Today we will look at different options we have for temperature sensing, try and understand their strength and weaknesses. By the end, we will figure out which technology is right for a given use case.
The following list doesn’t include temperature sensing devices or methods which are not generally integrated into a product, like infrared temperature measurement.
Options we have are:
- RTD (Resistance Temperature Detector)
- Temperature sensing ICs
- Digital Output
- Analog Output
Thermistors are a two-pin device that changes their resistance according to change in temperature. They can be used to measure temperature in -100°C to 500°C range.
If the thermistor’s resistance increases with an increase in the temperature it is called PTC or Positive Temperature Coefficient thermistor.
Similarly, if the thermistor’s resistance decreases with an increase in the temperature it is called NTC or Negative Temperature Coefficient thermistor.
Thermistors are the most cost-effective solutions to do basic temperature measurement. They have a fast and non-linear output. These are not very accurate but can be accurate when used in small ranges. They can produce large resistance change for a small change in temperature thus they are extremely easy to work with. Many times they are used in voltage divider circuit which feeds the output signal into either some ADC pin or in some control logic so when the temperature exceeds some action can be carried out. Self-heating needs to be taken care of during measurement.
A thermocouple produces voltage with a change in temperature. They are made up of two dissimilar metals. Depending upon which metals are used they are classified into different types, e.g. K, T, R, S, etc.
Different types of metal produce different output curves and have different operating temperature ranges. They can be used to measure temperature in -200°C to 2500°C range.
Thermocouples are rugged and can be used in a hazardous environment. They do not self-heat as they do not require an external power source. They are fast due to their low thermal mass. They are more accurate than Thermistors but are less accurate as compared to RTD (more about it shortly). They require a considerable amount of signal processing. They also need cold side temperature reading for compensation in high accuracy temperature measurement.
3. RTD (Resistance Temperature Detector)
Similar to thermister, RTD also changes its resistance when temperature changes but RTD has a very linear response. This makes RTD very accurate. It can be used to measure temperature in the range of -200°C to 850°C. RTD similar to thermocouple comes in various forms and shapes which make it more versatile. Though RTD is very accurate, working with RTD is a bit tricky. The change in resistance per degree celsius is low so it needs good low noise amplifiers.
Also for accurate current measurement, it requires a low drift current source but there are circuit topologies that take care of that. For a higher signal to noise ratio, you would like to have more current going through the RTD to produce higher voltage drop. But a high amount of current flowing through the RTD also leads to self-heating. Thus it is a balancing act of voltage drop across
Due to its versatility and accuracy, it is heavily used in various industries.
4. Temperature sensing ICs
It’s the same thing as its name suggest they are temperature sensing ICs. They can be used to measure onboard temperature within the range of -20°C to 150°C. You will find such ICs on your computer RAM to measure heat dissipation. They are normally categorized into two types digital or analog output. These types of ICs are also used with thermocouple to improve their accuracy.
Thermostats are switches that turn on/off at a certain temperature. These are very handy components because they are used for a very specific application. They don’t provide any reading when temperature changes, they just turn on/off when a certain temperature threshold is crossed.
Let’s take an example, I want to turn on a fan that is blowing over a heatsink when the heatsink’s temperature goes above 50°C. I will use a thermostat which is normally open and closes when the temperature goes beyond 50°C. This makes the systems extremely simple so the system has fewer failure points. Compared to something like using a thermistor a based circuit that will require voltage sensing with a comparator then a switch (Transistor) to turn on/off the fan. Thermostat reduces the number of components making assembly cheaper, make the systems easy to maintain.
Done! We have gone through most of the commonly used components for temperature measurement, though it is an extremely basic explanation of each of the components, you now have a better understanding of which device to use for a given application.