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Chapter 11 Electrical Engineering 235
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
variety of confi gurations to meet the specifi c
needs of each application. Switches are char-
acterized 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
locations. In Figure 11-17, a light can be turned on
or off from each of two SPDT switches.
Diodes
Standard diodes are designed to allow
current fl ow in only one direction. They can be
used as rectifi ers to change alternating current to
direct current. Diodes have two electrodes called
the anode and the cathode. The anode is made of a
positive (P-type) semiconductor material and the
cathode is made of a negative (N-type) semicon-
ductor material. For the diode to allow current
to fl ow, the correct polarity must be applied.
Polarity is the condition of being positive or
negative with respect to ground. The negative
power supply terminal must be connected to
the cathode (–) and the positive terminal must
be connected to the anode (+). This is called
forward bias. Current will fl ow in forward bias.
If the negative power supply terminal were
connected to the anode and the positive to the
cathode, a reverse bias condition would exist.
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Figure 11-16. 
These are common variable resistors.
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Figure 11-17. 
In this example, a light can be controlled from either of the two single-pole double-throw switches.
Light
Switch 2
Switch 1
120 V