Cover story
The design of a thermistor-based temperature sensing system
By Jellenie Rodriguez, Staff Applications Engineer, and Mary McCarthy, Staff Engineer, both at Analog Devices
A
resistance temperature detector (RTD) is a type of resistor with resistance that varies as a function of temperature. Thermistors operate in a similar manner to RTDs, but unlike RTDs which only have
a positive temperature coefficient, they can have positive or negative temperature coefficient (NTC). An NTC thermistor decreases its resistance as temperature increases, whereas a positive one (PTC) increases its resistance as temperature increases; see Figure 1. The temperature curve of an RTD is near-linear, and the sensor covers a wider temperature range than thermistors (between –200°C and +850°C), since thermistors have non-linear (exponential) characteristics. Thermistors can detect temperature changes much faster than RTDs, delivering faster feedback. Thus, they are commonly used in applications that require low cost, small size, faster response rate, higher sensitivity, and with restricted temperature range, like in monitoring electronic equipment, households and building controls, scientifi c laboratories, or the cold-junction compensation used for thermocouples in commercial or industrial applications. In most cases, precision temperature measurements use NTC thermistors rather than PTC ones. There are a few available PTC thermistors that are used in overcurrent input protection 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 (Curie point) is reached, above which a dramatic increase in resistance (several orders of magnitude) occurs within a few degrees Celsius. So, during an overcurrent condition, the PTC thermistor will exhibit a high amount of self-heating beyond the switching temperature and its resistance will increase dramatically, thereby resulting in a reduced input current. The switching point for a PTC thermistor is typically between 60°C and 120°C, which is not suitable for monitoring temperature measurements in some applications. NTC thermistors are available with nominal resistances at 25°C, ranging from a few ohms to 10MΩ. As shown in Figure 1, resistance changes per degree Celsius are more signifi cant for a thermistor than an RTD. The high sensitivity and high resistance values of thermistors make their front- end circuitry much simpler compared to RTDs, since a thermistor doesn’t need any special wiring confi gurations – it only uses a simple 2-wire confi guration.
06 February 2024
www.electronicsworld.co.uk
Figure 1: The response characteristics of thermistors vs. RTDs
Figure 2: A typical NTC thermistor measurement signal chain block
Figure 3: The current excitation of a thermistor
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