Ch 3 - Circuit Protection

7/29/2019 Ch 3 - Circuit Protection

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Circuit Protection

Protective Devices terminate current flow ina circuit.

Located in series within a circuit Excessive current flow results from a

decrease in circuit resistance.

Excessive current flow can damage

components and wiring. Shorts Undesirable, low resistance path for

current to flow.


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Fusible links Special section of wire

designed to melt(open)when current flow

exceeds its rating

Fusible link usually 4

wire sizes smaller thanthe circuit it protects

Must be replaced with

wire of same rating Located near starter or

Pos. battery terminal


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Fuses

Glass Cartridge

Minifuse

Autofuse

Maxifuse


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Fuse Ratings

Element within thefuse melts if current

exceeds its rating

See figure 3-3 on

page 53


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Circuit Breakers

Protects circuits thatare prone tooverloading

Some resetthemselves, othersmust be manuallyreset

Commonly usedwith head lights &power seats


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ECBElectronic Circuit Breakers

Solid state design no moving or replaceable

parts

Positive temperature coefficient resistanceincreases as current increases,

ECB resets when it cools

Commonly found in power window motors and

power door lock actuators


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Switches

Control current flow

through a circuit

Normally open nocurrent flow when switch

is at rest

Normally closed

current flow when switch

is at rest


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Switches

SPST Single pole single throw

Pole - number of input circuits

Throw

number of output circuits

SPDT Single pole double throw

Ganged switch several contacts move

together, affect several circuits

Mercury switch detects motion


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Relays

Device thatuses lowcurrent tocontrol ahigh currentcircuit

Electro-magneticswitch


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Solenoids Electromechanical

device that performs

work

Electromagnet that

moves an iron core

one way, an internal

spring resets core

Some solenoidsrequire reverse

polarity to reset core


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Variable Resistors

Vary input voltage or current to an output device

Stepped resistor several fixed resistor values

Rheostats two wire regulator of electricalcurrent Resistance value changes

Potentiometer three wire resistor that acts as a

voltage divider, produces a continuously variableoutput signal proportional to a mechanicalposition


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Electronic Components


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Diodes One way electrical valve

Zener Diode Allows voltage to pass in theopposite direction when voltage exceeds acertain limit


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LEDLight-Emitting Diode

Forward biased LED

that emits light

No filament, will lasta very long time

Used in IP clusters

and Tail lights

Requires very little

current to operate


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Clamping Diode

Used to suppress voltage spikes

Wired in parallel with a electromagnetic coil

A/C clutch

Connected to the circuit in reverse bias


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Transistor

Regulates current or

voltage and acts as

a switch or gate for

electronic signals

Solid state switching

device


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Phototransistors

A semiconductor device which conducts a current

proportional to the light incident on it. It behaves like a

normal transistor, except that it has a transparent top to its

case and a small current is produced by photons generating

electron-hole pairs in the base (photoelectric effect). This

small current is amplified by the transistor action Detects available light, used in automatic lighting systems


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Phototransistor

Used on GM power sliding doors to monitordoor movement and location


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Circuit Defects

Open A break in circuit continuity, stopscurrent flow

Short Current bypasses the normal circuit pathShort to ground

Short to power

Short to another circuit

High Resistance unwanted opposition tocurrent flow

Ch 3 - Circuit Protection

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