Fault Finding in Electronic Circuits: Visual Checks, Multimeter Testing, and Oscilloscope Usage

1. Unit 5 Learning Objective(s) L5.1a To understand the need for safe working practices when carrying out practical electronics L5.1b To understand relevant electrical and electronic principles L5.2 To be able to recognise and select components used in electrical and electronic circuits L5.3 To be able to construct an electronic circuit and understand its basic operating principles L5.4 To be able to test and find faults on electronic circuits

2. FAULT FINDING What To Look For Types Of Equipment

3. Lesson Objectives • Learning Task 4: Checking and Testing – P22+ • Learning Outcome 5.4 • To be able to test and find faults in electronic circuits

4. FAULT FINDING – VISUAL CHECKS If the component is not connected to the PCB then it will not function. Also check for solder crossing onto another component (a SHORT) as this will affect the circuit. SOLDERING Ensure that each component has been put in correctly according to its connection, i.e. positive and negative legs. POLARISATION +ve & –ve legs Take care when putting in components so that you do not damage them. DAMAGE

5. EQUIPMENT - MULTIMETER Select a range with a maximum greater than you expect the reading to be. Connect the meter, making sure the leads are the correct way round. If the reading goes off the scale: immediately disconnect and select a higher range. IMPORTANT ALWAYS disconnect the multimeter before adjusting the range switch. ALWAYS check the setting of the range switch before you connect to a circuit. NEVER leave a multimeter set to a current range (except when actually taking a reading).

6. V MULTIMETER VOLTAGE We can start by measuring a 9V alkaline battery. 1 If the range is set to les than 9V the reading will only show as “1”. Fix the range so that it's 20V, and try again.    SET AT 2V 9.60 For this new battery we get a reading of 9.6V. Remember that the battery voltage "9V" is just an average voltage. SET AT 20V

7. V MULTIMETER VOLTAGE Connected in PARALLEL between two points Connect the BLACK (-ve) lead to 0V (Normally the negative terminal of the battery or power supply.) Connect the RED (+ve) lead to the point you need to measure the voltage.

8. A MULTIMETER CURRENT Connected in SERIES Therefore the Circuit needs to be broken All the current following through the circuit must pass\through the multimeter.

9. Ω MULTIMETER RESISTANCE You can only test resistance when the device you're testing isnot powered. You can only test a resistor before it has been soldered into a circuit. Resistance is non-directional You can switch probes and the reading will be the same.

10. Ω CONTINUITY IF THE TWO POINTS ARE CONNECTED THERE IS A READING Test 1 shows that the two points are not connected. 1 Test 2 shows that these two points are connected 2

11. OSCILLOSCOPE An Oscilloscope is a type of electronic test equipment that allows signal to be viewed, usually as a two-dimensional graph. Normally these are very small signal values An Oscilloscope displays a Vt graph of voltage against time. Voltage Time The Oscilloscope has three main areas, the screen, vertical section and horizontal section. You do not need to worry about the trigger, we set this to AUTO!

12. OSCILLOSCOPE – SWITCHING ON Plug the oscilloscope into the main and switch on using the Green POWER Button The red LED will light up Alter the INTENSITY and FOCUS accordingly using the small screwdriver

13. OSCILLOSCOPE - SCREEN The screen displays the signal to us as voltage against time. The screen is shown as a grid, which relates to the settings we choose later. When switched on you can see the dot/line on the screen When working it has a resemblance to an electrocardiograph (ECG)

14. OSCILLOSCOPE – VERTICAL SECTION The vertical section allows us to change the Voltage setting which the oscilloscope uses. The VERT VAR dial allows us to alter the magnitude of the setting. This must start out fully Anticlockwise The POSITION dial allows us to alter the level of the output line The VOLTS/DIV dial allows us to change the increments of the grid Here the setting is 0.5V on the VOLTS/DIV dial

15. OSCILLOSCOPE – HORIZONTAL SECTION The horizontal section allows us to change the Time (time base) setting setting which the oscilloscope uses. The SWP VAR dial allows us to alter the magnitude of the output. This must start out fully Anticlockwise The POSITION dial allows us to alter the level of the output line The TIME/DIV dial allows us to change the increments of the grid Here the setting is 0.1ms on the TIME/DIV dial

16. LOGIC PROBE • If you get a steady light you have found your + or – voltage. • If blinking you have found a pulsed or switched signal. A logic probe is a hand-held pen-like probe used for analyzing and troubleshooting the logical states (Boolean 0 or 1) of a digital circuit. There are usually three different coloured LEDs on the probe's chassis; • A RED and GREEN LED indicate high and low states respectively • An AMBER LED indicates a pulse