CIRCUIT CONTROL DEVICES

LEARNING OBJECTIVES

Upon completion of this chapter you will be able to:

• State three reasons circuit control devices are used and list three general types of circuit control devices.
• Identify the schematic symbols for a switch, a solenoid, and a relay.
• State the difference between a manual and an automatic switch and give an example of each.
• State the reason multicontact switches are used.
• Identify the schematic symbols for the following switches: Single-pole, double-throw, Double-pole, single-throw, Double-pole, double-throw, Single-break, Double-break, Rotary, Wafer
• State the characteristics of a switch described as a rocker switch.
• State the possible number of positions for a single-pole, double-throw switch.
• Identify a type of momentary switch.
• State the type of switch used to prevent the accidental energizing or deenergizing of a circuit.
• State the common name for an accurate snap-acting switch.
• State the meaning of the current and voltage rating of a switch.
• State the two types of meters you can use to check a switch.
• Select the proper substitute switch from a list.
• State the conditions checked for in preventive maintenance of switches.
• State the operating principle and one example of a solenoid.
• State the ways in which a solenoid can be checked for proper operation.
• State the operating principle of a relay and how it differs from a solenoid.
• State the two types of relays according to use.
• 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 sure to "tag out" the switch to ensure that power is not applied to the circuit while you are working. When work has been completed, power must be restored to the circuit. This will allow you to check the proper operation of the circuit and place it back in service. After the circuit has been checked for proper operation, remove the tag from the power switch.

Many electrical devices are used some of the time and not needed at other times. Circuit control devices allow you to turn the device on when it is needed and off when it is not needed.

Some devices, like multimeters or televisions, require the selection of a specific function or circuit. A circuit control device makes possible the selection of the particular circuit you wish to use.

TYPES OF CIRCUIT CONTROL DEVICES

Circuit control devices have many different shapes and sizes, but most circuit control devices are either SWITCHES, SOLENOIDS, or RELAYS.

Figure 3-1 shows an example of each of these types of circuit control devices and their schematic symbols.

Figure 3-1. - Typical circuit control devices: RELAY COIL TERMINALS

Figure 3-1, view A, is a simple toggle switch and the schematic symbol for this switch is shown below it. Figure 3-1, view B, is a cutaway view of a solenoid. The schematic symbol below the solenoid is one of the schematic symbols used for this solenoid. Figure 3-1, view C, shows a simple relay. One of the schematic symbols for this relay is shown next to the relay.