuit is more intelligent, when a temperature control event occurs. When a specific high temperature threshold is exceeded preventative action can be taken by the system to lower the temperature. An example of this is turning on a fan.
Similarly, a temperature detection circuit can serve as the core of a temperature compensation function. Consider a system such as liquid measuring equipment. Temperature, in this case, directly affects the volume measured. By taking temperature into account, the system can compensate for changing environment factors, enabling it to operate reliably and consistently. There are four commonly used temperature sensor types:
1. Negative Temperature Coefficient (NTC) thermistor
A thermistor is a thermally sensitive resistor that exhibits a large, predictable, and precise change in resistance correlated to variations in temperature. An NTC thermistor provides a very high resistance at low temperatures. As temperature increases, the resistance drops quickly. Because an NTC thermistor experiences such a large change in resistance per °C, small changes in temperature are reflected very fast and with high accuracy (0.05 to 1.5 °C). Because of its exponential nature, the output of an NTC thermistor requires linearization. The effective operating range is -50 to 250 °C for gas encapsulated thermistors or 150°C for standard.
2. Resistance Temperature Detector (RTD)
An RTD, also known as a resistance thermometer, measures temperature by correlating the resistance of the RTD element with temperature. An RTD consists of a film or, for greater accuracy, a wire wrapped around a ceramic or glass core. The most accurate RTDs are made using platinum but lower cost RTDs can be made from nickel or copper. However, nickle and copper are not as stable or repeatable. Platinum RTDs offer a fairly linear output that is highly accurate (0.1 to 1 °C) across -200 to 600 °C. While providing the greatest accuracy, RTDs also tend to be the most expensive of temperature sensors.
This temperature sensor type consists of two wires of different metals connected at two points. The varying voltage between these two points reflects proportional changes in temperature. Thermocouples are non-linear, requiring conversion when used for temperature control and compensation, typically accomplished using a lookup table. Accuracy is low, from 0.5 to 5 °C. However, they operate across the widest temperature range, from -200 to 1750 °C.
4. Semiconductor-based sensors
A semiconductor-based temperature sensor is placed on integrated circuits (ICs). These sensors are effectively two identical diodes with temperature-sensitive voltage vs current characteristics that can be used to monitor changes in temperature. They offer a linear response but have the lowest accuracy of the basic sensor types at 1 to 5 °C. They also have the slowest responsiveness (5 to 60 s) across the narrowest temperature range (-70 to 150 °C).