Modern electronic systems use sensors to monitor parameters like temperature and provide protection from excessive temperature excursions with good accuracies, reasonable costs and often with low power consumption. For instance, expensive laptops with densely-packed circuits dissipate considerable power in the form of heat. Also, many compact and high-power portable equipment use cooling fans to keep junction temperatures at proper levels. These sensors provide temperature feedback to the system controller to make decisions such as over-temperature shutdown, turn on/turn off cooling fan, battery management, temperature compensation or general-purpose temperature monitoring system.
Most-common sensor technologies available to designers for measuring temperature within a system are thermistors, thermocouples, resistance temperature detectors (RTDs) and temperature-sensing ICs. Suitability of each technology for a system depends upon several factors such as: What device is to be measured? Is it ambient air temperature of an enclosure or an electronic component that may have high voltage present, or perhaps some part of an automobile engine? Important considerations driving the choice of sensors are temperature range to be sensed, overall cost, distance from sensor to instrument, cost of sensor, available area for the sensor to be mounted and so on.
Choosing the right sensor
Designing a temperature-sensing circuit involves many steps. One of the most crucial steps is to select an appropriate temperature transducer/sensor to match an application’s needs. Some key sensor characteristics to consider when selecting a sensor include temperature-measurement range, accuracy, response time, minimal temperature effect on the measured object and type of signal conditioning required. Other factors are long-term stability, mechanical ruggedness and cost. Some of these are discussed below.
Accuracy is needed in temperature sensing to enhance product reliability and performance. Assume that there is a system using a microcontroller (MCU) that operates up to 125°C before functioning abnormally. A better sensor with ±1°C accuracy will allow operation up to 123°C, whereas one with ±4°C accuracy will only allow operation up to 117°C. Many errors such as those introduced by the sensor, cabling or other hardware can affect the overall accuracy of measurement. Minimising these errors can lead to an increase in overall measurement accuracy.
Linearity is another important feature that affects accuracy in measurements. It defines the ability of a temperature sensor to give consistently-changing outputs over a range of temperatures. Linearisation techniques are often used to correct sensor non-linearity to achieve desired accuracy. All sensors require linearisation but to a different degree.
Signal conditioning is required with sensors as most of these provide low-level and non-linear outputs. To effectively and accurately measure signals, most analogue signals require some form of conditioning before these can be digitised and sent for further processing to the controller. For instance, filtering and amplification can dramatically improve the accuracy of thermocouple measurements. It is important that the required accuracy and resolution for an application is matched to the data acquisition and signal conditioning hardware that you select.
In the past, many complex analogue signal conditioning and calibration circuits were used that required manual calibration and precision resistors. Today, however, a digital design is used instead. Sensor outputs are often digitised by a high-resolution analogue-to-digital converter (ADC), which allows linearisation and calibration in software. This allows minimum operator involvement and also reduces cost and complexity.
Sensor excitation is another important concern. For instance, as RTDs and thermistors are resistive devices, you must supply these with an excitation current and then read the voltage across their terminals. If extra heat due to this cannot be dissipated, self-heating of the sensor is caused by the excitation current, which raises the temperature of the sensing element above the ambient temperature, thereby introducing errors in measurement. Power regulation is thus important to limit self-heating effects.
Stability defines how consistently a sensor maintains its accuracy over time. Generally, stability gets worsened by exposure to high temperatures. Wire-wound platinum and glass-encapsulated thermistors are the most stable, whereas thermocouples and silicon sensors are the least.
Temperature range varies for each sensor type irith the thermocouple family possessing the widest temperature range, spread across multiple thermocouple types.
Electrical noise can induce errors in temperature sensing. Mostly thermocouples and sometimes thermistors with very high resistances present this problem.
Sensor outputs vary by type of sensor. Most thermistors change resistance inversely-proportionally with temperature, thus the name negative temperature coefficient (NTC). Base metals such as platinum have positive temperature coefficient (PTC). Thermocouples have low milli-volt outputs that change with temperature. Silicon sensors are typically conditioned and come with a variety of digital outputs.
Cost is a major deciding factor. Although thermocouples are the least expensive and the most widely-used sensors, an NTC thermistor generally provides the greatest value for its price.
Sensitivity allows a sensor to sense very small changes in temperature.
Resolution dictates the smallest detectable change in temperature that a system can detect.
Flexibility allows a sensor to be configured into a wide variety of physical forms such as very small packages.
Response time determines how quickly a system can react to any change in temperature. Generally, the smaller the sensor, the faster the response time.
Thermocouples
Thermocouples are by far the most widely-used type of temperature sensors in the industry. Made by joining two wires of dissimilar metals, a thermocouple takes advantage of the voltage induced at the point of contact between the wires when these are heated. This voltage is proportional to temperature. Characteristics include wide temperature range (up to +2300°C), low cost, very low output voltage (about 40µV per °C for K type), reasonable linearity and moderately-complex signal conditioning. Signal conditioning for thermocouples requires a look-up table or algorithm correction.
