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88 ⏐⏐⏐ RESISTANCE
12-V battery
B+
Gang
switch
1 OFF
2 15 A
3
1 OFF
2
30 A
3
Headlights, etc.
also to get too hot. This concern is the primary reason for the thermal
protective element introduced above and appearing in Fig. 3.39(b). The
long, thin copper tubing in Fig. 3.39 is actually filled with an oil-type
fluid that will expand when heated. If too hot, it will expand, depress a
switch in the housing, and turn off the heater by cutting off the current
to the heater wire.
Dimmer Control in an Automobile
A two-point rheostat is the primary element in the control of the light
intensity on the dashboard and accessories of a car. The basic network
appears in Fig. 3.40 with typical voltage and current levels. When the
light switch is closed (usually by pulling the light control knob out from
the dashboard), current will be established through the 50-� rheostat
and then to the various lights on the dashboard. As the knob of the control
switch is turned, it will control the amount of resistance between
points a and b of the rheostat. The more resistance between points a and
b, the less the current and the less the brightness of the various lights.
Note the additional switch in the glove compartment light which is activated
by the opening of the door of the compartment. Aside from the
glove compartment light, all the lights of Fig. 3.40 will be on at the same
time when the light switch is activated. The first branch after the rheostat
contains two bulbs of 6-V rating rather than the 12-V bulbs appearing
in the other branches. The smaller bulbs of this branch will produce
a softer, more even light for specific areas of the panel. Note that the
sum of the two bulbs (in series) is 12 V to match that across the other
branches. The division of voltage in any network will be covered in
detail in Chapters 5 and 6.
Typical current levels for the various branches have also been provided
in Fig. 3.40. You will learn in Chapter 6 that the current drain
from the battery and through the fuse and rheostat approximately equals
the sum of the currents in the branches of the network. The result is that
the fuse must be able to handle current in amperes, so a 15-A fuse was
employed (even though the bulbs appear in Fig. 3.40 as 12-V bulbs to
match the battery).
Whenever the operating voltage and current levels of a component
are known, the internal “hot” resistance of the unit can be determined
using Ohm’s law, to be introduced in detail in the next chapter. Basi-
Panel switch
a
50 Ω, 1 W
b
+
6 V
–
+
6 V
Panel
lights
+
12 V
300 mA
–
Ashtray
light +
6 V
–
+
300 mA 6 V
–
–
FIG. 3.40
Dashboard dimmer control in an automobile.
Radio
display
Parking lights, side lights,
license plate light, etc.
300 mA + 500 mA
12 V
– Glove
compartment
light
R
G