Chapter 8 Electrical Engineering 149
The fourth band is the tolerance and in this
case it is silver, which has a value of 10%. This
means that the measured value of this resistor
could be 10% above or below 560 Ω.
In order to fi nd 10% of 560, do the following:
560 × 0.1 = 56
560 – 56 = 504
560 + 56 = 616
So the measured value of this 560 Ω resistor
with a 10% tolerance could be anywhere between
504 Ω and 616 Ω.
Variable Resistors
At times, such as in dimmer switches and fan
speed switches, it is important to vary the amount
of resistance. In these situations, resistors whose
resistance can be changed, known as variable
resistors, are used. Variable resistors have two
terminals and a wiper, which is a knob or a slid-
ing switch. As the wiper moves along a piece of
resistive material, usually carbon, the amount of
resistive material between the terminals changes,
and so does the resistance. Note the arrow on
the schematic for variable resistors. An arrow on
a schematic indicates that its value is variable.
Figure 8-16 shows common variable resistors.
Switches
Switches are used to control the fl ow of elec-
tricity in a circuit. They open and close (turn off and
on) the circuit or direct the fl ow of electricity into
a different circuit. Switches come in a wide vari-
ety of confi gurations to meet the specifi c needs of
each application. Switches are characterized by
the type of switch, the number of poles, and the
number of throws. The number of poles indicates
the number of paths for current fl ow into the switch.
The number of throws indicates the number of
paths leaving the switch. A single-pole single-throw
(SPST) switch has one path for current fl owing in
and one path fl owing out. This switch can only turn
the current on or off to one circuit. For example, a
switch on a lamp is usually a SPST switch. A single-
pole double-throw (SPDT) switch can direct the
current in one direction or the other. A common
use for the SPDT switch is a lighting circuit where a
light can be turned on or off from two different loca-
tions. In Figure 8-17, a light can be turned on or off
from each of two SPDT switches.
These are common variable resistors.
Figure 8-16.
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Light
Switch 2
Switch 1
120 V
Figure 8-17.
In this example, a light can be controlled from either of the two single-pole double-throw switches.
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