Construction Types for Thermistors

An NTC thermistor is a resistor that has a negative temperature coefficient, hence NTC. When using NTC thermistors resistance decreases as temperature increases. Typically, these thermistors are used as resistive temperature sensors and as current-limiting devices. For an NTC thermistor, the temperature sensitivity coefficient is roughly 5x higher than RTDs (resistance temperature detectors). NTC thermistors are used in temperature ranges between -40 and +150 (for chip type) +300 (for bead and glass encapsulated) Celsius.NTC Thermistor

NTC thermistors are frequently made from polymers or ceramics. Using different materials results in varied temperature responses and other characteristics. Temperature sensitivity of these sensors is expressed as a percent of change per degree in temperature. Different materials and production processes will change the value of temperature sensitivity, which usually ranges between -3% and -6% °C. When compared to RTDs, an NTC thermistor has a steeper resistance-temperature slope. Steep resistance offers a greater temperature sensitivity.

NTC thermistors are classified into three distinct groups, which depend on the production process implemented.

A bead type thermistor is made with a platinum alloy lead wire that is directly sintered onto a ceramic body. These types of thermistors offer quick response time, stronger stability, and allow for operations at a higher temperature than other thermistor types. Users must be careful because bead type devices are more fragile than their counterparts. Bead thermistors are sealed with glass, which protects them from handling dangers during assembly. They also improve measurement stability.

Glass encapsulated thermistors are sealed within an airtight glass bubble. They are designed to be mounted on printed circuit boards, especially when ruggedness is needed. The thermistors offer capabilities in temperatures above 150°C. Encapsulating an NTC thermistor helps to protect it from the environment and improves overall stability.

Disk/chip NTC thermistors are built with metalized surface contacts. They are the largest of the three types, and so they have slower reaction times. Although they might be slower, they do have a higher dissipation constant. Power dissipated by thermistors is supposed to be proportional to the square of a current. When compared to other types, disk and chip versions handle higher currents. These thermistors are made through pressing blended oxide powders into a round die. After, it is sintered at high temperatures and usually coated with epoxy.