Thermistors ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering


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Thermistors are devices made of metal oxide semi-conductor material which either increase in resistance with increasing temperature (a positive temperature coefficient) or decrease in resistance with increasing temperature (a negative temperature coefficient). Their resistance changes a lot for a small change in temperature and so they can be made into a small sensor and they cost less than platinum wire RTDs. The major difference between thermistors and RTDs is
linearity: thermistors are highly sensitive and nonlinear, whereas RTDs are relatively insensitive but very linear. For this reason, thermistors are typically used where high accuracy is unimportant. Many consumer-grade devices use thermistors for temperature sensors. The temperature range of thermistors is limited. They are only used for a typical range of -200°C to 1000°C and are commonly used in small hand held thermometers.

Due to their low cost, thermistors are used in many applications requiring information about process equipment for alarming and indication purposes. Such information, for example, would be the winding temperature of motors, or the temperature of bearings, or even the temperature of transformer windings. In such applications, the actual temperature may not be of as much concern as the change or rapid increase in temperature. It is for this reason that thermistors give vital alarming information about the state of equipment in motion. In the examples of motor winding and bearings, both show a significant increase in temperature at the onset of the failure due to insulation breakdown in the windings or excessive friction in the bearings. Because of their non-linearity and instability, thermistors are seldom used in continuous control applications.

Advantages of Thermistors:
  • Small sizes and fast response
  • Low cost
  • Suitability for narrow spans
Disadvantages of Thermistors:
  • More susceptible to permanent decalibration at high temperatures.
  • Use is limited to a few hundred degrees Celsius.
  • Respond quickly to temperature changes, especially susceptible to self-heating errors.
  • Very fragile
Additional learning resources:
Learn more about thermistor theory and operation from:
Learn the common terminologies used in thermistor applications from: Thermistor terminology