Thermistors vs. RTDs
As discussed in a previous article series, “Optimizing RTD temperature sensing systems Sensing System,” an RTD is a type of resistor with resistance that varies as a function of temperature. Thermistors operate in a similar manner to RTDs. Unlike RTDs that only have a positive temperature coefficient, thermistors can either have positive or negative temperature coefficients. A negative temperature coefficient (NTC) thermistor decreases its resistance as the temperature increases, while a positive temperature coefficient (PTC) thermistor increases its resistance as the temperature increases. Figure 1 shows the response characteristics of typical NTC and PTC thermistors and how they compare to an RTD curve.
In terms of temperature range, the RTD curve is near linear, and the sensor covers a much wider temperature range (commonly –200°C to +850°C) than thermistors due to the thermistor’s nonlinear (exponential) characteristics. RTDs are usually available in a well-known standardized curve while thermistor curves vary depending on the manufacturer. We will discuss this in detail in the Thermistor Selection Guide section of this article.
Thermistors are made up of composite materials, usually ceramic, polymer, or semiconductor (typically metal oxides), which are quite smaller and more inexpensive but not as rugged when compared with RTDs, which are made of a pure metal (platinum, nickel, or copper). Thermistors can detect changes in temperature much faster than RTDs, delivering faster feedback. Thus, thermistors are commonly used sensors in applications that require low cost, small size, faster response rate, higher sensitivity, and with restricted temperature range such as with monitoring electronic equipment, households and building controls, scientific laboratories, or the cold-junction compensation used for thermocouples in commercial or industrial applications.
In most cases, NTC thermistors rather than PTC thermistors are used in precision temperature measurement applications. There are a few available PTC thermistors that are used in overcurrent input protection circuits or as resettable fuses for safety applications. The resistance-to-temperature curve of a PTC thermistor exhibits a very small NTC region until its switching point (or Curie point) is reached, above which a dramatic increase in resistance of several orders of magnitude occurs within a span of a few degrees Celsius. So, during an overcurrent condition, the PTC thermistor will have a high amount of self-heating beyond the switching temperature and its resistance will increase dramatically, thereby resulting in a reduced current being input to the system and thereby preventing damage to occur. The switching point for a PTC thermistor is typically between 60°C and 120°C, which is not suitable for monitoring temperature measurements in a wide range application. This article focuses on NTC thermistors that can typically measure or monitor temperature from –80°C to +150°C. NTC thermistors are available with nominal resistances at 25°C, ranging from a few ohms to 10 MΩ. As shown in Figure 1, the resistance changes per degree Celsius is more significant for a thermistor vs. an RTD. The high sensitivity and high resistance values of thermistors make their front-end circuitry much simpler compared to RTDs as a thermistor does not need any special wiring configurations such as 3-wire or 4-wire to compensate for lead resistance. Thermistor design only uses a simple 2-wire configuration.
Read more: Optimizing thermistor-based temperature sensing systems: Challenges