Appliance Design - July 2008 - (Page 18)

HEatING ElEmENtS an exploded view of where the heater is placed in an LCD. Photo: Minco. an lCD heater outfitted with a wire resistive heater from Minco. the operation of the device. Typically, LCD’s are rated to 32 DegF, although some higherpriced, extended range LCDs are rated down to below 0 DegF. With a standard LCD, if the temperature drops just a couple degrees below freezing, say to 30 DegF, the liquid crystals can become sluggish and even freeze, says Brian Williams, Minco’s Global Heaters Product Marketing Manager. By adding 1 W to 2 W per sq. in., the heaters can keep LCDs operating in temperatures as low as –67 DegF. Of course, not all devices that are used outside require an LCD heater. Cell phones, for example, are often used outside in cold weather, but rarely use an LCD heater because of space and cost issues. Instead, those applications typically use waste heat generated by the chips on the PCBs to keep the crystals warm and eliminate condensation. And, LCDs heaters are not just for use in outside applications. There are indoor applications, including LCDs in use on industrial freezers and semiconductor applications in which the manufacturing process itself gets really cold, Williams says. Another application that he has seen is a ruggedized computer that went into a petrochemical facility. In this case, the computers needed to be washed down every night and the heater helped dry the display. In all of these applications, suppliers of electrically heated glass take great pains to make sure that the units are safe. This ranges from making sure that a towel warmer won’t get hot enough to burn a human hand, to insulating the coatings or the resistive wires to negate any electrical shock hazard. When thinking of utilizing heated glass in an application, it’s important to first understand how it is made. Glass, of course, is an insulator, so it must be modified with conductive material to make it a resistive heater. There are a number of different ways to accomplish this in terms of materials, configuration, and construction. Each of which has their own set of advantages and disadvantages. There are two common methods for achieving this: The application of fine, resistive wire to the glass. The use of metal oxides, infused into the glass, or applied to the surface as a coating or film. A simple version of a resistive wire can be found on the rear window defroster in automobiles. In fact, electrically heated glass was first developed in World War II to prevent aircraft windshields from frosting over and obscuring visibility. Minco takes this wire approach, but unlike the unsightly wire pattern on the back window of a car, the company uses very thin diameter wires that are far less conspicuous. The typical wires is about 0.001 in., but they can be as small as 0.0008 in. The wires are placed in a grid-like pattern and can match the shape of the glass. Typically, Minco tries to create a product that has 50 percent space with wires and 50 percent space with open area. The spacing of the wires can also be varied to target specific areas of the glass such as around the edges where mounting brackets are located. These brackets act like a heat sink, says Williams. By varying the density of the wire pattern across the plane of the glass, extra heat can be funneled to the edges to make up for this heat loss, a process that Williams calls profiling. Minco’s heaters can get as hot as 248 DegF. Williams says that the grid pattern can also be designed to consider the size of the LCD, where the information will be displayed and if there might be any cold spots. For instance, in addition to cold spots from mounting brackets, some electronics might use a cooling fan, which may cool one section of the LCD but not another section. The heater wires are encapsulated between two pieces of transparent material in a laminate “sandwich” construction, which protects them from damage or abrasion, permitting a rugged application. The time it takes to reach specified temperatures can be as short as 5 seconds in some applications. Williams says one of the benefits of a wire element system as compared to using metal oxide coated glass, is that the wire systems have better resistance tolerances. The heaters have resistance tolerances that start at +/- 10 percent, and can be built down to +/- 2 percent. By contrast, Williams says that heaters that use metal oxide to make the glass conductive might have resistance tolerances of up to 20 to 25 percent, depending on the uniformity of the coating on the glass. This is especially important in a battery-powered operation, he says. With a consistent tolerance, the effect of the LCD heater on the life of the battery can be estimated with some degree of accuracy. 18 applianceDESIGN July 2008 www.applianceDESIGN.com http://www.appliancedesign.com

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Appliance Design - July 2008

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