Temperature measurement and control is one of the most common applications of microcontroller-based data acquisition systems. Four types of sensors are commonly used to measure temperature in commercial and industrial applications. These are thermocouples, resistive temperature devices (RTDs), thermistors, and integrated circuit (IC) temperature sensors. Each sensor has its unique advantages and disadvantages and by understanding how these sensors work, and what types of signal conditioning are required for each, we can make more accurate and reliable temperature measurement, monitoring, and control.
The typical characteristics of various temperature sensors are:
• Thermocouples: these are inexpensive, and the most common temperature sensors with a wide range of temperature range. Thermocouples work on the principle that when two dissimilar metals are combined, a voltage appears across the junction between the metals. By measuring this voltage, we can get a temperature reading. Different combinations of metals create different thermocouple voltages and there is a wide range of thermocouples available for different applications. Thermocouples generate very low voltages, typically 50 !lV/oC. These low-level signals require special signal conditioning to remove any possible noise. Thermocouples have non-linear relationships to the measured temperature and as a result it is necessary either to linearize the characteristics or to use look-up tables to obtain the actual temperature from the measured voltage.
• RTDs: an RTD is a resistor with its resistance changing with temperature.
The most popular type of RTD is made of platinum and has a resistance of I00 0 at 0oC. Because RTDs are resistive devices, a current must pass through the RTD to produce a voltage that can be measured. The change in resistance is very small (about 0.4 0/oC) and special circuitry is generally needed tomeasure the small changes in temperature. One of the drawbacks of RTDs is their non-linear change in resistance with temperature.
• Thermistors: thermistors are metal oxide semiconductor devices whose resistance changes with temperature. One of the advantages of thermistors is their fast responses and high sensitivity. For example, a typical thermistor
may have a resistance of 50 k0 at 25oC, but have a resistance of only 2 k0 at 85oC. Like RTDs, a current is passed through a thermistor and the voltage across the thermistor is measured. Thermistors are very non-linear devices and look-up tables are usually used to convert the measured voltage to temperature. Thermistors are very small and one disadvantage of this is that they can be self-heating under a large excitation current. This of course increases the temperature of the device and can give erroneous results.
• IC temperature sensors: integrated circuit temperature sensors are usually 3- or 8-pin active devices that require a power supply to operate and give out a voltage which is directly proportional to the temperature. There are basically two types of IC temperature sensor: analogue sensors are usually 3-pin devices and give out an analogue voltage of typically I0 mV/oC which is directly proportional to the temperature; digital temperature sensors provide 8- or 9-bit serial digital output data which is directly proportional tothe temperature.
In this chapter we shall be looking at how we can interface various temperature sensors to our microcontroller in order to measure and display the ambient temperature.