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What are three reasons circuit control is needed?
- To remove power from a malfunctioning device
- To remove power from a device you wish to work on and restore power when the work is completed
- To turn devices on and off as the device is needed
- To select the function or circuit desired within a device.
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What are the three types of circuit control devices?
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What is the difference between a manual and an automatic switch?
- A manual switch must be turned on or off by a person.
- An automatic switch turns a circuit on or off without the action of a person (by using mechanical or electrical devices).
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What is one example of a manual switch?
A light switch, an ignition switch, television channel selector, etc.
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What is one example of an automatic switch?
A thermostat, an automobile distributor, a limit switch, etc.
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Why are multicontact switches used?
Multicontact switches make possible the control of more than one circuit or the selection of one of several possible circuits with a single switch.
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What classification of a switch is used when you describe it as a rocker switch?
The type of actuator.
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In describing a switch by the number of positions of the actuator, what are the two possible configurations for a single-pole, double-throw switch?
Two-position and three-position.
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What type of switch should be used to control a circuit that requires a temporary actuation signal?
A momentary switch.
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What type of switch is used if it is necessary to guard against a circuit being accidentally turned on or off?
A locked-position switch.
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What is the common name used for an accurate snap-acting switch?
A microswitch.
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What is the current rating of a switch?
The maximum current a switch is designed to carry.
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What is the voltage rating of a switch?
The maximum voltage allowable in the circuit in which the switch is installed.
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What two types of meters can be used to check a switch?
An ohmmeter and a voltmeter.
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If a switch must be checked with power applied, what type of meter is used?
A voltmeter.
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What should you check when performing preventive maintenance on a switch?
The switch operation for smooth and correct operation, the terminals for corrosion, and the physical condition of the switch.
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What is the operating principle of a solenoid?
The magnetic field created in a coil of wire and core will attract a soft iron plunger when current flows through the coil.
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What is one example of the use of a solenoid?
A starter motor and solenoid.
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If a solenoid is not operating properly, what items should be checked?
The connections, the plunger, the mechanism that the solenoid actuates, the energizing voltage, and the coil of the solenoid.
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What is the operating principle of a relay?
The magnetic field created in a coil of wire will attract aft armature causing a movement in sets of contacts.
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How does a relay differ from a solenoid?
The solenoid provides a mechanical movement of a plunger (a moveable core) while the core of a relay is fixed.
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What are the two classifications of relays?
Control relays and power relays (contactors).
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How can you determine if a relay is operating (changing from one position to the other)?
- By observing the movement of the contacts if the relay is open or sealed with a transparent cover.
- If the relay has an opaque cover, you can "feel" the operation of the relay by placing your finger on the cover.
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What items should be checked on a relay that is not operating properly?
- The coil should be checked for opens, shorts, or a short to ground
- Terminal leads should be checked for charred or burned insulation
- The contact surfaces should be checked for film, carbon, arcing, and contact spacing.
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What is used to clean the contacts of a relay?
A burnishing tool.
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What tool is used to set contact clearances on a relay?
A point bender
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