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“Sensor is a device that when exposed to a physical phenomenon (temperature, displacement, force, etc.) produces a proportional output signal (electrical, mechanical, magnetic, etc.)”.
The ‘Thermistor’ uses resistance to detect temperature.
Thermistors can measure temperatures across the range of -40 ~ 150 ±0.35 °C (-40 ~ 302 ±0.63 °F).
Typical operation resistances are in the kW range, although the actual resistance may range from few W to several MW.
Typical Thermistor Types
The adjoining figure shows typical types of thermistors.
The shape of the thermistor probe can take the form of a bead, washer, disk, or rod.
Basically, thermistors are broadly classified as Ceramic, PTC (positive temperature coefficient) and NTC (negative temperature coefficient) thermistors.
Basic Working Principle
The electrical resistance of metals depends on temperature.
By measuring the changing resistance, the temperature can be determined.
The change in resistance can easily be converted to an electrical signal transmittable.
A thermistor is made of semiconductor, a mixture of metal oxide.
Metals usually have a positive resistance coefficient with respect to temperature.
Unlike metals, the semiconductors have a negative resistance coefficient.
This is the main difference between a thermometer and a thermistor.
Thus, it can be said that a PTC Thermistor is similar to an Resistance Temperature Detectors (RTD).
Thus, thermistors are based on the principle of when the temperature of the resistors changes, the electrical resistance of the resistors will change correspondingly.
In Negative Temperature Coefficient (NTC) thermistors, when the temperature of the resistors increases, the resistance of the resistors will be decreased.
In Positive Temperature Coefficient (PTC) thermistors, when the temperature of the resistors increases, the resistance of the resistors will also be increased.
The PTC Working Principle
The PTC (Positive Temperature Coefficient) is a temperature sensitive semiconductor, which is made of doped polycrystalline ceramic on the basis of barium titanate.
The resistance of these thermistors increases sharply when a defined temperature is reached.
This property is the reason for the self-regulation characteristic, which the PTC heating elements make use of.
Due to the special Resistance-Temperature-characteristic, there is no additional temperature regulation or safety device necessary while reaching high heat-power level when using the low resistance area.
The PTC-heating element regulates the power sensitively according to the required temperature. The power input depends on the requested heat output.
The NTC thermistors which are discussed herein are composed of metal oxides.
The most commonly used oxides are those of manganese, nickel, cobalt, iron, copper and titanium.
As seen from the adjoining figures, the resistance of these thermistors decreases with the increase in temperature.
Sample Configuration in Application (PTC Thermistor)
As to their possibilities of application, PTC thermistors can be divided on the basis of their ‘function’ and their ‘application’.
Out of the so many possible applications, I would like to like to show the use of ‘PTC thermistors for over-current protection’.
It’s one of the simplest configurations and is very easy to understand.
Here, PTC thermistor is used in the form of a fuse which is connected in series with the load in the circuit.
Major Specifications in Thermistors
Resistance-Temperature Curves : Usually varies and is provided by the manufacturer.
Nominal Resistance Value : Usually varies and is provided by the manufacturer.
Resistance Tolerance : The standard tolerances available for each thermistor type are given on the specific product data sheet.
Beta Tolerance : The beta of a thermistor is determined by the composition and structure of the various metal oxides being used in the device.
Application of Thermistors
The thermistor is a versatile component that can be used in a wide variety of applications where the measurand is temperature dependent.
Depending on the type of application and the specific out put requirements, the PTC or the NTC Thermistor is used.
Thus, the application have to be broadly divided as PTC Thermistor applications and NTC Thermistor applications respectively.
Following are the various applications.
Application of PTC Thermistors
Apart from these, Power PTC thermistors are used as a ‘Fuse’ for Short-circuit and over-current protection.
They are used as a ‘switch’ for Motor start Degaussing.
They are used as a ‘temperature sensor’ in measurement and control & over temperature protection circuits.
They are used to limit temperature for motor protection and over temperature protection circuits.
They are also used as ‘level sensors’ and ‘limit indicators’.
NTC thermistors are used in General Industrial Applications such as Industrial process controls, Photographic processing, Copy machines, Soldering irons (controlled), Solar energy equipment, etc.
They are used in Consumer / Household Appliances like Thermostats, Burglar alarm detectors, Refrigeration and air conditioning, Fire detection, etc.
They are used in Medical Applications like Fever thermometers, Dialysis equipment, Rectal temperature monitoring, Respiration rate measurement, Blood analysis equipment, Respirators, etc.
Limitations of Thermistors
Limited temperature range, typically -100 ~ 150 °C (-148 ~ 302 °F).
Nonlinear resistance-temperature relationship, unlike RTDs which have a very linear relationship.
They can be affected by self-heating errors that result from excitation current being dissipated in the thermistor.
Thermistors are also relatively fragile, so they must be handled and mounted carefully to avoid damage.
Exposure to higher temperatures can de-calibrate a thermistor permanently, producing measurement inaccuracies.