Several temperature sensing techniques are used currently. The most common of these are thermocouples, thermistors and sensor integrated circuits (ICs). What is most suitable for your application depends on the required temperature range, linearity, accuracy, cost, features and the ease of designing the necessary support circuitry.
A thermocouple consists of two dissimilar metals joined together at one end, to produce a small unique voltage at a given temperature. The thermoelectric voltage, resulting from the temperature difference from one end of the wire to the other, is actually the sum of all the voltage differences along the wire from end to end.
Thermocouples are available in different combinations of metals or calibrations. The four most common calibrations are J, K, T and E. Each calibration has a different temperature range and environment, although the maximum temperature varies with the diameter of the wire used in the thermocouple. For example, a ‘type J’ thermocouple is made from iron and constantan wires.
Thermocouples are very popular because of their low thermal mass and wide operating temperature range, which can extend to about 1700°C with common types. However, sensitivity of thermocouples is rather small (of the order of tens of microvolts perºC). A low-offset amplifier is needed to produce a usable output voltage.
Thermistors are special solid temperature sensors that behave like temperature-sensitive electrical resistors. These are generally composed of semiconductor materials. There are basically two types of thermistors—negative temperature coefficient (NTC), which are used mostly in temperature sensing and positive temperature coefficient (PTC), which are used mostly in electric current control.
Thermistor exhibits a change in electrical resistance with a change in its temperature. The resistance is measured by passing a small, measured direct current through it and measuring the voltage drop produced thereby. When it comes to NTC-type, the negative coeffiient can be as large as several per cent perºC, allowing the thermistor circuit to detect minute changes in temperature, which could not be observed with a thermocouple circuit.
Low-cost thermistors often perform simple measurement (and trip-point detection) functions in low-end systems. Low-precision thermistors are often inexpensive. You can find thermitors that will work over a temperature range from about -100°C to +550°C, although most are rated for maximum operating temperatures from 100°C to 150°C. Simple thermistor-based set-point thermostat or controller applications can be implemented with very few components. Just the thermistor, a comparator and a few other components can do the job.
As thermistors are extremely non-linear devices that are highly dependent upon process parameters, and their performance may be degraded by self-heating, these have drawbacks in some applications. For example, resistance temperature function of a thermistor is very non-linear, so if wide range of temperatures are to be measured, you’ll find it necessary to perform sustantial linearisation.
There are a wide variety of temperature sensor ICs that are available to simplify the broadest possible range of temperature monitoring challenges. These silicon temperature sensors differ significantly from the above mentioned types in a couple of important ways.
The first is operating temperature range. A temperature sensor IC can operate over the nominal IC temperature range of -55°C to +150°C. The second major difference is functionality. A silicon temperature sensor is an integrated circuit, and can therefore include extensive signal processing circuitry within the same package as the sensor. There is no need to add compensation (or linearisation) circuits for temperature sensor Ics.
Some of these are analogue circuits with either voltage or current output. Others combine analogue-sensing circuits with voltage comparators to provide alert functions. Some other sensor ICs combine analogue-sensing circuitry with digital input/output and control registers, making them an ideal solution for microprocessor-based systems.
Digital output sensor usually contains a temperature sensor, ananalogue-to-digital converter (ADC), a two-wire digital interface and registers for controlling the IC’s operation.Temperature is continuously measured and can be read at any time. If desired, the host processor can instruct the sensor to monitor temperature and take an output pin high (or low) if temperature exceeds a programmed limit. Lower threshold temperature can also be programmed and the host can be notified when temperature has dropped below this threshold. Thus, digital output sensor can be used for reliable temperature monitoring in microprocessor-based systems.