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Gary Plimer 2005 Electrical Circuits / Electronics Electricity is one of the most important forms of energy available to man. It affects everyones lives.

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Presentation on theme: "Gary Plimer 2005 Electrical Circuits / Electronics Electricity is one of the most important forms of energy available to man. It affects everyones lives."— Presentation transcript:

1 Gary Plimer 2005 Electrical Circuits / Electronics Electricity is one of the most important forms of energy available to man. It affects everyones 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 Gary Plimer 2005 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 Gary Plimer 2005 Batteries & Voltage Supplies

4 Gary Plimer 2005 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 Gary Plimer 2005 Resistor Colour Code First and second colour bandDigitMultiplier Black0x 1 Brown1x 10 Red2x 100 Orange3x 1000 or 1 K Yellow4x 10 000 or 10 K Green5x 100 000 or 100 K Blue6x 1 000 000 or 1 M Violet7Silver means divide by 100 Grey8Gold means divide by 10 White9 Tolerances: brown 1% red 2% gold 5% silver 10% none 20%

6 Gary Plimer 2005 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 Gary Plimer 2005 Pupil Assignment Calculate the value of the following resistors: 1)blue – violet – brown – silver 2)orange – white – brown – gold 3)brown – black – red – gold 4)brown – black – green – brown What colours would the following resistors have? 1)270 R 2)1K5 3)33 K

8 Gary Plimer 2005 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 Gary Plimer 2005 Light Emitting Diode A light-emitting diode is a special diode that gives out light when current is flowing though it. LEDs are used as indicators to tell when a circuit (or part or 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.

10 Gary Plimer 2005 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 Gary Plimer 2005 Switch Contacts SPST SPDT DPST DPDT

12 Gary Plimer 2005 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 Build the circuit using the modular boards Your teacher will demonstrate how to connect the boards

13 Gary Plimer 2005 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, Kirchoffs 2 nd 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 + …

14 Gary Plimer 2005 Measuring Voltage Drops To measure d.c. voltage: Connect the black lead to the COM socket Connect the red lead to the V socket Make sure that d.c. is selected Place the lead probes on the points where the voltage is to be measured Digital Multimeter

15 Gary Plimer 2005 Measuring Voltage Drops V Note how voltage is measured over the components Make sure you take a note of the symbol for VOLTMETER

16 Gary Plimer 2005 Pupil Activity ( Voltage Drops) Task: Measure the voltage drop over the 2 bulbs. Enter your findings into a table. Bulb No.Voltage (v) 1 2 9V

17 Gary Plimer 2005 Pupil Activity ( Voltage Drops) Task: Measure the voltage drop over the 2 bulbs and resistor. Enter your findings into a table.

18 Gary Plimer 2005 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 Kirchoffs law?

19 Gary Plimer 2005 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..

20 Gary Plimer 2005 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

21 Gary Plimer 2005 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.

22 Gary Plimer 2005 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

23 Gary Plimer 2005 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.

24 Gary Plimer 2005 Ohms Law in Practice The task is to calculate the resistance of the lamp.

25 Gary Plimer 2005 Worked Example For the series circuit shown, calculate: a) The total resistance (RT) b) The circuit current (IC) c) The potential difference across both resistors (V1 and V2)

26 Gary Plimer 2005 Worked Example a)b) c)

27 Gary Plimer 2005 Pupil Problems For the circuit shown below calculate: a)The total resistance of the circuit b) The circuit current

28 Gary Plimer 2005 Pupil Problems For the circuit shown below calculate: a) The total resistance b) The circuit current c) The voltage drop across each resistor. d) Use Kirchoffs second law to verify your answers to (c).

29 Gary Plimer 2005 Pupil Problems For the circuit shown below calculate: a)The total resistance of the circuit b) The circuit current.

30 Gary Plimer 2005 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

31 Gary Plimer 2005 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.

32 Gary Plimer 2005 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.

33 Gary Plimer 2005 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.

34 Gary Plimer 2005 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.

35 Gary Plimer 2005 Worked Example The resistance values are R1 = 270 R, R2 = 100 R and for the buzzer 240 R. Calculate the resistance of the parallel branch and the total circuit resistance.

36 Gary Plimer 2005 Pupil Activity (Parallel Circuits) Task: Build the circuit, Measure the voltage over each of the bulbs. Enter your findings into a table.

37 Gary Plimer 2005 Current in Parallel Circuits There are two important points to remember about resistors in parallel. 1) The voltage drop across each resistor is the same. 2) The sum of the currents through each resistor is equal to the current flowing from the voltage source.

38 Gary Plimer 2005 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.

39 Gary Plimer 2005 Pupil Problems For the circuit shown below calculate: (a) The total resistance of the circuit (b) The circuit current.

40 Gary Plimer 2005 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).

41 Gary Plimer 2005 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).

42 Gary Plimer 2005 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

43 Gary Plimer 2005 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.

44 Gary Plimer 2005 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.

45 Gary Plimer 2005 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).

46 Gary Plimer 2005 Pupil Problems For the circuit shown calculate: The resistance of the parallel combination The total circuit resistance. The branch currents

47 Gary Plimer 2005 Pupil Problems For the circuit shown calculate: The total resistance The circuit current The branch current The voltage drop across each resistor.

48 Gary Plimer 2005 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.

49 Gary Plimer 2005 Power in Circuits Electrical power is measured in watts (W). Electrical power can be converted into other forms of power using electric circuits. For example the power used in overcoming electrical resistance can be converted into heat – this is the principle of an electric fire. The power in an electric circuit depends both on the amount of current (I) flowing and the voltage (V) applied. The formula for power in electric circuits is: Power = Voltage x Current (watts) P = V x I (W)

50 Gary Plimer 2005 Worked Example An electric household lamp consumes 60 watts from a 240 volt supply. Calculate the current drawn by the lamp and the resistance of the lamp.

51 Gary Plimer 2005 Pupil Problems In the following simplified circuit for a vacuum cleaner motor, calculate: The power consumption of the motor The voltage of the lamp The total power drawn from the power supply.

52 Gary Plimer 2005 Pupil Problems The torch circuit below is supplied with two 4.5 volt batteries connected in series, with the current being 20 mA. Determine: The resistance of the bulb The voltage across the bulb The total power drawn from the supply The power drawn by the bulb.

53 Gary Plimer 2005 Pupil Problems An electric iron rated at 800 W is connected to a 230 V supply. Calculate the maximum current drawn by the iron. What is the power used by the iron at half- heat setting? A kettle and a toaster use the same double socket. If the kettle draws a current of 10 A and the toaster 3 A, find the power used by each of the appliances. The two sockets are wired in parallel to a 230 V supply. An electric drill draws a current of 1.5 amps from a 110 volt supply. Calculate the power rating of the drill.

54 Gary Plimer 2005 Pupil Problems An emergency power generator has to drive 80 lamps. Each lamp takes 60 W at 230 V. Calculate the current through each bulb if: They are connected in series. They are connected in parallel. How many 150 W lamps can be connected in parallel to a 250 V supply through a 5 A fuse?

55 Gary Plimer 2005 Pupil Problems The rear screen heater in a car is connected to the 12 V system and draws a current of 2 A. Find the resistance of the circuit. In reality the 12 V, 0.5 A interior light is on the same circuit. State whether this is a parallel or series circuit and calculate the power and current when both lamp and heater are on.

56 Gary Plimer 2005 Voltage Dividers Input transducers are devices that convert a change in physical conditions (for example, temperature) into a change in resistance and/or voltage. This can then be processed in an electrical network based on a voltage divider circuit.

57 Gary Plimer 2005 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.

58 Gary Plimer 2005 Voltage Dividers Activity Measure the resistance of the 2 resistors from the last activity. Enter the values into the formula below and calculate Vo. Simulate the circuit using croc clips and measure Vo. Hopefully! The value of Vo should be the same in all three cases, (within reason).

59 Gary Plimer 2005 Worked Example

60 Gary Plimer 2005 Pupil Problems Calculate Vo in the following exercises

61 Gary Plimer 2005 Pupil Problems Calculate Vo in the following exercises

62 Gary Plimer 2005 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)

63 Gary Plimer 2005 Switch Contacts Types of switch contacts: SPST (Single Pole Single Throw) SPDT (Single Pole Double Throw)

64 Gary Plimer 2005 Pupil Activity Copy the circuit into your note book. Simulate the circuit using Croc Clips, then describe in your own words how the circuit operates.

65 Gary Plimer 2005 Analogue Transducers A thermistor is a device whose resistance varies with temperature. It is a temperature-dependent resistor. There are two main types. Negative temperature coefficient ( t or NTC) – where resistance decreases as temperature increases. Positive temperature coefficient (+t or PTC) – where resistance increases as temperature increases. The circuit symbols for and typical characteristics of the two types of resistor are shown on the next slide.

66 Gary Plimer 2005 Thermistor NTC is the most common thermistor

67 Gary Plimer 2005 Data Charts In your Data book you should find a graph which describes how the resistance of a thermistor changes with temperature. Your teacher will work through the use of the chart.

68 Gary Plimer 2005 Strain Gauges Strain gauges are really load sensors. They consist of a length of resistance wire and when stretched their resistance changes. Strain gauges are attached to structural members (beams, etc.) and as they are loaded, a reading on a voltmeter can be obtained.

69 Gary Plimer 2005 Light Dependent Resistor The LDR (sometimes called a photoresistor) is a component whose resistance depends on the amount of light falling on it. Its resistance changes with light level. In bright light its resistance is low (usually around 1 K). In darkness its resistance is high (usually around 1 M).

70 Gary Plimer 2005 Pupil Activity Use your Data Book to find the resistance of an ORP12 LDR for the following light conditions: a)10 Lux b)40 Lux c)100 Lux

71 Gary Plimer 2005 Pupil Activity 1)Copy the circuit shown below into your note book. 2)Using the Electrical Modular Boards, construct the voltage divider circuit. 3)Using a multimeter measure Vo. 4)Warm the thermistor up with your fingers and re measure Vo. 5)Describe the operation of the voltage divider. 6)Reverse the position of the thermistor and resistor. Repeat 3,4 & 5.

72 Gary Plimer 2005 Pupil Activity 1) Copy the circuit shown below into your note book. 2) Using the Electrical Modular Boards, construct the voltage divider circuit. 3) Using a multimeter measure Vo. Cover the LDR up with your hand and re measure Vo. 4) Describe the operation of the voltage divider. 5) Reverse the position of the LDR and resistor. Repeat 3,4 & 5. Describe what is happening.

73 Gary Plimer 2005 Pupil Activity A potentiometer configured as a variable resistor can be used in a circuit as a voltage or current control device. They are often used in voltage divider circuits to adjust the sensitivity of the input. Build a voltage divider using a potentiometer. Check its operation by measuring Vo from the voltage divider.

74 Gary Plimer 2005 Potentiometers Some more examples of potentiometers.

75 Gary Plimer 2005 Voltage Divider Sensitivity With an analogue sensor it is normally desirable to adjust the sensitivity of the circuit. Rather than using a fixed resistor we can replace it with a variable resistor (or potentiometer). This allows us to fine tune the sensitivity of the voltage divider.

76 Gary Plimer 2005 Pupil Activity To save money and inconvenience the residents want the outside light to come on when it gets dark. They also want to be able to adjust the sensitivity from summer to winter nights. Build the following circuit using modular circuit boards. Adjust the variable resistor so as Vo goes higher when your hand is about 100mm above the LDR

77 Gary Plimer 2005 Pupil Problems Calculate the voltages that would appear across each of the resistors marked X in the circuits below. 6v 0v

78 Gary Plimer 2005 Pupil Problems In each of the following voltage divider circuits determine the unknown quantity.

79 Gary Plimer 2005 Pupil Problems In each of the following voltage divider circuits determine the unknown quantity.

80 Gary Plimer 2005 Pupil Problems An NTC (negative temperature coefficient) thermistor is used in a voltage divider circuit as shown below. Using information from the graph shown, determine the resistance of the thermistor and hence calculate the voltage that would appear across it when it is at a temperature of: 80 C OR 20 C.

81 Gary Plimer 2005 Pupil Problems What would happen to the voltage across the thermistor in the circuit shown previously as the temperature increased? What would happen to the voltage across the resistor in the circuit shown previously as the temperature increased?

82 Gary Plimer 2005 Pupil Problems A thermistor (type 5) is used in a voltage divider circuit as shown below. The characteristics of the thermistor are shown in your Data Book. If the voltage V2 is to be 4.5 V at 100 C, determine a suitable value for R1. State whether V2 will increase or decrease as the temperature drops. Explain your answer

83 Gary Plimer 2005 Voltage Dividers We have seen that Voltage Dividers, divide the voltage depending on the value of resistors used. In addition, if we include a variable resistor, we can alter the sensitivity of the voltage divider. If we include a thermistor, we can measure changes in temperature. If we include a LDR, we can measure changes in light levels. If we include a potentiometer, we can measure changes in position.

84 Gary Plimer 2005 Transistors The transistor is a semiconductor device. This means that it is sometimes a good conductor of electricity and sometimes a poor one. A transistor is made up of three layers of semiconductor materials that are either n type or p type. There are two types of bipolar transistor available: pnp or npn. Transistors come in many variations and sizes. However, they all are reliable, efficient, small and relatively cheap.

85 Gary Plimer 2005 Transistors Collector Emitter Base NPN Bipolar Transistor A transistor is an electronic switch Transistors amplify current which enables them to drive heavy loads such as motors A voltage of 0.7V will switch on a NPN transistor

86 Gary Plimer 2005 Transistors Activity Base resistor value (K) Base/Emitter Voltage (mV) Base current ( A) Lamp on/off 2200 1000 470 220 100 47 33 22 10 1 Simulate the circuit using Croc Clips. Fill in the table shown below.

87 Gary Plimer 2005 Transistors Vin R Your teacher will work through a number of problems which will look at calculating: a)Base current b)Voltage drop over R c)Base – Emitter voltage M

88 Gary Plimer 2005 Transistors Activity 1k Build the following transistor circuit using modular boards. Adjust the voltage reaching the transistor base by altering the value the potentiometer. At what voltage does the transistor switch on? Measure the current flowing to the base. Now measure the current flowing in the collector leg. What is the transistor doing? 5V (B) 10K 5V (A) Buzzer

89 Gary Plimer 2005 Relays Although relays are often considered to be output devices, they are really output switches from electric or electronic circuits. When an electric current flows into the relay coil, the coil becomes an electromagnet. This electromagnet attracts the armature and moves the contacts. This movement provides the switching, just as the contacts in any other switch do.

90 Gary Plimer 2005 Relays The relay is a very useful device because it is the vital link between microelectronics and high-energy systems that require substantial amounts of current. The relay is perhaps the most commonly used switch for driving devices that demand large currents.

91 Gary Plimer 2005 Relays – Protection Diode As seen earlier, relays have a coil that is energised and de-energised as the relay switches on and off. During this process of switching, the coil can generate a large reverse voltage (called a back e.m.f.). This reverse voltage can cause considerable damage to components, especially transistors. The transistors and other sensitive components can be protected by the inclusion of a diode that provides a path for the current caused by the reverse voltage to escape.

92 Gary Plimer 2005 Solenoid A solenoid is another output transducer that has a coil inside. Circuits containing a solenoid require a protective diode as well.

93 Gary Plimer 2005 Pupil Activity - Relay Build the following circuit using modular boards Change the resistance of the thermistor by heating it up with your hand Listen carefully, you should hear the relay contacts opening and closing Now add a bulb and 5V power supply to the output side of the relay and heat the thermistor up again, (your teacher will demonstrate the correct connections). The bulb should light when the thermistor is hot. It might be necessary to replace the 10K with a potentiometer.

94 Gary Plimer 2005 DPDT Relay As electric motors normally draw larger currents, relays are ideal devices for such circuits. By using DTDP switching, relays can control the direction of rotation of motors. Simulate a sensing circuit using an LDR in a voltage divider Add a transistor driving circuit and a DPDT relay Connect the relay up so as the motor drives clockwise and anticlockwise depending on the amount of light hitting the LDR

95 Gary Plimer 2005 Motor Reversal Circuit

96 Gary Plimer 2005 Capacitors Capacitors are electronic components that store electricity for short periods of time within electronic circuits or networks. Electrolytic capacitors are polarity conscious. This means that they must be connected the right way round. The negative lead must be connected to zero volts with the positive terminal towards the higher voltage side of the circuit.

97 Gary Plimer 2005 Capacitors Polyester capacitors are for small-value uses and can be connected without regard to polarity. Capacitance in measured in farads, but because this is a very large measurement most capacitors are rated in F (microfarads) or in nF (nanofarads).

98 Gary Plimer 2005 Pupil Activity + 100uF 10 K 9V 0V Construct the following circuit Allow the capacitor to charge up Connect the end of the LED to 0V The LED should light up for a short period of time

99 Gary Plimer 2005 555 Integrated Circuit An integrated circuit (or IC) is simply an electronic package that contains a number of components on a silicon chip. The 555-timer IC that you are going to use is a very versatile chip that has many applications.

100 Gary Plimer 2005 Pupil Activity - Monostable Simulate the circuit shown, press the switch and observe the circuit operation. Monostable means one stable state. The light comes on but always goes back to its original state.

101 Gary Plimer 2005 ASTABLE CIRCUIT

102 Gary Plimer 2005 Pupil Activity - Astable Build the following circuit on Breadboard, describe its operation.


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