Page 329 - Understanding Automotive Electronics

2735 | CH 9 Page 316 Tuesday, March 10, 1998 1:24 PM

9 AUTOMOTIVE INSTRUMENTATION

surrounding area will be light, and the segments will be visible in the presence
of ambient light.
The LCD is an excellent display device because of its low power
requirement and relatively low cost. However, a big disadvantage of the LCD
for automotive application is the need for an external light source for viewing in
the dark. Its characteristic is just the opposite of the LED; that is, the LCD is
readable in the daytime, but not at night. For night driving, the display must be
illuminated by small lamps inside the display. Another disadvantage is that the
display does not work well at the low temperatures that are encountered during
winter driving in some areas. These characteristics of the LCD have limited its
use in automotive instrumentation.

VFD
VFDs use a phosphor (a One of the most common automotive display devices in use today is the
material that emits light vacuum-fluorescent display (VFD). This device generates light in much the
when bombarded by same way as a television picture tube does; that is, a material called phosphor
electrons). VFDs pro- emits light when it is bombarded by energetic electrons. The display uses a
vide readability over a filament coated with material that generates free electrons when the filament is
wide range of condi- heated. The electrons are accelerated toward the anode by a relatively high
tions. voltage. When these high-speed electrons strike the phosphor on the anode, the
phosphor emits light. A common VFD has a phosphor that emits a blue-green
light that provides good readability in the wide range of ambient light
conditions that are present in an automobile. However, other colors (e.g., red or
yellow) are available by using other phosphors.
The numeric characters are formed by shaping the anode segments in the
form of a standard seven-segment character. The basic structure of a typical
VFD is depicted in Figure 9.18. The filament is a special type of resistance wire
and is heated by passing an electrical current through it. The coating on the
heated filament produces free electrons that are accelerated by the electric field
produced by a voltage on the accelerating grid. This grid consists of a fine wire
mesh that allows the electrons to pass through. The electrons pass through
because they are attracted to the anode, which has a higher voltage than the
grid. The high voltage is applied only to the anode of the segments needed to
form the character to be displayed. The instrumentation computer selects the
set of segments that are to emit light for any given message.
VFD brightness can be Since the ambient light in an automobile varies between sunlight and
controlled by varying the darkness, it is desirable to adjust the brightness of the display in accordance
voltage on the accelera- with the ambient light. The brightness is controlled by varying the voltage on
tor grid. the accelerating grid. The higher the voltage, the greater the energy of the
electrons striking the phosphor and the brighter the light. Figure 9.19 shows
the brightness characteristics for a typical VFD device. A brightness of 200 fL
(foot-lamberts) might be selected on a bright sunny day, whereas the
brightness might be only 20 fL at night. The brightness can be set manually
by the driver, or automatically. In the latter case, a photoresistor is used to
vary the grid voltage in accordance with the amount of ambient light. A

316 UNDERSTANDING AUTOMOTIVE ELECTRONICS