NTC stands for "negative temperature coefficient". NTC thermistors are resistors with a negative temperature coefficient, which means that the resistance decreases as the temperature increases. They are mainly used as resistance temperature sensors and current limiting devices. The temperature sensitivity coefficient is about five times that of silicon temperature sensors (silicon transistors) and about ten times that of resistance temperature detectors (RTDS). Typical use of NTC sensors ranges from 55 to +200 °C.
    The nonlinear relationship between resistance and temperature exhibited by NTC resistors poses a great challenge to measuring temperature using analog circuits. However, the rapid development of digital circuits has solved this problem, by interpolating lookup tables or solving equations that approximate a typical NTC curve, the value can be calculated.

NTC thermistor definition
    An NTC thermistor is a thermistor that shows a large and predictable decline in resistance as the temperature of the resistance increases over the operating temperature range.

Characteristics of NTC thermistors
    Unlike RTDS (resistance temperature detectors), which are made of metal, NTC thermistors are usually made of ceramics or polymers. Different materials used in the manufacture of NTC thermistors result in different temperature responses and other different performance characteristics.

Temperature response
    Most NTC thermistors are usually suitable in? They are used in a temperature range of 55 to 200 °C, at which point they are read. Some special NTC thermistor families can be used at temperatures near zero (-273.15 °C), while others are specifically designed for temperatures above 150 °C.
    The temperature sensitivity of an NTC sensor is expressed as "percent change per degree Celsius" or "percent change per Kelvin." Depending on the material used and the production process, typical values for temperature sensitivity range from -3% to -6%/°C.

NTC and RTD resistance-temperature curves Compare characteristic NTC curves
    As can be seen from the figure, compared with platinum alloy RTDS, NTC thermistors have a steeper resistance-temperature slope, which means better temperature sensitivity. Even so, RTDS are still sensors with an accuracy of ±0.5% of the measured temperature, and they are useful in the temperature range of -200 to 800 °C, a wider range than NTC temperature sensors.

Comparison with other temperature sensors
    Compared to RTDS, NTC thermistors are smaller, more responsive, more resistant to shock and vibration, and cost less. Their degrees are slightly lower than RTDS. The accuracy of NTC thermistors is similar to that of thermocouples. However, thermocouples can withstand extremely high temperatures (about 600°C) and are therefore used instead of NTC thermistors in these applications. Even so, NTC thermistors are more sensitive, stable, and degree than thermocouples at lower temperatures, and have fewer additional circuits when used, so the total cost is lower. The cost is also lower because there is no need for signal conditioning circuits (amplifiers, level converters, etc.), which are often needed when dealing with RTDS, and thermocouples are always needed.

Self-heating effect
    The self-heating effect is a phenomenon that occurs when a current flows through an NTC thermistor. Since a thermistor is essentially a resistor, it dissipates electrical energy in the form of heat when a current flows through it. This heat is generated in the thermistor core and will affect the accuracy of the measurement. The extent to which this happens depends on the current flow, the environment (whether it is a liquid or a gas, whether there is any liquid flowing through the NTC sensor, etc.), the temperature coefficient of the thermistor, the total area of the thermistor, etc. The fact that the resistance of an NTC sensor, as well as the current flowing through it, depends on the environment is often used for liquid presence detectors, such as those in a storage tank.

Thermal capacity
    Heat capacity represents the amount of heat required to raise the temperature of a thermistor by 1°C, usually expressed in mJ/°C. When using an NTC thermistor sensor as a surge current limiting device, it is important to know the heat capacity because it determines the response speed of the NTC temperature sensor.

Curve selection and calculation
    The selection process of thermistor must consider the dissipation constant, thermal time constant, resistance value, resistance-temperature curve and tolerance and other important factors.
    Because the relationship between resistance and temperature (the RT curve) is highly nonlinear, some approximations must be utilized in practical system design.
    First-order approximation
    An approximation, also a simple approximation, is a first-order approximation, which means:
    Δ R = k? Δ T
    Where k is the negative temperature coefficient, ΔT is the temperature difference, and ΔR is the resistance change caused by the temperature change. This first-order approximation is valid only over a very narrow temperature range and can only be used for temperatures where k is almost constant over the entire temperature range.
    Beta formula
    The other equation gives satisfactory results in the range from 0 to +100°C to ±1 °C. It depends on a single material constant β that can be measured. The equation can be written as:
    R(T)=R(T0)? E beta (1 t? 1T0)
    Where R(T) is the resistance (in Kelvin) at temperature T, and R(T 0) is the reference point at temperature T. The Beta formula requires two-point calibration and is usually accurate to no more than ±5 °C over the entire useful range of NTC thermistors.
    Steinhart-Hart equation
    The closest approximation known to date is the Steinhart-Hart formula, published in 1968:
     1T=A+B? ln(R)+C? (ln(R))3 
    Where ln R is the natural logarithm (in Kelvin) of the resistance at temperature T, and A, B, and C are coefficients derived from experimental measurements. These coefficients are usually published by the thermistor vendor as part of the data sheet. The Steinhart-Hart formula is typically around ±0.15 °C in the range -50 to +150 °C, which is sufficient for most applications. If greater accuracy is required, the temperature range must be reduced, and accuracy better than ±0.01 °C can be achieved in the range of 0 to +100 °C.

Choose the correct approximation
    The formula chosen to derive the temperature from the resistance measurement needs to be based on the available computational power as well as the actual tolerance requirements. In some applications, first-order approximation is sufficient, while in others, even the Steinhart-Hart equation does not meet the requirements, and the thermistor must be calibrated point by point, numerous measurements made, and lookup tables created.
    Structure and characteristics of NTC thermistor
    Materials commonly involved in the manufacture of NTC resistors include oxides of platinum, nickel, cobalt, iron and silicon, used as pure elements or as ceramics and polymers. NTC thermistors can be divided into three categories, depending on the production process used.

Bead thermistor
    These NTC thermistors are made from platinum alloy leads sintered directly into a ceramic body. They typically have fast response times, better stability, and allow operation at higher temperatures than disc and chip NTC sensors, but they are more brittle. They are usually sealed in glass to protect them from mechanical damage during assembly and improve their measurement stability. Typical sizes range from 0.075 to 5 mm in diameter.

Disk and chip thermistors
    Disk thermistors These NTC thermistors have metallized surface contacts. They have greater resistance than beaded NTCS and therefore a slower reaction time. However, due to their size, their dissipation constant (the power required to raise the temperature by 1°C) is higher. Because the power dissipated by thermistors is proportional to the square of the current, they can handle higher currents better than bead thermistors. Disc thermistors are made by pressing an oxide powder mixture into a circular mold and then sintering it at high temperatures. Chips are typically manufactured through a casting process, in which a material slurry is spread out into a thick film, dried and cut into shape. Typical sizes range from 0.25 to 25 mm in diameter.

Glass encased NTC thermistor
    These are secret? NTC temperature sensor enclosed in an airtight glass bubble. They are designed for temperatures above 150 °C or for printed circuit board installations that must be rugged. Encapsulating the thermistor in glass improves the stability of the sensor and protects the sensor from environmental impacts. They are made by sealing beaded NTC resistors in glass containers. Typical sizes range from 0.4 to 10 mm in diameter.

Typical application
    NTC thermistors are widely used. They are used to measure temperature, control temperature and compensate temperature. They can also be used to detect the presence or absence of liquids, as current limiting devices in power supply circuits, for temperature monitoring in automotive applications, and many other applications. NTC sensors can be divided into three categories, depending on the electrical characteristics utilized in the application.

Resistance-temperature characteristics

    Applications based on resistance-temperature characteristics include temperature measurement, control, and compensation. These also include the use of NTC thermistors, so the temperature of the NTC temperature sensor is related to other physical phenomena. This type of application requires the thermistor to operate under zero power conditions, which means that the current passing through it is as low as possible to avoid heating the probe.

Current-time characteristics
    Applications based on current time characteristics include: time delay, surge current limitation, surge suppression, etc. These characteristics are related to the heat capacity and dissipation constant of the NTC thermistor used. Circuits typically rely on NTC thermistors to heat up as a current passes through them. At some point, it triggers some kind of change in the circuit, depending on its application.

Voltage-current characteristics
    Applications based on thermistor volt-current characteristics often involve environmental conditions or circuit changes that cause a change in the operating point on a given curve in the circuit. Depending on the application, this can be used for current limiting, temperature compensation or temperature measurement.

NTC thermistor symbol
    According to IEC standards, the following symbols are used to represent negative temperature coefficient thermistors.

NTC Thermistor (IEC standard)