A thermistor is defined as a kind of resistor whose electrical resistance varies with changes in temperature. Even though resistance property of all the resistors fluctuates slightly with temperature, but a thermistor is a device that is particularly sensitive to temperature changes. Thermistors are prepared so that the resistance changes drastically with temperature and this change can be from 100 ohms to more for one-degree change in temperature.
Fig. Thermistor symbol
Fig. Practically used thermistors
Thermistors are available in two configurations: First is NTC thermistors whose resistance decreases with an increase in temperature. Another is PTC thermistors whose resistance increases with increases in temperature.
NTC thermistors are commonly used in temperature measurement.
Thermistors are composed of materials whose resistance is known. As temperature increases, the NTC thermistor’s resistance increases in a non-linear fashion, following a particular curl. The shape of this resistance vs. temperature curl is determined by the properties of the materials that are used for the making of the thermistor.
Thermistors are available with a large variety of base resistances and resistance vs. temperature curves. Low-temperature applications (-55 to approx. 70°C) require lower resistance thermistors (2252 to 10,000Ω). Whereas higher temperature applications use higher resistance thermistors (above 10,000Ω).
Thermistors are well suited for use in environments where extreme conditions or electronic noise is present. They are available in different shapes: the ideal shape for a particular application depends on whether the thermistor will be surface-mounted or embedded in a system, and on the type of material being measured.
Common Applications of Thermistors
Thermistors are engaged in a broad array of commercial and industrial applications to measure the temperature of liquids, surfaces, and ambient gasses.
When enclosed in a protective probe that can be reliably sterilized, so they can easily be used in food and beverage industries, scientific laboratories and in R&D.
Heavy-duty probe mounted thermistors are best suitable for immersion in corrosive fluids and can be used in industrial processes, while vinyl-tipped thermistor mounts are used in outdoor applications or for biological applications.
Thermistors are also available with metal or plastic cage-style element covers, used for air temperature measurement.
NTC Thermistors: Here the term NTC refers to as negative temperature coefficient, of resistance means the temperature of the device decreases as temperature increases. These are non-linear; device. Self-heating phenomena can affect the resistance of an NTC thermistor. When a current flows through the NTC thermistor, it absorbs the heat causing its own temperature to rise.
Fig. Different types of NTC thermistors
General Characteristics of NTC thermistors
It has fast Response Time to (±1%).
NTC thermistors having an accuracy range of 0.05 to 0.20 ˚C with long-term stability. Other temperature sensors can float over time.
NTC thermistors are customizing to meet different application requirements.
NTC thermistors offer brilliant immunity to electrical noise and lead resistance more so than other types of temperature sensors.
Because of their small size and ease of production, NTC thermistors have proven to be very economical choices.
NTC thermistors are manufactured using a mixture of metal oxides such as nickel, manganese, copper along with binding agents and stabilizers. After this mixing, the material is pressed into wafer forms and sintered at extreme temperatures. Once wafers are prepared these are either dice into smaller chip style thermistors, or left in the form of a disc thermistor.
NTC Thermistor Working Principle
When the ambient temperature of any thermistor increases, its resistance decreases significantly. Generally, for every 1oC rise in temperature, there is a 5% decrease in resistance. So they are highly sensitive.
They can observe even a very small change in temperature which could not be observed by a thermocouple or RTD. This makes thermistors very useful for precise measurement of temperature, control, and compensation. These can be used in the temperature range of – 60oC to 300o
They have resistance in the range of 0.5 ohms to 0.75 M ohms.
Characteristics of Thermistors:
Three important characteristics of thermistors are:
Resistance – temperature characteristics,
Current – time characteristics.
Voltage – current characteristics.
Resistance Temperature Characteristics
As the temperature of the thermistor increases its resistance decreases exponentially.
Mathematical expression for the relationship between resistance of thermistor and temperature can be given by
RT1 = RT2 e [β (1/T1 – 1/T2)]
RT1 = Resistance of the thermistor at the T1 temperature
RT2 = Resistance of the thermistor at the T2 temperature
β = is a constant, whose value depends upon the material used in the construction of thermistor, typically its value ranges from 3500 to 4500.
Fig.Resistance Vs. Temperature characteristics
Voltage Current Characteristics
The voltage drop across a thermistor increases as current increases. It increases until it reaches the peak value after the peak occurs; it decreases with the increase in temperature.
Current and Time Characteristics
It is clear from the figure shown below that the time delay to reach maximum current is the function of the applied voltage. When the applied voltage, is decreased the time delay to reach the maximum current is also decreased.