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Selecting the Right Display for White Goods Applications
The days when all washers and dryers were finished in white enamel and had a couple of large electromechanical dials or controllers, are fading into the past. Today’s consumers are looking for increased variety, sophistication and added features in the major appliances they buy for their homes. As a result, there is a greater need, as well as opportunity for the incorporation of displays that enhance the ‘human-machine interface,’ allowing for more user control, more feedback of information and many more features.
History and Trends in White Goods Applications
Historically, the term ‘white goods’ has been applied to any large household appliance finished in white enamel. This includes washers, dryers, stoves, refrigerators, freezers, ovens, dishwashers and so on. Today, these products are available in virtually any color and virtually any material, such as brushed stainless steel and coated metal finishes.
In addition, the umbrella of ‘white goods’ has expanded to cover even more household appliances, including microwave ovens, food processors, bread makers, sandwich makers, and so forth.
While the number and types of appliances have grown over the years, there have been a number of general marketplace trends underlying this growth. Many consumers are interested in appliances which are more energy efficient, and use less water. The leading edge of this movement may be in Europe and Asia, but today, many American consumers are looking for appliances with higher energy efficiency as well.
Another trend in the white goods marketplace is the development of more high-end models. Affluent consumers are looking for the highest performance, the most features, and an up-scale appearance. White enamel simply won't 'cut it' in the kitchens and laundry rooms of today's homeowners.
To facilitate the incorporation of these features, consumers need a more sophisticated human-machine interface. The classic rotary electro-mechanical dial does not provide enough information, and is not a practical way for users to choose among the numerous set-up options on today's high-end appliances. A more comprehensive interface is needed, and a variety of display technologies are available for providing such an interface.
Facilitating the Transition to Electronic Displays in White Goods
Fortunately, technological trends in the white goods industry have made it easier to incorporate modern displays into these products. In the past, a washer or dryer was strictly an electro-mechanical device. Operated directly from the 110/220 VAC line, these older units used mechanical switches and solenoids to control their operation, with neon lamps for lighted indicators. All control functions were provided via these electromechanical components.
Newer generations of white goods, however, use solid-state controls. These controls, which rely on semiconductor devices, require a source of regulated, low-voltage DC power. Such a low-voltage power source is ideal (and necessary) for most display technologies. Because this power source has already been designed into a number of white goods appliances, these products are easily adaptable to incorporate display technology, without the significant incremental expense of adding a low-voltage DC power supply. In these cases, that power supply is already there.
Types of Displays Available
If we exclude simple 'on-off' indicators from our discussion, the types of displays suitable for use in white goods include:
Light-Emitting Diodes ('LEDs')
LEDs are semiconductor PN junctions (diodes) which emit light when current passes through them. Types of displays using LEDs include: Monochrome numeric displays: These are the '7-segment' displays traditionally seen on test equipment. They provide displays of the digits of 0 - 9 and often include supplemental information such as a decimal point, a '+' or '-' sign, and through the use of overlays, can also display information such as °F or °C. Monochrome dot matrix displays: These displays form characters through the use of a matrix of 5 x 7 dots or 5 x 8 dots; one for each character. RGB dot matrix displays. Like the monochrome versions, these, too, use a matrix of 5 x 7 dots or 5 x 8 dots. However, through the use of individually driven red, blue, and green chips within each pixel, these displays can create virtually any color.
This dot matrix LED display is excellent for
numerals and simple characters, where a bright,
rugged display is needed.
This 7-segment LED display includes both a
floating decimal point plus extra dots to
form a 'colon.' Displays like these can
indicate time and temperature.
Liquid Crystal Displays ('LCDs')
LCDs are a 'sandwich' of polarized glass with a liquid crystal solution in the middle, whose transparency can be controlled by an electric field applied to it. This transparency affects the amount of light which is transmitted or reflected (depending on display type), thereby controlling what is seen by the user. The individual elements of an LCD can be made in virtually any shape. Types of LCD systems include: Monochrome Numeric Displays: These are usually 7-segment type common-plane-based LCDs which are used widely from digital wrist- watches to microwave oven timers. The LCD image appears blue or black, on top of a grayish-white background. Four-digit numeric 7-segment devices are the most commonly used LCD display. Custom options are easily available. Passive Matrix Displays (PMLCDs): These displays are available in 5 x 8 dot matrix character or graphic types. The PMLCD can display characters as well as full graphic images and pictures. Recent passive matrix displays using color STN (CSTN) or dual-scan STN (DSTN) technology offers color with good clarity. Full color TFT (Thin Film Transistor) LCDs offer the highest contrast, color and resolution with excellent picture quality.
This monochrome 7-segment LCD includes provisions
for a floating decimal point, as well as units
of measure.
This monochrome graphic LCD can display numerals,
characters, static symbols, and pictograms.
On higher-end white goods, these displays can
communicate a wide range of information to
the user.
Vacuum Fluorescent Displays ('VFDs')
VFDs are 3-element vacuum tubes (triodes), with a filament, a control grid, and an anode. Electrons are emitted from the filament, and their arrival at the anode is controlled by the control grid. The anode emits light when the electrons hit it; therefore, the voltage on the control grid controls whether the display is on or off, as well as the brightness of the display. As with the LCD, the individual elements of an VFD can be made in virtually any shape. Types of displays using VFDs include:
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Monochrome Numeric: These are 7-segment displays, just like the LED and LCD versions.
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Monochrome Character: These are multi-segment displays. Because the individual elements in an VFDs can be of any shape, these devices can also display character which are formed to a high degree of accuracy.
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Monochrome Dot Matrix: These displays form characters through the use of a matrix of 5 x 7 dots or 5 x 8 dots; one for each character.
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Monochrome Graphic: These VFDs can display characters as well as full graphic images and pictures.
VFDs are available in tube form (the display only) or as VFD modules (the display plus all necessary driver circuitry).
The Characteristics of Each Display Type
LEDs
Since LEDs are solid-state devices, they are mechanically rugged, with the ability to handle shock and vibration levels far in excess of what would ever be experienced in a white goods application. They offer high visibility across the full range of ambient light levels, from total darkness to direct sunlight. They can be made very large, with intense brightness levels, and they provide a virtually unlimited viewing angle. Operating life is in excess of 100,000 hours, and 'failure' consists of a drop in brightness level to 80 percent of its initial value. Since they still function at this point, it is classified as a non-catastrophic failure.
Operating temperature range is -40°C to +85°C, ample for any white goods application. However, where even higher maximum temperatures are experienced, Lumex does offer some LED displays with a +105 °C temperature rating.
LEDs are available in a large number of colors including red, green, blue, orange, or yellow. They are also available in 2-color ('bi-color) and full-color ('RGB') versions.
Power consumption is on the order of 10 mA per LED. While this may be high for some battery-powered applications, it is not an issue for white goods. The drive voltage for LEDs is approximately 2V DC, which may possibly require voltage-conversion circuitry in a typical 5V environment.
Although the basic LED dies or 'chips' are quite small, there is a limit regarding how small the packaged LEDs can be made. This means that LEDs cannot be used for small displays where intricate characters (Chinese/Japanese) or detailed graphical elements are needed. (As an aside, RGB LED graphic displays can easily be created for signage, scoreboard/stadium monitors, and other large-size applications.)
For comparison purposes with the other display technologies, a typical 4-digit green LED display costs roughly $2 each in production quantities.
LCDs
LCDs can be made to display virtually anything, from simple numerals to complex Chinese/Japanese characters, to full graphics. Visibility can be very good in areas with bright ambient light. They are available in monochrome and full-color versions.
Some of the classic characteristics of LCDs are not a factor when it comes to usage in white goods applications. For example, LCD power consumption is very low in the microamp range; great for battery-powered devices, but not an issue with line-operated washers and dryers.
Passive matrix and monochrome LCDs have a temperature range of approximately -30°C to +80°C, narrower than LEDs, but still more-than-adequate for virtually any usage on white goods. However, if they are used in stoves or ovens, they should be protected or isolated from high temperatures of +70°C or more. Seven-segment LCDs can have a typical operating temperature range of -40°C to +85°C, depending on how they are driven.
Other considerations when using LCDs in white goods include: Their glass packaging is slightly more breakage-prone. They need an external light source ('backlight') to be visible in locations with very low ambient lighting. As you move from a 7-segment numeric display to a passive matrix display to an active matrix color display, the viewing angle decreases.
The operational life of a typical 7-segment numeric LCD is approximately 80,000 hours.
For comparison purposes with the other display technologies, a typical 1” LCD panel (no driver circuitry; just the LCD device itself) costs roughly $1 each in production quantities. Passive monochrome character or graphic modules range from $3 to $8 depending on type and quantity. Passive color (CSTN) modules can cost close to $30 each.
VFDs
Like LCDs, VFDs can be made to display virtually anything, from simple numerals to complex Chinese/Japanese characters, to full graphics. While not as bright as the brightest LEDs, VFDs offer excellent visibility in areas with high ambient light levels. Their viewing angle is virtually as wide as LEDs.
Most VFDs are monochrome devices, with a bluish-green color being most common, as well as the brightest. However, through the application of filters, or the use of different materials in the anode, other monochrome color choices include blue, green, yellow-green, orange and red. Recently, RGB (full color) graphic displays have been introduced.
Like the classic vacuum tubes they resemble, VFDs require a number of power supply voltages. The filament requires an AC drive voltage of approximately 60V at 50 - 60 Hz. This is usually supplied via a center-tapped transformer, with a bias voltage applied to the center tap. The anode and grid also require DC drive voltages. Approximately 60V DC at several hundred mA are generally adequate to drive a VFD display tube. By contrast, VFD modules can include all necessary power supply voltages; even incorporating the function of the center-tapped transformer; into one integrated package. A typical VFD module operates from a single source of +5V DC at several amps. Operating temperature ranges for VFDs can be as wide as -40°C to +85°C, so there are virtually no temperature limitations for VFDs used in white goods, provided they are not in close proximity to burners on stoves, or hot air vents on ovens.
The glass 'vacuum tube' construction of VFDs requires some extra care during assembly, but once installed, they are typically no more fragile (as far as the end user is concerned) than LCDs.
The operational life of a VFD depends on many variables, but is generally in the range of 25,000 hours.
For comparison purposes with the other display technologies, a typical 4-digit VFD display costs roughly $4 each in production quantities.
The general characteristics of the three display types are listed in the table. They are all referenced to application in white good appliances. (See Table 1.)
Choosing the Right Display for the Application
With this background as a point of departure, how does a designer choose which type of display to incorporate into a stove, oven or dishwasher?
As general guidelines, the following characterizations can be helpful:
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LEDs high brightness and saturated color are good for safety warnings and/or display of basic information. They are very rugged and have virtually unlimited lifetime.
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LCDs can display more complex information. Backlight often needed. Full color graphic TFT displays are excellent for high-end models, but they are pricey.
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VFDs provide the display capabilities of LCDs with brightness levels approaching that of LEDs. Generally, these are the most expensive. Complex power supply-drive issues can be avoided by using today's integrated VFD modules.
Generally, the choice of which display to use comes down to that basic criteria:
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Type of information to be displayed
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Budget available for the display
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Marketing/appearance considerations
Type of Information to be Displayed
When graphics or intricate characters must be displayed, LEDs are ruled out, and LCDs or VFDs become the display of choice. If warning messages, such as 'hot surface' must grab the user’s attention, LEDs offer the intensity and saturated color that is ideal for that purpose.
Budget Available for the Display
In an apples-to-apples comparison of the most basic display types, simple LCD panels are the least expensive, followed by LEDs, VFDs and monochrome or color LCD modules. This rank ordering may change if the appliance already includes a microcontroller and can provide serial or parallel data in a form the display can use directly. This might allow the use of a controller-less display, reducing the cost. However, the designer must look at the offsetting cost of providing the required power supply, if display modules are not used.
When a budget is extremely tight, displays are generally not used at all. Instead, simple on-off indicators, such as individual LEDs, neon glow indicators or indicating mechanical switches, are employed.
Marketing/Appearance Considerations
Because an LED, LCD or VFD could technically be used in virtually any white goods appliance, the choice often comes down to a marketing decision on product features or desired price points. Which display type will provide the appearance that the stylist is hoping to convey? Which display type will offer features or benefits to the end user, which will translate into a competitive advantage? Which display type will allow the quickest integration into the appliance, allowing the quickest time to market? In consumer goods, these types of decisions are often more important than the specific technical details behind each display type.
Typical Selections
Although today's display technologies offer a wide degree of flexibility and offer designers almost unlimited choices, Lumex does see some general trends among the appliance manufacturers:
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Ovens: VFDs or LEDs most common
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Washers & Dryers (high-end): LCDs, particularly graphic devices
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Microwaves: Could be LEDs, LCDs or VFDs, all are applicable
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Dishwashers: LCDs most common
One thing we don't see is a mix of display types, e.g. LEDs, LCDs and/or VFDs, all in the same appliance. Such a mix is too much design work, and it is too expensive. Also, we don't see any incandescent displays or indicators. They simply have too limited a lifetime, with a continual need for replacement.
As white goods continue to grow in sophistication, there will be in increased push toward the use of displays. This is likely to result ultimately in the use of full graphic displays for the highest-end models, with character displays in most others. The choice of specific type will be made by the marketing positioning of the appliance. The continuing growth in the use of modules will reduce the burden on appliance designers to 'start from scratch' when designing displays into their products, and more so if microcontrollers are present to manage the functions and features of the appliance at hand.