Introduction The term "Thermistor" is used to describe a range of electronic components whose principle characteristic is that their electrical resistance changes in response to changes in their temperature. The word "Thermistor" derives from the description "thermally sensitive resistor". Thermistors are further classified as "Positive Temperature Coefficient" devices (PTC devices) or "Negative Temperature Coefficient" devices (NTC devices). PTC devices are devices whose resistance increases as their temperature increases. NTC devices are devices whose resistance decreases as their temperature increases. NTC thermistors are manufactured from proprietary formulations of ceramic materials based on transition metal oxides. A discrete thermistor such as a chip, disc or rod is a fundamental electrical component. Alpha () (Temperature Coefficient): Alpha, a
material characteristic, is defined as the percentage resistance
change per degree Centigrade. Alpha is also referred to as the
temperature coefficient. For Negative Temperature Coefficient (NTC)
Thermistors, typical values of alpha are in the range 3%/°C to
6%/°C. The temperature coefficient is a basic concept in thermistor
calculations. Because the resistance of NTC thermistors is a
nonlinear function of temperature, the alpha value of a particular
thermistor material is also nonlinear across the relevant
temperature range. Where RT is the resistance of the component at the relevant temperature T (°C), dR/dt is the gradient of the Resistance vs Temperature curve at that temperature point, and alpha is expressed in units of "percentage change per degree Centigrade". (Note: In some texts the "100" term is omitted from the equation, but it is understood or implied in the units in which alpha values are specified.) Thermal Time Constant (T.C.):
When a
thermistor is being used to monitor the temperature of it’s
environment then the accuracy of measurement of the resistance of
the thermistor is critical.
The definition of Thermal Time Constant arises from the exponential nature of the rate of transfer of heat between the thermistor and the medium that it is monitoring. It is similar in principle to the definition of time constants in describing the responses of systems where physical effects have an exponential response with respect to time. Graph # 8 illustrates determination of T.C. for the thermistor of the previous example using a strip chart recorder. When the thermistor is transferred from a 25°C oil bath to a 75°C oil bath it’s resistance will change and the voltage drop across it can be measured using the chart recorder. By measuring the graph and the speed of the chart recorder the T.C. for the device in a stable oil bath environment can be determined. Time
Constant recording of a thermistor element
The value of resistance of
a thermistor that is measured in a physical system depends on the
power dissipated in the thermistor due to the measurement method and
also on the thermal characteristics of a dynamic temperature system.
It is important to consider both effects in implementing thermistor
sensing systems.
