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Solution Guide
TDK's NTCRP series (NTC thermistors) are used in a wide range of applications, including electric vehicle drive motors, batteries, and industrial equipment temperature detection, which require high reliability. This article explains thermal response, a very important characteristic of NTC thermistors, and introduces the method of measuring the response of the NTCRP series.

Product Overview of NTCRP Series

This product is a glass-encapsulated NTC thermistor sealed in a PPS case. The case at the tip of the product has been made as compact as the smallest in the industry by resin injection molding. Depending on the environment/shape of the mounting location, you can choose from two types: rectangular type and cylindrical type. The rectangular type is suitable for mounting on flat surfaces such as square wires for motors and busbars, while the cylindrical type is ideal for wrapping round copper wires into bundles. If you have any questions about the installation method, please feel free to contact us, and our experienced technical staff will propose the optimal mounting solution for you.

Figure 1: NTCRP Series Product Dimensions

What is Thermal Time Constant and Thermal Responsiveness of NTC Thermistors?

What is Thermal Time Constant?
The thermal time constant refers to the measure of thermal responsiveness. It represents how the temperature of a sensor initially at temperature T1 changes over time when placed in an environment with a different temperature, T2 (either in air or water). This temperature change over time is approximated by an exponential function.
T(t) = T2 + (T1 - T2) * e^(-t/τa)
At time t = τ,
T(τ) = T1 + (T2 – T1) * (1 – 1/e)
Thus, at time τ, the temperature change of the sensor is 63.2% of the temperature difference T1-T2. This time τ is defined as the thermal time constant.

What is Thermal Responsiveness?
Thermal responsiveness refers to the speed of response to actual temperature changes of a heat source.
NTC thermistors that quickly react to temperature changes of a heat source and exhibit a change in resistance output are considered to have high responsiveness. Generally, thermal responsiveness is influenced by the physical size and shape of the thermistor, the type of material, and the manufacturing process.

Figure 2 : Temperature change of the sensor when the ambient temperature rised from T1 to T2. (approximation of exponential function)

Thermal Time Constant Measurement Test Using Silicone Oil

Test Method

In silicone oil, a thermistor is moved from 25°C to 85°C. By observing the resistance-time characteristics of the thermistor when its temperature is changed from 25°C to 85°C, the time it takes for the thermistor's temperature to reach 63.2% (62.9°C) of the change is measured.

Figure 3 : Thermal Time Constant Test Method(In Silicon Oil)
Test Results (Comparison between our standard NTCRP series and similar products from other companies)

The data below shows the results of the thermal time constant measurement test using silicone oil. Smaller numbers indicate faster response time, and TDK products have shown faster results compared to products from other companies. The fast response time allows for instant and accurate temperature measurement.

Measurement products Similar products
from other companies
NTCRP series
Thermal time constant
measurement result
4.8 sec 4.2 sec

Test Method for Measuring Thermal Time Constant by One-Sided Heating

Test Method

In air, a thermistor is moved from 25°C to the temperature of a hot plate set at 85°C. The side of the silicone tube in contact with the hot plate is left open. By observing the resistance-time characteristics of the thermistor when its temperature is changed from 25°C to 85°C, the time it takes for the thermistor's temperature to reach 63.2% (62.9°C) of the change is measured.

Figure 4: Thermal Time Constant Test Method(On hot plate)
Test Results (Comparison between our standard NTCRP series and similar products from other companies)

The results of the one-sided heating measurement test showed a faster response compared to similar products from other companies.
The TDK NTCRP series uses proprietary materials with high thermal conductivity for the case resin and filler material, enhancing responsiveness.

Measurement products Similar products
from other companies
NTCRP series
Thermal time constant
measurement result
18.1 sec 12.2 sec

Summary

We have introduced the measurement methods/results of thermal time constant using silicon oil for the NTCRP series, as well as the measurement methods/results using one-sided heating. If you have any inquiries or questions regarding the design, development, or purchase requests for the NTCRP series, please feel free to contact us."