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PRACTICAL ELECTRONICS
MASTERCLASS
(Mr Bell)
1
Fault finding
• A fault is when a piece of
equipment does not work the way
it had been intended too.
• Fault finding is a job in itself, in
which electricians need to
dismantle broken equipment and
find the fault and fix it.
• Faults can occur due to many
reasons, such as:– Broken components
– Surge in power
– Poorly soldered joints (dry-joint)
Open and Closed Circuits
• Open circuit: is when the normal electrical path
of current in a circuit has been interrupted or
broken. This can be caused by:
–
–
–
–
–
Faulty solder joints (dry joint)
Broken wires or tracks
Components missing or placed in incorrectly (LED?)
Wrong value of components (resistors)
Fuses blown
• Closed circuit: When the electrical path of
current can flow in an uninterrupted path and
works like it was intended.
Short Circuit
•
Short circuit: When the electrical path of current crosses over
onto another unintended path. This can cause a higher level
of current to flow in to components that could then damage
it.
•
Hence why we incorporate fuses, circuit breakers and earth
wire in to electrical appliances, to prevent overloads in
current.
• This circuit shows a blue track
which has crossed over the
circuit, bypassing the globe
and decreasing the resistance
in the circuit and increasing
the current to a dangerous
level, which could cause
overheating and a fire.
However the fuse would
prevent this by breaking the
circuit.
Testing checklist – what to do
1.
2.
3.
4.
5.
6.
7.
First understand how the circuit works, i.e. what each part does.
Is the power supply switched on and is the polarity of the power
supply correct.
Inspect the circuit for obvious concerns, like:- Broken wires
- missing components
- fuses blown
Is their power at each part of the circuit? (use multimeter)
Check each component to see if current is travelling through it
If it is a component, then replace one at a time and re-test after each
replacement.
Always keep a log of what you have done and think logically
Multimeter
•
Multimeters are very useful test instruments. They are multi, in that they
can measure voltage (voltmeter), current (ammeter), resistance
(ohmmeter) and be used as a continuity tester (establishing if an electrical
path can be established).
LCD
Power button
Rotary switch
Banana jacks
Red ammeter
jack
Black COM
jack
Testing probes
Red voltmeter
jack
AC and DC settings
•
Multimeters can measure both AC and DC current/voltage, but what does
AC DC mean? And no its not the band.
AC means Alternating
Current , which is current
or Voltage that changes
direction, usually many
times in one second. This
is used in wall outlets in
your house and power
sockets.
The symbol on the
multimeter is a wave
symbol.
DC means Direct Current,
which is current or Voltage
that always flows in one
direction (+ to -). This is
what batteries use and it is
what we will use in the
classroom.
The symbol on the
multimeter is a straight line
with a dotted line below it.
What is resistance?
• Resistance is how difficult it is for
electricity to flow through
something.
• The higher the resistance the more
difficult it is for electricity to flow.
• A resistor is used to resist current
to an LED.
• Resistance is measured in ohms.
• Named after Georg Ohm (German
physicist and mathematician) in
1854.
• The symbol for ohms is Ω (the
capital Greek letter for omega).
What is voltage?
• Voltage is how hard electricity
is being “pushed” through a
circuit.
• The higher the voltage, the
harder the electricity is being
pushed.
• Voltage is measured in volts.
• It is named after Alessandro
Volta (Italian physicist) in 1880.
• The symbol for volts is V.
• Alessandro also invented the
first battery.
Circuit Symbol
What is current?
• Current is how much electricity
is flowing through the circuit.
• The higher the current, the
more electricity is flowing.
• Current is measured in
amperes.
• Named after André-Marie
Ampère in 1836
(mathematician and physicist)
• The symbol for amperes is A.
Circuit Symbol
Measure the current for resistor A,
B and C
A
C
B
•
•
•
Keep the crocodile clips
and battery connected
Record your measured
values in your log book
Remember to write in your
units
Continuity tester
• The multimeter
can also be used
as a continuity
tester.
• This allows you to
determine if an
electrical path
exists.
• Used to help test
for open, closed
and short circuit
conditions.
1. Turn the rotary
switch to
2. Insert the black
lead into the COM
socket
3. Insert the red lead
(+) into the VΩ
socket