Thermistors vs. RTDs and Thermocouples

The thermistor is a widely used temperature sensor and an attractive alternative to thermocouples and RTDs — but only in some applications.

Temperature is the most commonly assessed and measured physical parameter; if you add all the consumer, industrial, commercial, medical, food-related, product tests and measurements, and other places where it needs to be sensed. In some cases, the reading is just a data point primarily required to inform the user (as in “What’s the temperature outside right now?”); in many cases, it is part of a closed-loop system that regulates and maintains the system temperature at a desired setpoint or enable adjustment of acquired data to correct and compensate for ambient-temperature variations.

Due to the importance of assessing temperature, many fundamental types of transducers are available to sense temperature and create an electrical signal as an analog to represent that temperature. These include well-known transducers such as thermocouples, resistance-temperature detectors (RTDs), and solid-state sensors, as well as more “esoteric” schemes such as infrared sensing, optical sensors, Bragg reflectors, and hot-wire anemometers; there are also complicated schemes for measuring extreme cold (cryogenics) or intense heat (such as plasmas).

This FAQ will examine the thermistor, a basic, relatively low-cost, reliable temperature sensor that is easy to use and interface with. It will also examine thermistors in the context of two major alternatives: thermocouples and RTDs.

Q: What is a thermistor?
A: A thermistor is a sensing component with a resistance that changes with changes in the ambient temperature. It consists of a metal oxide semiconductor pressed into a small bead, disk, wafer, or other container and coated with epoxy or glass. Thermistors are very accurate and effective sensors for measuring temperature, although only over a limited range.

Q: Why are they called thermistors?
A: The name is derived from the phrase THERMaIly sensitive reslSTORS.

Q: What are some of their critical attributes?
A: They provide the best sensitivity of any comparable measurement device and are well-suited for capturing and conveying more minor temperature changes. They also generally have a much higher resistance than an RTD. Unlike an RTD, a thermistor is typically a negative temperature coefficient device, which means its resistance decreases with increased temperature.

Q: The obvious question is, how do they compare, at a high level, to RTDs and thermocouples?
A: Figure 1 provides the overview.

Figure 1. This high-level overview provides a relative comparison of key characteristics of thermocouples, RTDs, and thermistors. (Image: National Instruments)

Q: How are they made? What is their schematic diagram symbol?
A: Thermistors are made using a mixture of metals and metal oxide materials. Once mixed, the materials are formed and fired (sintered) into the required shape. The thermistors can then be used “as-is” as disk-style thermistors. Or further shaped and assembled with lead wires and coatings to form bead-style thermistors, as shown in Figure 2.

Figure 2. (left) A thermistor can look like a plain tiny bead with two lead wires; (right) it is represented on the schematic diagram by either the IEC or ANSI symbol. (Image: Omega Engineering; DevExplained)

Q: Where are thermistors used?
A: They are used in many consumer and moderate-range situations where temperature extremes are not a consideration, such as air conditioners, HVAC systems, and water systems.

Q: What does their “packaging” look like?
A: Epoxy coatings are usually used for lower-temperature devices (typically -50 to 150°C, while glass coatings are used for higher-temperature applications (typically -50 to 300°C). Lower-end, non-harsh consumer applications such as air conditioners often use them as a bare-bead package with attached copper leads that can be soldered to the circuit board. More rugged installations can be packaged in probes.

Q: What is the size of a thermistor?
A: They can be quite small, as small as a pinhead. This makes them easier to place and assures that their thermal mass is low and their response time is quick.

Q: What are the basic types of thermistors?
A: There are two basic types. The negative temperature coefficient (NTC) thermistor is the most common for temperature sensing. Its resistance decreases as the temperature increases, and vice versa. With the positive temperature coefficient (PTC) thermistor, the resistance increases as the temperature increases. The PTC version is rarely used for temperature measurement but has other uses.

Q: Where is the PTC version used?
A: The PTC thermistor is used as a current-limiting device or fuse. Instead of using a change in ambient temperature to induce a change in resistance and thus allow temperature measurement, the PTC device is often used in reverse mode. An increase in current flow, such as to a motor that is failing or stalled, causes an increase in self-heating, which in turn causes an increase in resistance. This increase impedes the flow of current, acting like a current-limiting valve. Note that this FAQ does not discuss the details of PTC units or their current-limiting application; it focuses entirely on NTC thermistors and temperature sensing.

Q: What are some of the key characteristics of thermistors?
A: A thermistor’s sensitivity is many times greater than an RTD’s, but its useful temperature range is more limited. NTC thermistor resistance drops dramatically and non-linearly with temperature increase.

Q: Are there standards for thermistors similar to those for RTDs and thermocouples?
A: Unlike thermocouples and RTDs, which have industry and regulatory standards for resistance vs. temperature and other vital attributes, thermistors are not standardized and are much more linked to their vendor. This has not stopped their adoption in suitable applications.

Thermistor performance

Q: What’s the temperature vs. resistance transfer function of a thermistor?
A: Each thermistor material provides a different resistance vs. temperature curve. Some materials provide better stability, while others have higher resistance so that they can be fabricated into larger or smaller thermistors.

Q: What is a top-tier performance parameter for a thermistor?
A: Many manufacturers list a beta (β) constant indicating the change in resistance between two specified temperatures. This constant and the resistance at 25°C can be used to identify a specific thermistor curve.

Q: What is the nominal resistance of a thermistor?
A: It varies with the vendor and specific model, but may be as high as several thousand ohms. This high value provides a larger output than RTDs (typically at around 100 ohms) with the exact measuring current and often eliminates the need for lead-wire resistance compensation, which is mandated for temperature sensors in some configurations.

Relatively low-temperature applications (-55 to approximately 70°C) generally use lower-resistance thermistors (2200 Ω to 10,000 Ω). Higher-temperature applications often use higher-resistance thermistors (above 10,000 Ω) to optimize the resistance change per degree at the required temperature.

Unlike RTDs or thermocouples with relatively small nominal resistance, thermistors have many standard resistances depending on the range. Commonly used values include 2252 Ω, 3 kΩ, 5kΩ, 10kΩ, 30 kΩ, 50 kΩ, and even 1 MΩ.

Q: What is the linearity of a thermistor’s resistance change vs. temperature change?
A: In contrast to RTDs, where resistance changes nearly linearly, NTC thermistors have a highly nonlinear change in resistance seen in Figure 3. The designer must accommodate this in the system and either work with nonlinearity or devise some linearization scheme. However, their higher resistance change per degree of temperature provides a more excellent resolution, providing clear advantages over a modest sensing range.

Figure 3. The temperature vs. resistance (or voltage, for a thermocouple) curve for the three sensors is very revealing and shows their stark differences. (Image: Omega Engineering)

Q: How accurate and stable is a thermistor?
A: Thermistors are among the most accurate types of temperature sensors, and high-performance ones can offer accuracy of ±0.1°C or ±0.2°C, depending on the sensor model. However, this accuracy comes at a “price” because it can only be achieved over a fairly limited temperature range, such as 0°C to 100°C. That is often not a problem for many consumers and other applications.

Q: How does the thermistor compare to thermocouples and RTDs regarding key performance attributes when choosing the “best” or most appropriate temperature sensor for an application?
A: Figure 4 provides a good summary. However, as with all such summaries, exceptions and exceptional cases or custom units may contradict these generalizations.

Figure 4. This table reveals the comparative performance attributes of the three sensor types in more detail. (Image: National Instruments)

Q: What does the basic interface circuitry for a thermistor look like?
A: Many circuits and even dedicated ICs exist for this application, and they all use either a source driving current into the device and measuring the voltage drop or a voltage source placed across it and measuring current flow. The basic circuit can then be expanded with calibration and compensation functions, fault and wire-break detection, an A/D converter, and more.

Q: Can you show examples?
A: Figure 5 shows a basic current source driving the thermistor and a voltage source. Although they look similar, there is a distinct difference in the current source setup compared to a voltage source (VREF) and what is measured in each case.

Figure 5. The current-source interface (left) and the voltage-source interface (right) are simple and almost identical. Still, they are very different in their stimulus/response interaction with the thermistor. (Image: Analog Devices)

Conclusion

Thermistors provide designers with a viable option for temperature measurement in the right situations and applications. They are often judged against thermocouples and RTDs, and they can be cost-effective, high-performance sensors in many applications, but they are not appropriate or suitable for others.