eFunda: Introduction to Thermistors
engineering fundamentals Thermistors: Introduction
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Overview

Similar to Resistance Temperature Detectors (RTD), the Thermistor (Bulk Semiconductor Sensor) uses resistance to detect temperature. However, unlike an RTD's metal probe where the resistance increases with temperature, the thermistor uses ceramic semiconducting materials which respond inversely with temperature. Examples of thermistors are shown in the following schematic.

Typical Thermistors

Typical thermistor sensors can measure temperatures across the range of -40 ~ 150 ±0.35 °C (-40 ~ 302 ±0.63 °F). The shape of the thermistor probe can take the form of a bead, washer, disk, or rod as illustrated in the above figure. Typical operation resistances are in the kW range, although the actual resistance may range from several MW to several W.

Pros and Cons

  • Pros:
  - High accuracy, ~±0.02 °C (±0.36°F), better than RTDs, much better than thermocouples.
  - High sensitivity, ~10 times better than RTDs, much better than thermocouples. As a result, lead wire and self-heating errors are negligible.
  - Small in size compared to thermocouples.
  - Response time shorter than RTDs, about the same as thermocouples.
  - Reasonable long term stability and repeatability.
  • Cons:
  - Limited temperature range, typically -100 ~ 150 °C (-148 ~ 302 °F).
  - Nonlinear resistance-temperature relationship, unlike RTDs which have a very linear relationship.

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