Electrical Circuits / Electronics

1. Electrical Circuits / Electronics • Electricity is one of the most important forms of energy available to man. It affects everyone’s lives in many ways. If you take time to think about your everyday life you will realise that our lives are full of devices that depend upon electricity. • Some important terms: • Electric current • Electric current is the name given to the flow of negatively charged particles called electrons. • Current is measured in amperes, usually referred to as ‘amps’ (A). Current is the rate of flow of electrical charges (called electrons) through a circuit.

2. Electrical Circuits Voltage In most circuits a battery or voltage supply is used to drive the electrons through the components. Voltage is measured in volts (V). Resistance All materials conduct electricity. The materials that conduct electricity well are called conductors and those that are poor conductors are called insulators. Metals are good conductors while rubber and glass are good insulators. Resistance is therefore a measure of how much voltage is required to let a current flow. Resistance is measured in ohms ().

3. Batteries & Voltage Supplies

4. Components - Resistors Fixed Resistor Symbol Resistors are basic components in electrical and electronic circuits. They limit the amount of current flowing in circuits or parts of circuits. Resistors are roughly cylindrical and have coloured stripes. They also have connection wires sticking out of each end. The stripes indicate the value of the resistors. The colours represent numerical values according to a special code. Although the symbol for ohms is ‘’ it is often shown as a capital R; that is, 270 ohms can be expressed as either 270  or 270 R.

5. Resistor Colour Code

6. Resistor Value Calculation If the colours on the resistor are: 1st band  red 2nd band  violet 3rd band  brown 4th band  gold Then its value is: 2(red) 7(Violet) x 10(Brown) with a 5% tolerance (Gold) i.e. 270ohms 5% tolerance.

7. Pupil Assignment • Calculate the value of the following resistors: • blue – violet – brown – silver • orange – white – brown – gold • brown – black – red – gold • brown – black – green – brown • What colours would the following resistors have? • 270 R • 1K5 • 33 K

8. Diodes Diodes are devices that allow current to flow in one direction only. Current will flow through the diode only when the anode (positive side) is connected to the positive side of the circuit and the cathode (negative side) is connected to the negative side of the circuit.

9. Light Emitting Diode A light-emitting diode is a special diode that gives out light when current is flowing through it. LEDs are used as indicators to tell when a circuit (or part of a circuit) is working. You can tell the cathode of an LED as it is the short leg and there is a ‘flat’ on the plastic casing. LED’s use less energy than bulbs, hence the reason we see their use in torches now.

10. Switches Switches are useful input devices (or transducers) that have metal contacts inside them to allow current to pass when then they are touching. There are several ways in which the contacts in mechanical switches can be operated. The main types are  push-button, toggle, key, slide, magnetic (reed) and tilt. These switches are ‘digital’ input devices as they can only be on or off.

11. Switches • Switches are useful input devices (transducers). • There are several ways in which the contacts in mechanical switches can be operated. Such as push button, key, slide, toggle, magnetic (reed) and tilt. • These switches are digital input devices as they can only be on or off. • The contacts on a switch can be NO or NC (normally open, normally closed)

12. Switch Contacts Types of switch contacts: SPST (Single Pole Single Throw) SPDT (Single Pole Double Throw)

13. Switch Contact Use DPDT SPST SPDT DPST

14. Pupil Activity We have now seen a number of common electronic components. Lets now try and combine some of these into a working circuit. Copy the circuit into your workbook simulate the circuit using. Add voltmeters / Ammeters and measure the voltage drop over each component. How would you write up a test plan and results for this circuit?

15. Series Circuits When components are connected end to end, as in the last activity, we say they are connected in series. This leads to an important law, Kirchoff’s 2nd Law The sum of voltages dropped across each component (V1, V2 ) is equal to the total voltage supply in the circuit. VT = V1 + V2 + V3 + …

16. Measuring Voltage Drops V Note how voltage is measured over the components Make sure you take a note of the symbol for VOLTMETER

17. Pupil Activity (Voltage Drops) Task: Measure the voltage drop over the 2 bulbs. Enter your findings into a table. 9V

18. Pupil Activity (Voltage Drops) Task: Measure the voltage drop over the 2 bulbs and resistor. Enter your findings into a table.

19. Prototype Board Prototype Board is used to test circuits prior to manufacturing the circuit in large numbers. Build a series circuit using 2 resistors of different values as shown by your teacher. Using the multimeter, check the voltage drop over each resistor. Do the results confirm Kirchoff’s law?

20. Circuit Simulation As in Pneumatics, it is possible to simulate electrical circuits. In this case we will use a program called Crocodile Technology. Your teacher will demonstrate the use of Croc Clips to simulate the circuit shown below..

21. Measuring Current Current is measured through components or parts of circuits, as shown in the circuit diagram opposite. Note that it is necessary to ‘break’ the circuit and connect the meter in series with the components. Take a note of the symbol for an Ammeter

22. Current measurement Using circuit simulation, measure the current flowing through all three components in the LED circuit. In a series circuit the current flowing through all components is the same. Try placing the meter at different parts of the circuit to prove this. In parallel circuits the same current does not always flow through each component  you will find out about this later.

23. Measuring Resistance Connect two resistors in series on a prototype circuit board and measure the overall resistance. You should find that Rtotal = R1 + R2 And the general rule for finding the sum of any amount of resistors in series is Rtotal = R1 + R2 + R3 + Rn

24. OHMS LAW Ohms law can be used to calculate theoretical Voltage drops, Current and Resistance in circuits. Using the triangle shown opposite, we can rearrange the formula to obtain V or I.

25. Ohms Law in Practice The task is to calculate the resistance of the lamp.

26. Worked Example • For the series circuit shown, calculate: • The total resistance (RT) • The circuit current (IC) • The potential difference (DROP) across both resistors (V1 and V2)

27. Worked Example a) b) c)

28. Pupil Problems • For the circuit shown below calculate: • The total resistance of the circuit • The circuit current • The voltage drops over the resistors

29. Pupil Problems • For the circuit shown below calculate: • The total resistance • The circuit current • The voltage drop across each resistor. • Use Kirchoff’s second law to verify your answers to (c).

30. Pupil Problems • For the circuit shown below calculate: • The total resistance of the circuit • The circuit current.

31. Pupil Problems A circuit has three resistors in series. Their values are 15 R, 24 R and 60 R. Calculate the total resistance of the circuit. Two resistors are connected in series. Their values are 25 R and 75 R. If the voltage drop across the 25 R resistor is 4 volts, determine the circuit current and the supply voltage

32. Series Circuits One of the problems with series circuits is if a component fails, then the whole circuit fails. Consider a set of bulbs connected in series. If one of these bulbs fail, then current cannot flow through the circuit, hence the remaining bulbs will fail to light also.

33. Parallel Circuits Parallel circuits are circuits where there is more than one path for electricity to flow along or that have more than one ‘branch’. Each branch receives the supply voltage, which means that you can run a number of devices from one supply voltage. A good example of a simple parallel circuit is a set of Christmas-tree lights where all the bulbs require a 230 volt supply.

34. Parallel Circuits Activity Parallel circuits can be arranged in many ways, but are normally set out so that you can easily see the parallel ‘branches’. A simple parallel car-alarm circuit is shown below with the switches wired up in parallel. Simulate the circuit shown below, then describe its operation in your note book.

35. Resistors in Parallel Connect two resistors in parallel on a prototype circuit board and measure the overall resistance The formula to calculate the theoretical value of resistors in parallel is shown below.

36. Worked Example Calculate the resistance of the parallel branch and the total circuit resistance. The resistance values are R1 = 270 R, R2 = 100 R and for the buzzer 240 R.

37. Pupil Activity (Parallel Circuits) Task: Build the circuit, Measure the voltage over each of the bulbs. Enter your findings into a table.

38. Current in Parallel Circuits • There are two important points to remember about resistors in parallel. • The voltage drop across each resistor is the same. • The sum of the currents through each resistor is equal • to the current flowing from the voltage source.

39. Worked Example The resistance values are R1 = 270 R, R2 = 100 R and for the buzzer 240 R. Your teacher will work through this problem on the white board.

40. Pupil Problems For the circuit shown below calculate: (a) The total resistance of the circuit (b) The branches and circuit current.

41. Pupil Problems For the circuit shown below calculate: (a) the total resistance of the circuit (b) the circuit current (c) the current flowing though R1 (10 R) (d) the current flowing through R2 (24 R).

42. Pupil Problems For the circuit shown below calculate: (a) the total resistance of the circuit (b) the circuit current (c) the current flowing through R1 (660 R). (d) the current flowing through R2 (470 R).

43. Pupil Problems A 6 R resistor and a 75 R resistor are connected in parallel across a voltage supply of 12 V. Calculate the circuit current. A 440 R resistor is connected in parallel with a 330 R resistor. The current through the 440 R resistor is 300 mA. Find the current through the 330 R resistor

44. Combined Series & Parallel Consider the combined series and parallel circuit shown in the figure below. You can see that R2 and R3 are connected in parallel and that R1 is connected in series with the parallel combination.

45. Combined Series & Parallel • Some points to remember when you are dealing with combined series and parallel circuits are: • The voltage drop across R2 is the same as the voltage drop across R3 • The current through R2 added to the current through R3 is the same as the current through R1 • The voltage drop across R1 added to the voltage drop across R2 (which is the same as across R3) would equal the supply voltage Vs.

46. Worked Example 2 • For the combined series and parallel circuit shown, calculate: • The total circuit resistance (RT) • The circuit current (IC) • The voltage drop across resistor R1 (VR1) • The current through resistor R2 (I2).

47. Pupil Problems • For the circuit shown calculate: • The resistance of the parallel combination • The total circuit resistance. • The branch currents

48. Pupil Problems • For the circuit shown calculate: • The total resistance • The circuit current • The branch current • The voltage drop across each resistor.

49. Pupil Problems • For the circuit shown calculate: • The total resistance of the circuit • The circuit current • The current through each resistor • The voltage drop across each resistor.

50. Voltage Dividers Activity Build a voltage divider circuit using any 2 values of resistor. Using the multimeter measure the voltage drop over R2. This voltage is known as Vo or the output voltage from the divider.