A thermistor is a temperature-sensitive resistor whose electrical resistance changes significantly with temperature. This characteristic makes it useful for detecting temperature variations and converting them into an electrical signal. Thermistors are mainly classified as NTC (Negative Temperature Coefficient) and PTC (Positive Temperature Coefficient) types, depending on how their resistance changes with temperature.
Because of their high sensitivity, compact size, fast response, and relatively low cost, thermistors are widely used in temperature measurement, circuit protection, temperature compensation, heating control, HVAC systems, automotive equipment, battery monitoring, and industrial applications. This article explains the thermistor symbol, working principle, types, construction, resistance-temperature characteristics, and major applications.
What is a Thermistor?
A thermistor is a type of thermal resistor whose electrical resistance increases or decreases with a change in temperature. Thus, the thermistors are thermally sensitive resistance whose resistance changes with a change in the thermistor’s body temperature. We will discuss the working principle of thermistors. How PTC and NTC thermistors function and the applications of thermistors will be discussed in the later sections.
Thermistor Symbol
The thermistor symbol used in American and international standards is shown in the figure below.
A thermistor is a passive circuit element whose resistance changes with temperature. Like other passive components, it does not provide electrical energy or require an external power source specifically to produce its temperature-dependent resistance characteristic.
Thermistors are generally small, sturdy, and inexpensive and can provide accurate temperature measurement within their specified operating range. However, their performance may become less reliable at extremely high or low temperatures. Therefore, thermistors are most suitable for applications where accurate temperature measurement is required within a defined temperature range.
Working Principle of Thermistor
The resistance of the thermistor changes with a change in the thermistor’s body temperature. The resistance of the thermistor does not vary linearly with changes in temperature. The thermistor has a non-linear resistance temperature curve.
The resistance of the thermistor can be measured using a resistance meter(Ohm-meter). By knowing the exact relationship between the change in the resistance and temperature, the temperature can be derived by measuring the resistance of the thermistor at a particular temperature.
The change in the thermistor resistance with temperature depends on the type of material used for thermistor construction. The plot between temperature and resistance of a thermistor is as given below. Â
From the above graph, we can measure the temperature by measuring the resistance of the thermistor. The procedure of temperature measurement is as follows. Â
- Measure the resistance of the thermistor by using an Ohm-meter.
- Draw a vertical line across from the resistance on the y-axis and drawing a vertical line down from where this horizontal line intersects with the graph, we can hence derive the temperature.
Types of ThermistorÂ
The thermistors are classified according to increase or decrease in thermistor resistance with variation in temperature. There are two types of thermistors.
- Negative Temperature Coefficient (NTC) Thermistor
- Positive Temperature Coefficient (PTC) Thermistor
NTC Thermistor
The NTC thermistor has a negative temperature coefficient. Its resistance decreases with an increase in temperature and increases when temperature decreases. Thus, the resistance and temperature are inversely proportional in the NTC thermistor. The electric current flow through the NTC thermistor increases with the increase in temperature.
With the increase in temperature, a large number of charge carriers or free electrons collide with valence electrons of another atom. The valence electrons, which gain sufficient energy, will break the bonding with the parent atom and move freely from one place to another. The electrons that move freely from one place to another are called free electrons.
Thus, the free electrons increase due to the rapid collision of the free electrons with the atom. The small increase in temperature produces millions of free electrons. The more free electrons cause a rapid increase in electric current. Thus, the small increase in temperature causes a rapid decrease in the temperature and allows a large current flow through the thermistor.
Resistance-temperature equation of NTC Thermistor
The resistance of the NTC thermistor decreases with an increase in temperature. The relationship between the temperature and resistance is governed in the NTC thermistor by the following mathematical expression.
The equation of the thermistor is highly nonlinear. A standard NTC thermistor usually exhibits a negative thermal resistance temperature coefficient of about 0.0045 per degree Kelvin.
The Beta (β) value of a thermistor is an indicator of the slope of the resistÂance-temperature curve characteristic.
The higher value β shows a good relationship between the resistance and temperature. A small increase in temperature will cause a decrease in the resistance if the value of β is high. Thus, higher sensitivity and accuracy can be achieved if the value of β is high.
From equation (1), the temperature coefficient of the thermistor can be given as;
Equation (2) shows that the thermistor has a negative temperature coefficient.
If β= 4000 K and T = 298 K, then the αT = 0.0045/∘K temperature coefficient. The temperature coefficient of the thermistor is much higher than the sensitivity of the platinum RTD.
PTC Thermistor
The resistance of the PTC thermistor increases with the increase in temperature. PTC thermistors are made from doped polycrystalline ceramic. Thermistors with a Positive Temperature Coefficient (PTC) are also called posistors. The plot between the resistance and temperature of a PTC thermistor is given below.
The PTC thermistors are not as popular as NTC thermistors. The PTC thermistors are used in circuit protection. When current passes through the PTC thermistor, it causes heating. In a PTC thermistor, this heating up will also cause an increase in resistance. This creates a self-reinforcing effect that drives the resistance upwards, therefore limiting the current. Thus, the PTC thermistor is used as a current limiting device.
Thermistor Construction
The thermistors are made with a sintered mixture of metallic oxides like manganese, cobalt, copper, iron, nickel, uranium, etc. These oxides are blended in a suitable proportion and compressed into the desired shape, and they are heat-treated to recrystallize.
Different shapes and sizes of thermistors are available on the market. Thermistors are available in the form of beads, disks, and washers. The size of the thermistor in the form of beads is in the range of 0.15 millimeters to 1.5 millimeters in diameter. Thermistors in the form of disks and washers are made by pressing the thermistor material under high pressure into flat cylindrical shapes with diameters from 3 millimeters to 25 millimeters.
The typical size of a thermistor is 0.125mm to 1.5 mm. The thermistors are available in the nominal values of 1K, 2K, 10K, 20K, 100K, etc. The resistance value of the thermistor is at a temperature of 25oC. The major advantages of thermistors are their small size and relatively low cost.
Applications of Thermistor
A thermistor is a temperature-sensitive resistor whose resistance changes significantly with temperature. Because this change can be measured and used in control circuits, thermistors are widely used for temperature measurement, protection, compensation, and automatic control. Their small size, fast response, and relatively low cost make them suitable for both electronic equipment and industrial systems.
Temperature Sensing
One of the most common applications of a thermistor is temperature measurement. A change in temperature produces a corresponding change in thermistor resistance, which can be detected by an electronic circuit.
Thermistors are commonly used in:
- Digital thermometers and thermostats
- Temperature monitoring circuits
- Air-conditioning and climate-control equipment
- Medical and laboratory instruments
- Electronic equipment temperature sensors
Thermistor Temperature Measurement Circuit
A thermistor can be connected in series with an ammeter and a battery to create a simple temperature-sensing circuit. When the temperature changes, the resistance of the thermistor also changes. This variation in resistance causes a corresponding change in the current flowing through the circuit. The ammeter can be calibrated directly in terms of temperature, allowing it to indicate the temperature instead of the current. With suitable calibration, the circuit can provide a resolution of approximately 1°C.
For improved sensitivity and measurement accuracy, a bridge circuit can be used with the thermistor. The bridge configuration detects small changes in thermistor resistance more effectively than a simple series circuit, making it suitable for precise temperature measurement.
At around 25°C, a standard bridge arrangement can provide a sensitivity of approximately 18 mV/°C. In the described circuit, a 4 kΩ thermistor is used as the temperature-sensitive element. As the temperature varies, the thermistor resistance changes, producing a corresponding change in the bridge output voltage that can be measured and calibrated in terms of temperature.
Temperature Compensation
Thermistors are widely used for temperature compensation in electronic circuits. Because their resistance changes significantly with temperature, they can be incorporated into a circuit to counteract temperature-related variations in other components. This helps maintain a more stable and consistent circuit performance over a range of operating temperatures.
A thermistor generally has a negative temperature coefficient (NTC), meaning its resistance decreases as temperature increases. In contrast, many conductors and semiconductor components exhibit a positive temperature coefficient (PTC), in which their resistance increases with temperature. This opposite temperature behavior makes an NTC thermistor useful for compensating temperature-induced changes in such components.
For applications operating up to approximately 125°C, a disc-type thermistor can be used as the compensating element. The thermistor is positioned close to the component whose temperature needs to be compensated, such as a copper meter coil, so that both experience nearly the same temperature variation.
Inrush Current Limiting
NTC thermistors are widely used to reduce the high inrush current that occurs when electrical equipment is switched on.
When the equipment is initially powered, the thermistor is relatively cold and has a higher resistance, which limits the initial current. As current flows through it, the thermistor heats up and its resistance decreases, allowing the equipment to operate normally.
This application is common in:
- Power supplies
- LED drivers
- Transformers
- Motors and electrical equipment
- Electronic appliances
Heating and Temperature Control
Thermistors can be used as feedback elements in heating systems to monitor temperature and regulate the heating process.
The control circuit continuously detects the thermistor’s resistance and adjusts the heating element accordingly. This can prevent excessive temperature rise and improve energy efficiency.
Thermistor-Based Temperature Control Circuit
The basic thermistor temperature-control circuit is illustrated below. It uses a 4 kΩ thermistor as part of an AC-excited bridge circuit. Any change in temperature alters the resistance of the thermistor, causing the bridge to become unbalanced and producing a corresponding output voltage.
The resulting unbalance voltage is fed to an amplifier, which increases the signal to a level sufficient to operate a relay. The relay then switches the heating circuit ON or OFF, thereby regulating the temperature. This feedback arrangement allows the thermistor to act as the sensing element while the relay controls the heating load.
Thermistor-based temperature controllers offer several useful characteristics. They provide high sensitivity and stable operation, while their relatively simple circuit arrangement makes them easy to implement. Another important advantage is their rapid response to temperature changes, allowing the control system to react quickly when the temperature moves away from the desired set point. In precision applications, such circuits can achieve very fine temperature control when properly designed and calibrated.
Liquid Level Detection
Thermistors can be used for liquid-level detection in specialized systems. The principle is based on the difference in heat transfer between the thermistor when it is surrounded by air and when it is immersed in a liquid.
The resulting temperature and resistance change can be detected by a control circuit to determine whether the thermistor is submerged.
Refrigeration and HVAC Systems
Thermistors are extensively used in refrigeration and HVAC systems to measure temperature at different locations.
They can monitor:
- Room temperature
- Evaporator temperature
- Condenser temperature
- Refrigerant-related temperatures
- Air inlet and outlet temperatures
The measured temperature is used by the control system to regulate compressors, fans, and other components.
Automotive Temperature Monitoring
Thermistors are commonly found in automotive electronic systems for monitoring temperatures. They can measure engine coolant, intake air, transmission components, and other temperatures.
The temperature information can be supplied to the vehicle’s electronic control system for functions such as engine control, cooling-fan operation, and protection against excessive temperature.
Battery Temperature Monitoring
Thermistors are important components in battery management systems (BMS). They monitor the temperature of battery cells and provide feedback to the charging and protection circuits.
Temperature monitoring helps prevent charging or discharging under unsafe thermal conditions and can contribute to improved battery life and reliability.
Gas Detection
Specialized thermistor-based sensing arrangements can be used for detecting changes associated with gases. In such systems, a change in the surrounding gas or a gas-related reaction can alter heat transfer and consequently change the thermistor temperature and resistance.
These arrangements can be used in certain gas-sensing and monitoring applications, although dedicated gas sensors are more common for many modern systems.
Industrial Process Control
Thermistors are used in industrial equipment where accurate temperature monitoring is required. They can provide temperature feedback for processes involving heating, cooling, manufacturing, and material processing.
Their small size and rapid response make them useful for monitoring temperatures at specific points within machinery and process equipment.
Consumer Electronics
Thermistors are widely used in electronic products to monitor and protect against excessive temperature. Devices such as computers, laptops, smartphones, chargers, and power adapters may use thermistors as part of their thermal-management systems.
The temperature signal can be used to control cooling fans, reduce operating power, or initiate protective shutdown when the temperature becomes excessive.
Weather Stations
Thermistors can be used in weather-monitoring equipment to measure ambient air temperature. Their resistance changes with changes in surrounding temperature, allowing an electronic circuit or data-acquisition system to calculate and record the atmospheric temperature.
They are suitable for compact weather stations because they are small, inexpensive, and capable of providing a relatively fast response to temperature changes.
Conclusion
A thermistor is a temperature-sensitive resistor whose resistance changes significantly with temperature. Its nonlinear resistance-temperature characteristic makes it particularly useful for temperature measurement, monitoring, compensation, and protection applications. Thermistors are mainly classified into NTC and PTC types. NTC thermistors decrease in resistance as temperature increases, while PTC thermistors increase in resistance with rising temperature.
Due to their small size, fast response, high sensitivity, and relatively low cost, thermistors are widely used in electronic circuits, HVAC and refrigeration systems, automotive equipment, battery management systems, industrial process control, and consumer electronics. Understanding the working principle, characteristics, construction, and applications of thermistors helps in selecting the appropriate type for a particular temperature-sensing or protection application.
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