Transcript Electrical Principles 2

Level 2
Electrical and Electronic
3
Conductors and insulators.
• A material that freely allows the
passage of an electrical current is
called a conductor, whereas a material
that resists the flow of current is
called an insulator.
Conductors and insulators
Factors affecting the resistance
of a conductor.
•
•
•
•
Length of the conductor.
Cross sectional area
The material the conductor is made of
The temperature of the conductor
Factors affecting the resistance of
a conductor.
What is the main conductor used
in motor vehicles?
COPPER
Reasons why copper is preferred,
• Very low resistance to electron flow.
• Malleability. Describes a material that
means it can be easily shaped.
• Ductility. The property of a material
that allows it to be drawn into wires.
The main insulator used is PVC
because of its,
• High resistance to electron
flow.
• Resistance to water, fuel, oil
and other contaminants.
Conductors and insulators
Earth return
• List the reasons
why the vehicles
chassis is used as
the earth return
circuit.
• Cheaper, less wire
is required
• Lighter, less wire
means less weight
Insulated earth return
• List vehicles where
the vehicle frame
could not be used
as the earth
return.
• Vehicles with
composite bodies
• Fuel and chemical
tankers
Cable sizes.
• The quoted size of a cable refers to
the number of strands and the wire
diameter.
• Example. 9/0.3 = 9 strands of wire,
each being 0.3mm diameter.
• As a guide, a 0.3mm strand of wire will
carry approximately 0.5 amps of
current.
Cable sizes.
• 14/0.3 8.75amps General applications such
as, side lights, indicators
• 28/0.3 17.5amps Headlamps, horn, heated
windows.
• 65/0.3 35amps
• 84/0.3 42amps
• 97/0.3 50amps
• 120/0.3 60amps Charging wire from
alternator to battery depending on
alternator output
• List 2 reasons why it is important to
select the correctly rated cable for
each application.
• To ensure enough power is supplied to
the consumer with minimal losses
• Over rating of the cable will increase
vehicle weight and cost.
Fuses
• a fuse contains a metal
strip which is designed
to overheat and melt
when subjected to a
specified excessive
level of current flow,
breaking the circuit
and stopping the
excessive current flow
from potentially
damaging the more
valuable components
circuit breakers.
• Circuit breakers are
not destroyed by
excess current. A
bimetallic strip heats
up and bends, opening a
set of contacts and
breaking the circuit.
Circuit breakers can be
either manual or
automatic reset types
depending on the type
of circuit they are
protecting. Electric
windows, window
Blade fuse colours
• Current rating.
3
4
5
7.5
10
15
20
25
30
Colour code.
Violet
Pink
Clear/beige
Brown
Red
Blue
Yellow
Neutral
Green
Electrical terminals and connectors
• What is the meant by the term
connectors?
• Connectors join wires together
• What is the meant by the term
terminals?
• Terminals are the point at which
the cables connect to a component.
Electrical terminals and connectors
Relays.
Relay
Relay operation
Simple Relay Components
2 4
Relay switches high currents.
Construction
Windings wrapped
around iron core to
form electromagnet.
Winding
s
Return
spring
Contains fixed and
moving contacts.
Spring ‘returns’ moving
contact to ‘home’ position.
1
3
Soft iron
core
Moving
contact
(Armature)
Fixed
contact
Next
Simple Relay Operation
(one normally open contact)
Operation:
12V supply is
available at switch.
Switch closes, magnetic
field attracts armature,
contacts close.
Current flows
across contacts.
12V
Switch opens,
return spring pulls
contacts apart.
Next
5 Pin Relay
COM
N.O.
Contains 3 high current terminals: N.O.,
N.C. and COM, and 2 coil terminals.
N.C.
Operation:
Current flows across N.C. and COM
contacts when relay not energized.
Switch closes, magnetic
field attracts armature, N.O.
and COM contacts close.
Current flows across
N.O. and COM contacts.
Switch opens, contacts
return to home position.
Commonly found in
automotive circuits.
Next
Briefly describe the operation
of a relay
• When a small current energizes
the electro-magnet, it attracts
an armature blade and closes
contact points. Current can then
flow across the points to power
the main circuit.
Electrical faults. The 3 main faults to
occur in motor vehicle wiring systems are;
•
•
•
Open circuit, where the circuit is
broken and no current can flow.
Short circuit is where a fault has
caused a wire to touch another
conductor and the current uses this
easier path to complete the circuit.
A high resistance in part of the
circuit, such as a dirty connection,
which will reduce the amount of
current that can flow.