EPCOS has developed an advanced temperature sensor for Sub-Zero Freezer’s new line of premium built-in refrigeration appliances—the new custom M2025 NTC probe. Now Sub-Zero is able to fill 100 percent of its thermistor sensor needs with components from EPCOS.

Sub-Zero Freezer Co., a manufacturer of premium refrigeration appliances, and EPCOS started working together several years ago. Sub-Zero had been using the EPCOS M2020 sensor in an injection-molded casing with great success when it embarked on a new design for its PRO 48 built-in refrigerator line.

The new design called for a more reliable evaporator, which is used to measure and regulate the refrigerator’s temperature and defrost cycles. However, the integration of the new evaporator caused a design hiccup associated with its sensor opening size. Unlike the previous evaporator, which featured an 8-mm opening for the sensor, the new evaporator had an opening of 6.5 mm. This smaller opening was no longer compatible with the EPCOS M2020 probe that Sub-Zero was previously using.

PRODUCT PROFILE: REFRIGERATION SENSORS

The M2020 family of sensors for temperature measurement in refrigerators and deep freezers features an injection-molded design that provides an impermeable seal and unmatched reliability. They are available in diameters ranging from just 5.4 mm to 8 mm.   The new M2025 sensor (middle) was developed with a diameter of 6.5 mm and a rounded tip to facilitate assembly. Its beveled back end ensures a proper seal.   M2025 key data: Climatic category 40/80/56 (IEC 60068-1) Rated resistance at 0 °C (Rn) 32,654 Ω Tolerance ±2 % B-value (B25/100) 3980 K Thermal time constant in water (Tthl) approx. 25 s

Supplying a solution

Sub-Zero’s engineers had run into a roadblock. Their solution: adapt the sensor instead of the evaporator, as the associated costs of the probe component were considerably lower than that of the evaporator. Because the sensor needed to be placed inside the evaporator to help detect cold spots and fluctuation in temperature, Sub-Zero’s designers knew they needed to decrease the component’s diameter.

To do so, the OEM, based in Madison, Wisconsin, went straight to the source — EPCOS. Sub-Zero’s choice to continue working with EPCOS was based on the company’s reputation and especially the reliability of its products, according to Josh Becker, reliability manager. “The reliability of these sensors in the field supported the test data and the promise EPCOS made to assist us in finding a solution,” he explains.

A major supplier to several global appliance makers that include BSH Bosch und Siemens Hausgeräte, Electrolux, GE and Whirlpool, EPCOS knew it needed to maintain the reliability of the M2020, but in a smaller package. Moreover, the new sensor would feature the same temperature range, thermal time, dissipation constants and resistance values as the M2020.

A sensible design

Because the specifications were the same and Sub-Zero was pleased with the injection-molded design of the M2020 component, EPCOS started work on designing the M2025, a miniature version of the M2020. Once specifications and terms were agreed on, EPCOS initiated its design process that started with constructing a completely new injection-molding tool to build the newly designed M2025 sensor. The two-cavity tool features a pre-mold cavity to position and center the NTC element and cable and a final mold cavity to define the final shape of the M2025 sensor.

Unlike traditional potted thermistor designs, EPCOS’ probe offered unmatched reliability and impermeability. Standard potted designs feature a sensor that is attached to a cable, which is then inserted into a molded casing. This, however, often creates material separation that can break the seal between the sensor cable and casing, ultimately decreasing the sensor’s function.

The new design calls for an injection-molded casing, where the sensor is encapsulated to the cable and a double-jacketed wire (Fig. 1). “During the injection mold process, the cable material and the over-molded material fuse together to create a seal,” Mike Williams, marketing engineer, NTC thermistors for EPCOS, says.

Because the M2020 sensor design is injection molded, a new machine tool needed to be built to accommodate the smaller head diameter size. Additionally, a smaller NTC element was used and a smaller diameter cable was used to help achieve the specified 6.5-mm size. Both the pre-mold and final mold processes were modified to obtain the smaller size. To make installation easier, EPCOS put a rounded tip on the new M2025.

Unmatched reliability

With reliability as a top priority, EPCOS subjected the sensors to a brutal battery of eight tests that included temperature cycling, storage in water tests and rapid temperature sensing in air trials.

The probes underwent harsh testing in the temperature cycling test: the M2025 was stored at -30º C air temperature and transferred into 10 ºC conditions every 15 minutes for 50,000 cycles. The water storage tests subjected the component to 30 ºC water while a 10-DC voltage was switched on and off every 30 minutes for 4000 hours (Fig. 2). In the rapid temperature sensing test, the sensor endured 10 minutes in -40 ºC temperature air that then switched within 30 seconds to 80 ºC air for 1000 cycles.

	FIGURE 2: RELIABILITY TEST
Drift of rated resistance during water immersion at 30 °C for an extended period of 5000 hours.

In these and the other five tests conducted by EPCOS, the M2025 needed to perform at less than a 2-percent resistance drift and show no signs of damage. In all cases, the new smaller-sized component passed.

While EPCOS was performing its own quality tests, Sub-Zero was also conducting tests of its own. “We conducted accelerated life-cycle testing in a very harsh environment. The device was completely submerged in water and our testing apparatus cycled it from a completely frozen to thawed stated in a matter of minutes,” says Becker. Sub-Zero also integrated 1000 sensors in PRO 48 units for field testing. The results from both EPCOS and Sub-Zero showed zero test failures. The sensor is already in production in the PRO 48 and will be carried over to other lines in the next few years, making EPCOS the premier supplier of NTC thermistors for the OEM.

	STATEMENT
“The relationship has been a success. EPCOS takes a regimented approach to product development—a philosophy we share.” Josh Becker, Reliability Manager at Sub-Zero

In a refrigeration application, the NTC thermistor is an integral component to the appliance as it detects temperature variance and prevents overheating or freezing more than necessary. This is achieved by way of a wheatstone bridge and a four-band resistor with an NTC. The resistor measures voltage input on one side while the other measures voltage output. As the temperature rises, the NTC resistance decreases and the sensor bridge connected to an MCU controls the evaporator function, depending on the temperature reading.

Quality and customization

Just like the M2020 and M2025, EPCOS can customize its offerings with various shapes, sizes and sensing applications. The supplier’s popularity proves its product claims: EPCOS’ commitment to reliability doesn’t end with its initial rigorous testing. The company continues to support its products by testing a set number of parts with each batch and conducting outgoing inspection tests that examine total sensor length, strip length, nominal resistance value and data value.

First integrated in the PRO 48, the sensor will soon be used across all of the OEMs refrigeration product lines. Sub-Zero says it is able to provide a more reliable appliance to its customers with the use of the EPCOS M2025. “The relationship has been a success. EPCOS takes a regimented approach to product development—a philosophy we share,” Becker says. “They not only offer unique and innovative products, but also provide the test data to support their claim of superior quality and reliability.”

PROFILE: SUB-ZERO FREEZER COMPANY

Founded in 1943 in Madison, Wisconsin, Sub-Zero Freezer Company built the first freestanding freezer. Since then, Sub-Zero has become a recognized leading manufacturer of premium built-in home refrigerators. Over time, the company has refined its early concept and has brought to market a comprehensive line of built-in models, including undercounter models, refrigerators for wine storage, and the new all stainless steel PRO 48 line.