How to Select an NTC Thermistor

NTC Thermistors are small, rugged ceramic semiconductors manufactured from compositions of the oxides of metals. Oxides of manganese, nickel, cobalt, copper, and/or iron are common. Each NTC thermistor composition or mix, has a specific ratio of metal oxides which defines the thermistor’s physical dimensions, R/T curve, and its resistance at 25 °C.

NTC thermistors exhibit a relatively large change in resistance vs. temperature, typically on the order of -3 % to -6 % per °C, providing a much greater sensitivity or signal response to changes in temperature when compared to other temperature sensors, such as thermocouples and RTD’s.

With the variety of physical configurations, R25 values and temperature/resistance tolerances, NTC thermistors are the most versatile temperature sensor available. North Star Sensors manufactures its NTC thermistors from the highest grade raw materials and controls every step of the manufacturing process.

Leads – Depending on the thermistor configuration, lead styles vary. Typical lead gage varies from 38 AWG to 26 AWG. Leads may be insulated or uninsulated. Common lead materials are copper, copper alloy, and nickel.

Coating/Encapsulation – Epoxy coatings are typical. Some thermistor styles are encapsulated in a polyimide tube or plastic cup.

R25 Values – Typical R25 values range from 50 Ω to 100 kΩ. An R25 of 10 kΩ is one of the most common configurations.

Point-Matched Tolerance – A point-matched tolerance is a ± % resistance tolerance at specific temperature point. The most common reference temperature point is at 25 °C.

Temperature Tolerance – A temperature tolerance is a ± degree C tolerance specified over a temperature range. Typical temperature tolerances range from ± 0.5 °C to ± 0.1 °C

Series – The first two characters indicate the basic thermistor style which would specify parameters such as lead type and over all length.

Coating/Encapsulation – The third character indicates the type of coating/encapsulation.

R25 – The fourth and fifth character represent the first significant numbers of the resistance at 25 °C while the sixth number represents the number of trailing zeros. The seventh character “R” represents “resistance.”

R/T Curve – The eighth and ninth character represent the R/T Curve and the NTC characteristic at 25 °C. For example, R/T Curve 44 represents an R/T Curve material with an NTC, or a negative temperature coefficient, of -4.4 %/°C at 25 °C.

Tolerance – For temperature tolerances, the tenth character, “T”, represents a temperature tolerance while the eleventh and twelfth characters represent a temperature tolerance and a temperature range, respectively. For point-matched tolerances, the tenth character, “P”, represents a point-matched tolerance at 25 °C while the twelfth character represents a ± % resistance tolerance at the given temperature point.

Configuration – The first character represents the style of the contacts

Electrode – The first character represents the conductive material in the electrical contact.

Size Code – The size code represents the length, width, and height of the thermistor chip including the height of the electrode material. Height is considered the dimension from one electrode to the other if one electrode is facing down and the other is pointed upward.

R25 – The first two characters signify the starting numbers of the resistance at 25 °C while the next number represents the number of trailing zeros. The final character “R” represents “resistance.”

R/T Curve – This represents the R/T Curve and the NTC characteristic at 25 °C. For example, R/T Curve 44 represents an R/T Curve material with an NTC, or a negative temperature coefficient, of -4.4 %/°C at 25 °C.

Tolerance – For point-matched tolerances, the starting character, “P”, represents a point-matched tolerance at 25 °C while the next character represents a ± % resistance tolerance at the given temperature point.

Packaging – This represents the packaging of the product