3-2
15.
State the operating principle and one example of a solenoid.
16.
State the ways in which a solenoid can be checked for proper operation.
17.
State the operating principle of a relay and how it differs from a solenoid.
18.
State the two types of relays according to use.
19.
State the ways in which a relay can be checked for proper operation and the procedure for
servicing it.
CIRCUIT CONTROL DEVICES
Circuit control devices are used everywhere that electrical or electronic circuits are used. They are
found in submarines, computers, aircraft, televisions, ships, space vehicles, medical instruments, and
many other places. In this chapter you will learn what circuit control devices are, how they are used, and
some of their characteristics.
INTRODUCTION
Electricity existed well before the beginning of recorded history. Lightning was a known and feared
force to early man, but the practical uses of electricity were not recognized until the late 18th century. The
early experimenters in electricity controlled power to their experiments by disconnecting a wire from a
battery or by the use of a clutch between a generator and a steam engine. As practical uses were found for
electricity, a convenient means for turning power on and off was needed.
Telegraph systems, tried as early as the late 1700s and perfected by Morse in the 1830s, used a
mechanically operated contact lever for opening and closing the signal circuit. This was later replaced by
the hand-operated contact lever or "key."
Early power switches were simple hinged beams, arranged to close or open a circuit. The blade-and-
jaw knife switch with a wooden, slate, or porcelain base and an insulated handle, was developed a short
time later. This was the beginning of circuit control devices.
Modern circuit control devices can change their resistance from a few milliohms (when closed) to
well over 100,000 megaohms (when open) in a couple of milliseconds. In some circuit control devices,
the movement necessary to cause the device to open or close is only .001 inch (.025 millimeters).
NEED FOR CIRCUIT CONTROL
Circuit control, in its simplest form, is the application and removal of power. This can also be
expressed as turning a circuit on and off or opening and closing a circuit. Before you learn about the types
of circuit control devices, you should know why circuit control is needed.
If a circuit develops problems that could damage the equipment or endanger personnel, it should be
possible to remove the power from that circuit. The circuit protection devices discussed in the last chapter
will remove power automatically if current or temperature increase enough to cause the circuit protection
device to act. Even with this protection, a manual means of control is needed to allow you to remove
power from the circuit before the protection device acts.
When you work on a circuit, you often need to remove power from it to connect test equipment or to
remove and replace components. When you remove power from a circuit so that you can work on it, be
