What is an NTC thermistor?

　　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 approximately five times that of silicon temperature sensors (silicon oxide) and ten times that of resistance temperature detectors (RTDs). NTC sensors are typically used in the range of -55°C to 200°C.

　　While most NTC thermistors are generally suitable for use in the temperature range of -55°C to 200°C and they provide the most accurate readings, there are special NTC thermistors available for temperatures close to absolute zero (- 273.15) °C) and products specially designed for use above 150 °C.

　　The temperature sensitivity of an NTC sensor is expressed as "percent change per degree Celsius". Typical values for temperature sensitivity range from -3% to -6% per °C, depending on the specifics of the material used and the production process.

　　Compared to platinum alloy RTDs, NTC thermistors have steeper resistance-temperature slopes, which translates into better temperature sensitivity. Even so, RTDs are still the most accurate sensors, with an accuracy of ±0.5% of the measured temperature, and they are used over a temperature range between -200°C and 800°C, a much wider range than NTC temperature sensors.

　　Comparison with other temperature sensors

　　Compared to RTDs, NTCs offer smaller size, faster response, greater shock and vibration resistance, and lower cost. They are slightly less accurate than RTDs.

　　Compared to thermocouples, the accuracy of both is similar; however, thermocouples can withstand very high temperatures (around 600°C) and are used in this application, where they are sometimes referred to as pyrometers. But NTC thermistors offer higher sensitivity, stability, and accuracy than thermocouples at lower temperatures, and use less additional circuitry, resulting in a lower overall cost. Costs are further reduced because NTCs do not require the signal conditioning circuits (amplifiers, level shifters, etc.) that are often required when dealing with RTDs.

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