Circuit ProtectionCircuit Protection Protective Devices – terminate current flow in

a 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.

Fusible linksFusible links Special section of wire

designed to melt(open) when current flow exceeds it’s rating

Fusible link usually 4 wire sizes smaller than the circuit it protects

Must be replaced with wire of same rating

Located near starter or Pos. battery terminal

FusesFuses Glass Cartridge

Minifuse

Autofuse

Maxifuse

Fuse RatingsFuse Ratings

Element within the fuse melts if current exceeds its rating

See figure 3-3 on page 53

Circuit BreakersCircuit Breakers Protects circuits that

are prone to overloading

Some reset themselves, others must be manually reset

Commonly used with head lights & power seats

ECB – Electronic Circuit BreakersECB – Electronic Circuit Breakers

Solid state design – no moving or replaceable parts

Positive temperature coefficient – resistance increases as current increases,

ECB resets when it cools Commonly found in power window motors and

power door lock actuators

SwitchesSwitches

Control current flow through a circuit

Normally open – no current flow when switch is at rest

Normally closed – current flow when switch is at rest

SwitchesSwitches

SPST –Single pole single throwPole - number of input circuits

Throw – number of output circuitsSPDT – Single pole double throwGanged switch – several contacts move

together, affect several circuitsMercury switch – detects motion

RelaysRelays Device that

uses low current to control a high current circuit

Electro-magnetic switch

SolenoidsSolenoids Electromechanical device that performs work

Electromagnet that moves an iron core one way, an internal spring resets core

Some solenoids require reverse polarity to reset core

Variable ResistorsVariable Resistors Vary input voltage or current to an output device Stepped resistor – several fixed resistor values Rheostats – two wire regulator of electrical

current – Resistance value changes Potentiometer – three wire resistor that acts as a

voltage divider, produces a continuously variable output signal proportional to a mechanical position

Electronic ComponentsElectronic Components

DiodesDiodes One way electrical valve Zener Diode – Allows voltage to pass in the

opposite direction when voltage exceeds a certain limit

LED – Light-Emitting DiodeLED – Light-Emitting Diode Forward biased LED

that emits light No filament, will last

a very long time Used in IP clusters

and Tail lights Requires very little

current to operate

Clamping DiodeClamping Diode

Used to suppress voltage spikes Wired in parallel with a electromagnetic coil –

A/C clutch Connected to the circuit in reverse bias

TransistorTransistor

Regulates current or voltage and acts as a switch or gate for electronic signals

Solid state switching device

PhototransistorsPhototransistors

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

PhototransistorPhototransistor

Used on GM power sliding doors to monitor door movement and location

Circuit DefectsCircuit Defects Open – A break in circuit continuity, stops

current flow Short – Current bypasses the normal circuit path

Short to groundShort to powerShort to another circuit

High Resistance – unwanted opposition to current flow