SWITCHES Basic Elements of a Circuit Three Essential Elements - - PowerPoint PPT Presentation

SWITCHES

Basic Elements of a Circuit

• Three Essential Elements

– Load – Complete Path – Power Source

• Additional Elements

– Control Device – Protective Device

• A switch is a component that allows us to control

whether the current is on or off in a circuit.

• A closed switch has practically zero resistance.
• An open switch has nearly infinite resistance.

Basic Switch Function

• To open and close a circuit

–Open (no connection) –Closed (connection)

• Control device

–Does not consume power

Schematic Terms

Pole-

• It is defined as the number of completely isolated

circuits that can be controlled by the switch.

• It refers to the number of input circuits of the switch

Pole Throw

Throw-

• It is defined as the number of closed contact positions

that exist per pole.

• It refers to the number of output circuits of the switch

Types of Switches

• Single-pole, single throw(SPST)
• Single-pole, double throw(SPDT)
• Double-pole, single-throw (DPST)
• Double-pole, double-throw(DPDT)

Single Pole, Single Throw

• It can control the current in only one circuit, and there

is only one closed contact position, hence the name single-pole, single-throw

• 1 pole, 1 throw
• Example- room lights

SPST Animation

Single Pole, Double Throw

• SPDT
• 1 pole
• 2 throws, 2 closed positions

SPDT Animation

Double Pole, Single Throw

• DPST
• It is used to control two completely separate

circuits simultaneously.

• 2 poles, 1 throw
• Not 2 throws because of

mechanical connection.

• Both armatures make

contact at same time- one closed position

DPST

DPST Animation

Double Pole, Double Throw

• DPDT
• A DPDT switch is used to reverse the

polarity of voltage across the terminals of a dc motor. (Reversing the polarity reverses the direction of the motor.)

• 2 poles
• 2 separate closed positions

DPDT Animation

Switch Styles

Toggle Slide

Switch Styles

Rocker Push On/Off

Other Switches

• Momentary contact switches

–Push button switches

• Rotary switches

Momentary Contact

• Push Button Normally Open
• PBNO
• Red in color

PBNO

Momentary Contact

• Push Button Normally Closed
• PBNC
• Black in color

PBNC

Rotary Switches

• Used for complex circuits
• Can have multiple throws

Rotary Switch

• Example- Stereo input selector

#1) ?P?T

#2) ?P?T

#3) ?P?T

#4) ?P?T

#1) 3P1T

#2) 2P5T

#3) 3P2T

#4) 3P3T

Review

• What are the three basic parts of a circuit?
• What else can a circuit include?
• What is the main function of a switch?
• What is the difference between a throw and a

pole?

• What other types of switches are available?

FUSES

Fuses

• A

fuse protects the circuit components against excessive current.

• Excessive current melts the fuse element,

blows the fuse, and opens the series circuit before damage can occur to the components

• r wiring.
• Slow-blow fuses are designed to open only on

a continued overload, such as a short circuit, rather than a temporary current surge.

Testing Fuses

• When measured with an ohmmeter, a good fuse

has practically zero resistance. An open fuse reads infinite ohms.

• When measured with a voltmeter, a good fuse

has zero volts across its two terminals. If there is significant voltage across the fuse, it is open.

SAMPLE FUSES

Glass-cartridge fuse Fuse holder Panel-mounted fuse holder Circuit Board Fuses

CIRCUIT BREAKERS

• A circuit breaker can be used in place of a fuse to protect

circuit components and wiring against the high current caused by a short circuit.

• It is constructed of a thin bimetallic strip that expands with

heat and in turn trips open the circuit.

• The advantage of a circuit breaker is that it can be reset once

the bimetallic strip cools down and the short circuit has been removed.

• Almost all new residential house wiring is protected by circuit

breakers rather than fuses.