Circuit 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 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

Fuses  Glass Cartridge  Minifuse  Autofuse  Maxifuse

Fuse Ratings  Element within the fuse melts if current exceeds its rating  See figure 3-3 on page 53

Circuit 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 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

Switches  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

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

Relays  Device that uses low current to control a high current circuit  Electro- magnetic switch

Solenoids  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 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 Components

Diodes  One way electrical valve  Zener Diode – Allows voltage to pass in the opposite direction when voltage exceeds a certain limit

LED – 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 Diode  Used to suppress voltage spikes  Wired in parallel with a electromagnetic coil – A/C clutch  Connected to the circuit in reverse bias

Transistor  Regulates current or voltage and acts as a switch or gate for electronic signals  Solid state switching device

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

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

Circuit Defects  Open – A break in circuit continuity, stops current flow  Short – Current bypasses the normal circuit path Short to ground Short to power Short to another circuit  High Resistance – unwanted opposition to current flow