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High reliability NTC sensor replaces bi-metal devices with much higher reliability
Tuesday, June 13, 2017

By Ros Kruger, Director, Supplier Marketing Europe – Emech, Sensors and Power, TTI, Inc

 

SEN_High-temp_650_3500x2623_300dpiIn the past, measurement of temperatures in excess of 500°C had primarily been performed using platinum temperature sensor elements. Now, a new NTC high-performance ceramic element is available from TDK enabling operation up to maximum temperatures of 650°C. Applications include the automotive sector, industry and domestic appliances such as exhaust gas recirculation, 3D printers and ovens. The new NTC element is characterized by high resolution and an extremely robust construction. With a sensitivity a of –0.72%/K at 600°C, it provides a significantly better resolution and therefore more accurate temperature measurement – in comparison platinum temperature sensors only have a sensitivity of 0.10%/K. During long term testing at an average application temperature of 300 °C, the aging behaviour is excellent; typical resistance drift is in the range of ±0.2K. The qualification has been based on automotive standard AEC-Q200. Thanks to NTC technology, the use of precious metals like platinum or gold can be avoided completely.

The new NTC element is based on a glass-encapsulated, high-performance perovskite ceramic with inflexible terminals made of aluminium oxide and a conducting path made of silver. The nominal temperature is 200°C, and the tightest tolerance about ±1K at 200°C. As a design option, the NTC element can be delivered with additional lead wire for welding connections or on a lead frame as a variant that includes a connector for easier mounting, e.g. for the injection molding process. Having been based on AEC-Q200 and adapted temperatures of up to 650°C, the new NTC element is suitable for applications like exhaust gas recirculation. These systems are used in diesel engines in order to reduce the formation of toxic nitrogen oxide in the combustion process. In 3D printers the new NTC element can monitor the correct temperature in the printer chamber and protect critical areas of the equipment and material against overheating. Last but not least, high-temperature sensors are required in household appliances like ovens for catalytic cleaning.

TDK’s perovskite NTC ceramic element is a completely new design. The NTC thermistor chip is connected to an alumina rod, metalized on two opposite sides. Glass encapsulation protects the NTC ceramic chip including the interconnection to the alumina rod electrodes. The electrodes and interconnections are silver-based. Furthermore, all materials of the sensor element are lead- and halogen-free and comply with all RoHS requirements with no exemptions.

The NTC element has a total length of 16 mm and a maximum diameter of 1.8 mm and is designed for measurements in an operating temperature range from –40°C to +600°C. The maximum temperature can be extended to 650°C for 1000 hours. The rated resistance is defined at 200°C with minimum tolerance. The specified resistance value at the rated temperature is 5000Ω. This specification allows a temperature measurement accuracy of about ±1 K at the rated temperature of 200°C. Besides providing high stability up to 650°C, the R/T curve allows higher sensitivity at elevated temperatures. Compared to platinum temperature sensors that have a sensitivity of +0.35%/K, the NTC element reaches a sensitivity of –4.1%/K at 25°C, one order of magnitude more.

The qualification has been in accordance with automotive standard AEC-Q200. The maximum temperature for testing was extended to 650°C for the specified maximum operating temperature. For long term testing, e.g. storage at 300°C (10 000 hours), as an average application temperature, a typical resistance drift is in the range of about 0.2K. This very low variation in resistance is result of a reliable ceramic combined with a robust new design.

TDK offers a wide range of NTC elements through its distributor TTI Europe.  The standard families have been qualified according to the automotive standard AEC Q200. The NTC families include chip devices NTC S860 with resistance values at 25° C of between 5kΩ up to 100kΩ and a chip geometry from 0.45 up to 1.70mm. Leaded NTC devices incorporate a glass encapsulated element for temperatures between -55°C up to 300°C and resistance values at 25°C from 2kΩ up to 230kΩ. Response time is very short. Glass encapsulated NTC elements can be optionally insulated.
For temperatures up to 155°C, epoxy-coated Minisensors or leaded NTC devices are available. Minisensors feature bendable wires (stranded wire on request), resistance values at 25°C of between 2kΩ and 100k Ω, short response times and have versions with insulated lead wires.
Leaded NTC sensors are available in a wide range of resistance values from 1kΩ up to 470kΩ, and are suited for taping for automated mounting and provide cost effective solutions for many industrial and automotive applications.

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