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