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issue: May 2007 APPLIANCE Magazine

Sensors and MCUs
Sensing the Perfect Fit

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by Jill Russell, Contributing Editor

Sub-Zero’s Pro 48 (top) built-in refrigerator features nearly 30-cubic-feet of storage space and integrated LED lighting throughout the interior. Originally released in 2005, the updated Pro 48 will feature the new M2025 sensor (bottom) from Epcos, Inc. The thermistor was modified for a smaller diameter, bringing it from 8 mm to 6.5 mm, to fit the new Pro 48 evaporator.

The need for a more reliable sensor prompted Sub-Zero/Wolf to commission a smaller, custom thermistor for its refrigerators.

Major appliance maker Sub-Zero Freezer Co./Wolf Appliance Co. (Madison, Wisconsin, U.S.) had been using a sensor supplied by Epcos, Inc. in refrigerators for several years. Sub-Zero says the M2020 thermistor from Epcos worked just fine in its refrigerator line. But when the OEM redesigned its built-in refrigerator line, it saw the opportunity to increase the reliability of the sensing component and decided to seize it.

The sensor project was spurred when the company integrated a new, more reliable evaporator into its Pro 48 built-in refrigerator line. In the previous design, the M2020 thermistor fit into an 8 mm opening in the evaporator, where it measured temperatures to regulate the unit’s defrost cycles.

Although the new evaporator was chosen because it was more reliable, it did pose one problem—the thermistor opening was smaller and the component size now needed to be 6.5 mm to fit in the evaporator.

Engineers decided to see what could be done with the thermistor, instead of the evaporator, to make the application work. According to Brian McCain, senior design engineer for Sub-Zero, it was important that the thermistor be kept inside the evaporator to sense certain cold spots and prevent the unit from freezing up.

Sub-Zero went to the Iselin, New Jersey, U.S.-based Epcos from the start— the M2020 had proven to be an effective component previously and the supplier had quite a good reputation, according to Josh Becker, manager of reliability for Sub-Zero. Becker was the one responsible for specifying both the M2020 and newly designed thermistor.

“The main driver for doing business with [Epcos] was that we knew they could offer something more reliable as opposed to lower cost, etc.,” Becker explains. “Our main concern was reliability. The standard device was reliable and we didn’t want to lose that.”

The two companies met to discuss what options would achieve the smaller thermistor head diameter. Here, they decided on component specifications and chose to keep the same resistance value of the previous part on the soon-to-be designed thermistor. Epcos soon came back to Sub-Zero with the custom-designed thermistor, the M2025, configured with a smaller head diameter while including all the reliability features of the old component.

The previously used M2020 was an injection-molded design, meaning the sensor is literally encapsulated to the cable and wire insulation, preventing any moisture from permeating the device. The new sensor was based on the same construction technique and also featured a double-insulated (double jacketed) wire, adding to the sensor’s reliability.

Epcos says that even though they provided Sub-Zero with an injection-molded thermistor with the M2020, it needed to change its production process to a proprietary, two-step process in order to achieve the smaller diameter size of the sensor. Additionally, Epcos utilized a smaller diameter cable for the wiring.

“The materials bind together after the molding and fuses, so that no moisture can enter the sensor,” Mike Williams, marketing engineer, NTC thermistors for Epcos, explains. “This results in zero field failures due to moisture penetration.”

And the companies have the testing results to prove it. “They could provide what other [suppliers] couldn’t, and that was data from the field and a test room environment,” Becker tells APPLIANCE. “We also conducted rigorous testing in our own laboratory to prove the reliability of the device.”

As of press time, Sub-Zero was still waiting for prototypes, but Epcos provided preliminary prototypes so that Sub-Zero could conduct its own testing and practice the assembly process. Epcos conducted its own testing as well, and both those tests and Sub-Zero’s preliminary results showed zero field failures.

The partnership, according to both Sub-Zero and Epcos, was so successful that Epcos will now be replacing another sensor that was previously provided by a competing sensor supplier. “This sensor provides 100 percent of the company’s sensor requirements” Williams says. “All sensors [in Sub-Zero refrigerators] are now from Epcos.”

The M2025 is scheduled to be in production in Sub-Zero’s built-in refrigerator line, including the Pro 48, as soon as final testing is completed. Sub-Zero says its goal is to introduce the sensor across the entire platform.

“The Pro 48 is a high quality, highly reliable product and at the heart of the refrigerator is this sensor that helps the appliance determine what the temperature is and control cooling,” Becker says. “This, combined with all the other components, allows us to provide a superior food preservation appliance.”


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