Power Control 423
“pickup” or “make” current. The current through the relay coil that al-
lows the magnetic field to become weak enough to release the armature is
known as the “break” or “dropout” current.
There are two types of contacts used in conjunction with most re-
lays—normally open (NO) and normally closed (NC). The NO contacts
remain open when the relay coil is de-energized, and are closed only when
the relay is energized. The NC contacts remain closed when the relay is
de-energized, and are open only when the coil is energized.Solenoids
A solenoid, shown in Figure 15-6 is an electromagnetic coil with
a movable core that is constructed of a magnetic material. The core, or
plunger, is sometimes attached to an external spring. This spring causes
the plunger to remain in a fixed position until moved by the electromag-
netic field that is created by current through the coil. This external spring
also causes the core or plunger to return to its original position when the
coil is de-energized.
Solenoids are used for a variety of control applications. Many gas
and fuel oil furnaces use solenoid valves to automatically turn the fuel
supply on or off upon demand. Most dishwashers use one or more sole-
noids to control the flow of water.Specialized Relays
There are many types of relays used for electrical power control.
General-purpose relays are used for low-power applications. They are rel-Figure 15-5. Simplified diagram of the construction of a relay that is used to con-
trol a motor