Schottky diode key to tight temperature control

Mansfield, OH —As appliance and HVAC engineers push the design envelope, they are striving for ways to achieve more affordable and accurate temperature control. On one hand, economical, traditional bimetal and bulb-and-capillary sensors cannot always provide the degree of accuracy required in products such as under-the-sink hot water heaters. Electronic control modules (ECM) coupled with thermistors, on the other hand, provide greater control, but the cost is often prohibitive.

"Up until now, precise temperature control has been available, but at a cost. And products such as appliances are very cost-sensitive items," says Martin Leslie, manager of new product development at Therm-O-Disc.

Taking advantage of a normally negative feature of the Schottky diode—the fact that its characteristics change with temperature—engineers at Therm-O-Disc have developed the patented Silicon Temperature Control (STC). An alternative to existing temperature control devices, it achieves a workable tradeoff between accuracy and cost.
 

Due to the slower thermal response of a bimetal sensor, the variation around set point will always be greater than that of an electronic sensor. A small variation is desirable, particularly in appliances such as cofee brewers, that require temperature control.

Two important characteristics in temperature sensors are set point and differential. Set point is the desired temperature setting; differential is the variation in set point at which the sensor is able to detect a temperature change. In both cases, bimetal and bulb/capillary typically have tolerances around±5C, whereas the tolerances on the STC are±1C over an operating range of 10–150C. What this performance difference means in an HVAC application, for example, is that the bimetal device will allow higher room temperature fluctuations, potentially causing inhabitants of the room discomfort or annoyance.

Bimetal works through the bonding together of two leaves made of dissimilar metals to form a composite leaf. As the leaf is heated and cooled it bends one way or the other—due to the difference in the metals' thermal expansion rates—activating a set of electrical contacts.

In contrast, the STC uses a Schottky diode as the temperature sensor. As the temperature fluctuates, the diode exhibits a sharp voltage change that can be as high as 400 mVdc/°C across a 10-150C operating range. With such a high mV dc/°C ratio, circuits can easily be incorporated to detect temperature changes that are as small as 1C.

"Circuit designers have long been aware of the fact that diode characteristics change with temperature. However, these designers don't tend to work in the temperature control industry," says Leslie. "Conversely, temperature control designers don't normally use diodes. Once we [Therm-O-Disc] realized that a diode could be used to sense temperature change the design challenge was to scour the diode world for an off-the-shelf diode that was best for temperature sensing."

Additional Details

Contact Martin Leslie, Product Manager—New Product Development, Therm-O-Disc Inc., 1320 South Main St., Mansfield, OH 44907; 419-525-8500; martin.leslie@tod.com; www.thermodisc.com.

Typical Applications

Hot water heating applications

Brewing appliances

Home and commercial heating/air conditioning units

Automotive cabin heating/air conditioning control