Thermistors
Introduction
A thermistor is a type of resistor whose resistance exhibits a large and predictable change in response to variations in temperature, enabling its use as a precise temperature-sensing device in electronic circuits.[1] Typically constructed from sintered semiconductor materials such as metal oxides, thermistors operate on the principle that temperature alters the conductivity of their base material, resulting in a nonlinear resistance-temperature relationship.[2] There are two primary types: negative temperature coefficient (NTC) thermistors, in which resistance decreases as temperature increases, and positive temperature coefficient (PTC) thermistors, in which resistance increases with rising temperature.[1]
The concept underlying thermistors traces back to 1833, when Michael Faraday observed the semiconducting behavior of silver sulfide, noting its resistance variation with temperature.[3] Practical development occurred in the 1930s, with Samuel Ruben inventing the modern thermistor in 1930 and receiving U.S. Patent No. 2,021,491 in 1935 for its application in temperature measurement.[4][5] Since then, thermistors have evolved into compact, cost-effective components, often with standard resistance values like 10 kΩ at 25°C for NTC types, and they provide high sensitivity—up to several percent change per degree Celsius—over limited ranges typically from -50°C to 150°C.[6] Their fast response time, on the order of seconds or less, stems from their small size and direct thermal coupling.[1]
Thermistors find extensive applications in temperature monitoring, circuit protection, and compensation across industries. In consumer electronics, they measure battery or ambient temperatures; in automotive systems, they support engine management; and in HVAC equipment, they enable precise control.[6] PTC variants are particularly valued for inrush current limiting in power supplies, where their resistance surge prevents overloads during startup, while NTC types excel in low-temperature sensing due to their steep resistance curve.[7] Compared to resistance temperature detectors (RTDs), thermistors offer superior sensitivity and lower cost but suffer from nonlinearity, requiring calibration or linearization circuits for accuracy over wide ranges.[6]
Fundamentals
Definition and Principles
A thermistor is a resistor whose resistance varies significantly with temperature, typically by several percent per degree Celsius, in contrast to standard metallic resistors that exhibit much smaller changes of about 0.4% per °C.[8] This temperature-dependent behavior makes thermistors suitable for precise temperature sensing in various applications. The term "thermistor" derives from the combination of "thermal" and "resistor," highlighting its core functionality as a temperature-sensitive electrical component.[9]