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Thermistor: Types, Working Principle and Applications

Updated: Mar 29

What is a thermistor?


The name "thermistor" is created by combining the words "thermal" and "resistor". As a passive component with temperature-sensitive properties, it is primarily employed as a temperature sensor.



Thermistor
Thermistor

A thermistor is a type of resistor that undergoes a significant change in resistance in response to variations in temperature.



Temperature and Resistance characteristics of Thermistor
Temperature and Resistance characteristics of Thermistor

Every type of resistor exhibits a certain level of temperature dependency, a characteristic encapsulated by their temperature coefficient. Thermistors, specifically, can achieve a high current coefficient. In thermistors, resistance decreases as temperature rises, leading to what is commonly known as a negative temperature coefficient (NTC). Conversely, thermistors with a positive temperature coefficient are referred to as positive temperature coefficient thermistors (PTC).



Symbol of thermistor
Symbol of thermistor


Types of Thermistors


Thermistors are usually ceramic semiconductors. They are composed mainly of metal oxides that are dried to obtain the desired form factor. The characteristic behavior of a thermistor is determined by the types of oxides.


Two types of thermistors are as follows:


  • NTC Thermistors: cobalt, nickel, iron, copper, and manganese are the primarily used materials.

  • PTC Thermistors: barium, strontium, and lead titanates are the primarily used materials.


NTC Thermistor


A NTC thermometer is used when a change in resistance over a large temperature range is required. It is used as a temperature sensor in the range of -55 °C to 2200 °C. Even though they can be produced for measuring either a much lower or higher temperature, they have quick response, reliability, robustness, and a low price.



Temperature and Resistance characteristics of NTC Thermistor
Temperature and Resistance characteristics of NTC Thermistor


PTC Thermistor


When a sudden change in resistance at a specific temperature is required. They exhibit a sudden change or increase in resistance above a defined temperature (Curie temperature). The typical switching temperature is in the range of 60 °C to 120 °C. They are used for self-regulating heating elements, self-resetting, and over current protection.




Temperature and Resistance characteristics of PTC Thermistor
Temperature and Resistance characteristics of PTC Thermistor


The working principle of a thermistor


The fundamental working principle of a thermistor is that resistance is dependent on temperature. The resistance of a thermistor is measured by an ohm meter (a device used for measuring electrical resistance). It should be noted that the thermistors do not read values, but the resistance varies with respect to temperature. Depending on the substance applied to a device, the amount of resistance is calculated.


Thermistors are non-linear sensors. For thermistors, a significant change in temperature coefficient resistance is required for temperature measurement. Thermistor has to be placed in the device body for which temperature measurement needs to be done and will be connected to an electrical circuit. When device temperature changes, the thermistor's resistance will also be changed, which will be recorded by the connected circuit and calibrated against the temperature that is set. Thermistors have two wires. One of the wires is connected to the excitation source, which is used for measuring the voltage of the thermistor. The primary advantage of thermistors is their capability to deliver a great change in resistance, providing us with a more sensitive and precise reading.


Thermistor Packages


Several types of thermistor packages and sizes are available. The most common thermistor package is the radial-leaded type, which is mainly constructed from epoxy. In more challenging environments,glass-encapsulated packages prove to be a better choice. Integrated packages, such as lugs, probes, and threaded housings for easy monitoring, are available.


Thermistor Package Examples


  • Radial leaded

  • Axial leaded

  • Glass

  • Threaded

  • Probe


Differences and similarities among thermistors, sensors, variable resistors, and rheostats


Each component serves a unique function in electronic circuits:


Functions and applications


Thermistors


  • Primarily used for temperature sensing.

  • Found in thermostats, climate control systems, and medical equipment.


Sensors


  • Broad applications in detecting light, motion, temperature, pressure, etc.


Variable Resistors


  • Used for tuning circuits, adjusting volume, and controlling voltage.


Rheostats


  • Specifically designed for controlling current in circuits.


Adjustment Methods


Thermistors


  • There is no manual adjustment; resistance changes with temperature.


Sensors


  • No manual adjustment; response based on physical parameters.


Variable Resistors


  • Manual adjustment by the user.


Rheostats


  • Manual adjustment for precise current control.


Similarities


Resistance Variation


  • Thermistors change resistance with temperature.

  • Variable resistors allow manual adjustment.

  • Rheostats provide manual control over current.


Circuit Applications


  • Thermistors are used in temperature-sensitive circuits.

  • Variable resistors adjust resistance in electronic applications.

  • Rheostats control current flow in electrical circuits.


Advantages of a thermistor


Lower cost


The most important reason for the popularity of thermistors is that they are lower in cost. At a minimal cost, it has the ability to provide an accurate date for a small temperature range.


Smaller Size


It is a compact design, and thermistors are fabricated in various forms.

  • Beads

  • Discs

  • Rods


Though their size is small, they are extremely durable and long-lasting.


Inrush control


Inrush is nothing, but when you switch on the device, it is changed with an exceptionally high amount of current. Without proper protection, harmful results will occur to the device. To protect sensitive circuits, NTC thermistors are used as inrush current limiters (ICL). Inrush currents can damage the capacitors, harm power switch contacts, and destroy rectifier diodes. PTC thermistors are also used as ICLs and for protecting overcurrent. Inductive electrical equipment such as transformers, motors, and ballast lighting experience. Inrush can be controlled by connecting thermistors in series, which limit the initial current to a safe value. NTC thermsitors are used because of their low values of cold resistance.


Protection from overheating


In an electrical circuit, the current flow produces heat, which is dissipated. This resultant heat increases the temperature of the resistor. By using a thermistor, definite resistance is reached, and hence the heat is reduced.


Assorted Uses


The main use of a thermistor is temperature measurement, yet it can also be used as a means of measuring pressure, power, and liquid levels. They are used as overload protectors and can provide malfunction-related warnings.


Precise Accuracy


Thermistors will be installed at a specific and definite distance from the circuit. This arrangement eradicates errors in readings because of the resistance in lead. The readings of thermistors are more precise as they are operated over a small temperature range. For small temperature changes, they respond quickly.


Fast Response Time


Since thermistors can respond to slight incremental temperature changes, they provide instant data with a small amount of delay. This kind of property is due to the small temperature ranges they monitor.


Easy customization


There are several varieties of thermistors that can be changed, adapted, or configured for any type of temperature application. Their various types and sizes allow them to be utilized in any kind of operation, condition, or situation.


Applications of Thermistor


Thermistors, or thermal resistors, are temperature-sensitive resistors that find applications in various electronic and industrial systems due to their ability to change resistance with temperature. Here are some common applications of thermistors:


Temperature Sensing


  • Thermistors are widely used as temperature sensors in electronic devices, such as thermostats, temperature controllers, and climate control systems.

  • They are employed in medical equipment to monitor and regulate body temperature.


Temperature Compensation


  • Thermistors are utilized in circuits to provide temperature compensation. They can help maintain stable operating conditions by adjusting other components' characteristics based on temperature changes.


Inrush Current Limiting


  • Thermistors with a high resistance at low temperatures can be employed in power supply circuits to limit inrush current when a device is powered on. As the device warms up, the thermistor's resistance decreases.


Heating Elements Control


  • Thermistors can be integrated into heating elements to control the temperature, preventing overheating and improving energy efficiency.


Liquid Level Measurement


  • In certain applications, thermistors can be used to measure the level of liquids by detecting temperature variations due to changes in the thermal conductivity of the liquid.


Refrigeration and HVAC systems


  • Thermistors are found in refrigerators and HVAC systems for temperature monitoring and control, ensuring that the systems operate within specified temperature ranges.


Automotive Applications


  • Thermistors are used in automotive systems for temperature monitoring of engine components, coolant, and intake air. They contribute to the efficient operation of the engine and control systems.


Battery Management


  • In battery management systems, thermistors are used to monitor the temperature of batteries. This helps prevent overheating and ensures safe charging and discharging.


Gas Detection


  • Certain thermistors are sensitive to changes in gas composition. They can be used in gas detection systems by monitoring the temperature changes associated with specific gas reactions.


Industrial Process Control


  • In industrial settings, thermistors play a crucial role in monitoring and controlling temperature in processes such as manufacturing, chemical reactions, and material processing.


Consumer Electronics


  • Thermistors are used in various consumer electronics, such as laptops and smartphones, for thermal management. They help prevent overheating by adjusting fan speeds or triggering thermal shutdowns.

Weather Stations


  • Thermistors are employed in weather stations to measure ambient temperatures accurately.

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