PRACTICAL ELECTRONICSMASTERCLASS

(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 do1. First understand how the circuit works, i.e. what each part

does.2. Is the power supply switched on and is the polarity of the

power supply correct.3. Inspect the circuit for obvious concerns, like:-

- Broken wires- missing components- fuses blown

4. Is their power at each part of the circuit? (use multimeter)5. Check each component to see if current is travelling through it6. If it is a component, then replace one at a time and re-test

after each replacement.7. 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).

Testing probes

Power button

LCD

Rotary switch

Red ammeter jack

Black COM jack

Red voltmeter jack

Banana jacks

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

A

BC

Measure the current for resistor A, B and C

• 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