Thermistors

Thermistors are components whose resistance is a function of temperature.  Typically they are described by their resistance at 25 deg. C and one or more parameters which describe the relationship between temperature and resistance.  The image shows a bead thermistor which has a resistance of 4.7 kΩ at 25 deg. Centigrade.  Thermistors can be NTC (negative temperature coefficient) in which case the resistance decreases with temperature or PTC (positive temperature coefficient) in which case resistance increases with temperature.  Additional parameters are the temperature range, thermal response time, tolerance and power dissipation.  An important application of thermistors is as temperature sensing devices.

Relationship between Temperature and Resistance

The relationship between temperature and resistance is non-linear and is described by the Steinhart-Hart Equations.  The curve below is for a 4.7 kΩ device with a negative temperature coefficient:

This particular device would be good for monitoring a domestic hot water system where temperatures are typically in the range 10 to 60 degrees C.

Steinhart-Hart Equations

The Steinhart-Hart equations is:

The temperature is expressed in deg. K (273.15+T expressed in deg. C). The equation can be rearranged in many ways as appropriate to the application and the data available.

 If appropriate to the application, the third term can be dropped and  if the resistance (R₀) at a reference temperature (T₀) is known, then the equation can be re-arranged such that:

In the above equations, temperatures are expressed in deg. C.  This form of the equation can be used for devices which are described by their resistance (R₀) at 25 deg. C (T₀) and the B parameter.

Reference

Wikipedia

Page Updated: 22-Oct-2008

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