Overview
NTC and thermistors
NTC (negative temperature coefficient) refers to thermistors whose resistance decreases as temperature rises. NTC thermistors are temperature-sensitive resistive devices made from semiconductor ceramics.
Materials and Manufacturing
These materials are produced by thoroughly mixing, forming, and sintering two or more metal oxides, such as manganese, copper, silicon, cobalt, iron, nickel, and zinc, to create semiconductor ceramics that exhibit a negative temperature coefficient. The resistivity and temperature characteristics depend on material composition ratios, sintering temperature, and structural state. Non-oxide NTC materials such as silicon carbide, tin oxynitride, and silicon nitride have also emerged.
History
In 1834, scientists first observed that silver exhibits a negative temperature coefficient. In the 1930s, copper(I) oxide and copper(II) oxide were found to have negative temperature coefficient behavior and were used for temperature compensation in aeronautical instruments. With the development of transistor technology, research on thermistors progressed significantly. By the 1960s, NTC thermistors were established for practical use. NTC thermistors have been widely used for temperature measurement, temperature control, and temperature compensation.
Applications
Thermistors are also used in instrument circuit temperature compensation and thermocouple cold-junction compensation. The self-heating property of NTC thermistors enables automatic gain control and can be used in RC oscillator amplitude stabilization circuits. When the self-heating temperature far exceeds ambient, the resistance also depends on ambient air conditions, so this characteristic is used to make sensing elements for flow meters, gas analyzers, and thermal conductivity detectors.
Power NTC thermistors are commonly used for inrush current suppression. These are high-power disk-shaped thermistors used in circuits with capacitors, heaters, and motor start loads. When the supply is applied, large transient surge currents can occur; the high initial resistance of the NTC suppresses excessive current and protects the power circuitry and loads. As the device heats during normal operation, its resistance drops to a low value and does not affect normal circuit operation.
B-Value Characteristics
The B-value (B-value) is one of the defining parameters of an NTC thermistor. In simple terms, the B-value is the material coefficient that indicates how sharply resistance changes with temperature. Different B-values over the same temperature interval indicate different material properties. The B-value reflects the curvature of the resistance-versus-temperature curve and serves as a sensitivity index for that temperature interval.
Qualitatively, a larger B-value corresponds to a steeper curve, meaning greater change in resistance per degree and higher sensitivity. A smaller B-value corresponds to a flatter curve and lower temperature sensitivity. Note that the B-value is a parameter calculated to approximate the resistance-temperature characteristic over a specific temperature interval for a given thermistor.
Because the B-value reflects the resistance change between two temperature points, it can be calculated by the standard relation shown in the formula images included below. Unless otherwise specified, the B-value is typically measured over the [25, 50] °C range using zero-power resistance values at T1 = 25 °C (298.15 K) and T2 = 50 °C (323.15 K).
Unless otherwise noted, B-values are usually specified for the [25, 50] °C range, calculated from zero-power resistances at the temperatures given above.
e = 2.718281828459045
Temperature Coefficient
The temperature coefficient of an NTC thermistor is defined as the relative change in resistance with respect to temperature change. The simplest conversion formula to compute the temperature coefficient alpha from the B-value is shown in the image below.
The final result includes a negative sign because the device exhibits a negative temperature coefficient. Without the negative sign, the result would represent a positive temperature coefficient.
Resistance Characteristic Tables
Manufacturers provide B-values when supplying thermistor electrical characteristics, but resistance-characteristic tables are still necessary. The B-value allows calculation of resistance for many temperatures, but it only represents the material behavior over a specified temperature interval. The full nonlinearity of resistance versus temperature may not be accurately captured across the entire temperature range by a single B-value. Therefore, even if thermistors from different vendors have identical B-values, their resistance-versus-temperature curves can still differ. For accurate design, manufacturers should provide resistance tables across the intended measurement temperature range.
Example resistance-versus-temperature characteristics demonstrate that three thermistors with identical B-values can still have different resistance values across temperatures. This shows that the B-value is only an interval-specific parameter and does not fully define the thermistor across all temperatures. Thus, in practical applications, providing a resistance table over the working temperature range is the correct approach.
B-Value Range and Customization
B-values typically range from about 1800 K to 5800 K. Common measurement intervals are [25, 50] °C, [25, 85] °C, [0, 100] °C, or [0, 50] °C. Some manufacturers provide custom B-values for specific NTC thermistor requirements.
Example Calculation
Consider a customer order for 10,000 NTC thermistors with a temperature accuracy of ±0.5 °C. The part number MF52A-103 has a B-value of approximately 3950 K based on R25 at 25 °C (298.15 K).
Using the standard relation between B-value and temperature coefficient alpha, the temperature coefficient at 25 °C can be calculated. The example images below show the calculation steps and results.
The calculated temperature coefficient alpha at 25 °C is approximately -4.44%/°C. Using alpha and the device accuracy, the temperature tolerance at 25 °C can then be derived as shown in the image below.
Electrical Characteristics
Example specifics: R25 = 50 kΩ ±2%. That is, the resistance at 25 °C is 50 kΩ with a tolerance of ±2%. B25/50 = 3950 K ±2%, meaning the average B-value between 25 °C and 50 °C is 3950 K with a tolerance of ±2%.
