Thermistor – Working, Symbol, Types & Applications

If you are looking for a temperature sensor that is cost effective and accurate, your first choice may be a thermistor. It is a type of resistor whose resistance changes with change in temperature.  This article deals with the working, types and applications of a thermistor. Lets start! What is a Thermistor? Coined from the words “THERmally controlled resISTOR”, thermistor is a temperature controlled resistor. The resistance offered by this solid state temperature controlled device depends on the ambient temperature. All resistors do have a temperature dependency that is given by their temperature coefficients. For most of the resistors (fixed and variable), this temperature coefficient is kept very low, such that the change in temperature does not significantly affect their resistance. On the other hand, the temperature coefficient of thermistor is considerably high, thus their resistance change with respect to change in temperature. Since in a thermistor, physical changes (change in its temperature) tend to change its electrical properties (like resistance),it can be also referred to as transducer. A thermistor is mostly made from sensitive semiconductor based metal oxides with metallised or sintered connecting leads onto a ceramic disc or bead. Thus we can define a thermistor as: “A two terminal solid state thermally sensitive transducer, that allows a significant change in its resistive value with respect to change in ambient temperature.” Some practical Thermistors are shown in figure:   What are the types of Thermistors? As already discussed, temperature dependence of a resistor is defined by its temperature coefficient. According to this, the thermistors are classified into two categories based on the type of temperature coefficient. There are two types of temperature coefficient namely negative temperature coefficient and positive temperature coefficient. The ceramic semiconductor material used for each type of thermistor, differs, as the temperature coefficient is dependent on the material used. Let’s discuss about each in brief! NTC Thermistor: Definition – NTC or negative temperature coefficient thermistor is a device whose resistance decreases with increase in temperature. These types of resistor usually exhibit a large, precise and predictable decrease in resistance with increase in temperature. Material Used for Construction – Unlike other resistors (fixed or variable), these are made of ceramics and polymers, which composed of metal oxides that are dried and sintered to obtain a desired form factor. In case of NTC thermistor, cobalt, nickel, iron and copper oxides are preferred NTC Thermistor Symbol – The symbol for NTC thermistor is given as:   Characteristic Curve – A typical NTC thermistor gives most precise readings in the temperature range of -55oC to 200oC. However some specially designed NTC thermistors are used at absolute zero temperature(-273.15oC) and some can be used above 150oC.The figure below shows the characteristic curve of a NTC thermistor:   From the figure we can say that they have a steep resistance temperature curve, denoting good temperature sensitivity. However due to the nonlinear relationship between resistance and temperature, some approximations are utilized to design practical system. Out of all the approximations, the simplest one is: 𝛥R = k𝛥T, where k is the negative temperature coefficient of the Thermistor. Heat dissipation: Like any resistor, Thermistor also dissipates heat whenever a significant amount of current flows through it. This heat is dissipated in the core of the Thermistor, therefore it can tamper the precision of the device. Heat capacity: The heat required to increase the temperature by 1oC in a NTC Thermistor, is called its heat capacity. It defines the response speed of NTC Thermistor and hence its knowledge is needed to decide where it has to be used. PTC Thermistor: Definition – PTC or Positive temperature coefficient Thermistors are those...
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