Presentation on theme: "Electrical Circuits / Electronics"— Presentation transcript:
1Electrical 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 currentElectric 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.
2Electrical 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).ResistanceAll 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 ().
4Components - Resistors Fixed Resistor SymbolResistors 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.
5First and second colour band Resistor Colour CodeFirst and second colour bandDigitMultiplierBlackx 1Brown1x 10Red2x 100Orange3x 1000 or 1 KYellow4x or 10 KGreen5x or 100 KBlue6x or 1 MViolet7Silver means divide by 100Grey8Gold means divide by 10White9Tolerances:brown 1%red 2%gold 5%silver 10%none 20%
6Resistor Value Calculation If the colours on the resistor are:1st band red2nd band violet3rd band brown4th band goldThen its value is: 2(red) 7(Violet) x 10(Brown) with a 5% tolerance (Gold) i.e. 270ohms 5% tolerance.
7Pupil Assignment Calculate the value of the following resistors: blue – violet – brown – silverorange – white – brown – goldbrown – black – red – goldbrown – black – green – brownWhat colours would the following resistors have?270 R1K533 K
8DiodesDiodes 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.
9Light Emitting DiodeA 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.
10SwitchesSwitches 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.
12Pupil ActivityWe 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 workbookBuild the circuit using the modular boardsYour teacher will demonstrate how to connect the boards
13Series CircuitsWhen 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 LawThe sum of voltages dropped across each component (V1, V2 ) is equal to the total voltage supply in the circuit.VT = V1 + V2 + V3 + …
14Measuring Voltage Drops To measure d.c. voltage:Connect the black lead to the ‘COM’socketConnect the red lead to the ‘V’ socketMake sure that ‘d.c.’ is selectedPlace the lead probes on the pointswhere the voltage is to be measuredDigital Multimeter
15Measuring Voltage Drops Note how voltage is measured over the componentsMake sure you take a note of the symbol for VOLTMETER
16Pupil Activity (Voltage Drops) Task:Measure the voltage drop over the 2 bulbs. Enter your findings into a table.Bulb No.Voltage (v)129V
17Pupil Activity (Voltage Drops) Task:Measure the voltage drop over the 2 bulbs and resistor. Enter your findings into a table.
18Prototype BoardPrototype 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?
19Circuit SimulationAs 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..
20Measuring CurrentCurrent 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
21Current measurementUsing 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.
22Measuring ResistanceConnect two resistors in series on a prototype circuit board and measure the overall resistance.You should find thatRtotal = R1 + R2And the general rule for finding the sum of any amount of resistors in series isRtotal = R1 + R2 + R3 + Rn
23OHMS LAWOhms 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.
24Ohms Law in PracticeThe task is to calculate the resistance of the lamp.
25Worked Example For the series circuit shown, calculate: The total resistance (RT)The circuit current (IC)The potential difference across both resistors (V1 and V2)
27Pupil Problems For the circuit shown below calculate: The total resistance of the circuitThe circuit current
28Pupil Problems For the circuit shown below calculate: The total resistanceThe circuit currentThe voltage drop across each resistor.Use Kirchoff’s second law to verify your answers to (c).
29Pupil Problems For the circuit shown below calculate: The total resistance of the circuitThe circuit current.
30Pupil ProblemsA 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
31Series CircuitsOne 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.
32Parallel CircuitsParallel 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.
33Parallel 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.
34Resistors in ParallelConnect two resistors in parallel on a prototype circuit board and measure the overall resistanceThe formula to calculate the theoretical value of resistors in parallel is shown below.
35Worked ExampleCalculate 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.
36Pupil Activity (Parallel Circuits) Task:Build the circuit, Measure the voltage over each of the bulbs. Enter your findings into a table.
37Current 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 equalto the current flowing from the voltage source.
38Worked ExampleThe 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.
39Pupil Problems For the circuit shown below calculate: (a) The total resistance of the circuit(b) The circuit current.
40Pupil 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).
41Pupil 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).
42Pupil ProblemsA 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
43Combined 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.
44Combined 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 acrossR3The current through R2 added to the current through R3 is thesame as the current through R1The voltage drop across R1 added to the voltage drop across R2(which is the same as across R3) would equal the supply voltageVs.
45Worked Example 2For 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).
46Pupil Problems For the circuit shown calculate: The resistance of the parallel combinationThe total circuit resistance.The branch currents
47Pupil Problems For the circuit shown calculate: The total resistance The circuit currentThe branch currentThe voltage drop across each resistor.
48Pupil Problems For the circuit shown calculate: The total resistance of the circuitThe circuit currentThe current through each resistorThe voltage drop across each resistor.
49Power = Voltage x Current (watts) Power in CircuitsElectrical power is measured in watts (W).Electrical power can be converted into other forms ofpower using electric circuits. For example the power usedin overcoming electrical resistance can be converted intoheat – this is the principle of an electric fire.The power in an electric circuit depends both on theamount 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)
50Worked ExampleAn electric household lamp consumes 60 watts from a 240 volt supply. Calculate the current drawn by the lamp and the resistance of the lamp.
51Pupil ProblemsIn the following simplified circuit for a vacuum cleaner motor, calculate:The power consumption of the motorThe voltage of the lampThe total power drawn from the power supply.
52Pupil ProblemsThe 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 bulbThe voltage across the bulbThe total power drawn from thesupplyThe power drawn by the bulb.
53Pupil ProblemsAn electric iron rated at 800 W is connected to a 230 V supply.Calculate the maximum current drawn by the iron. What is thepower used by the iron at half- heat setting?A kettle and a toaster use the same double socket. If thekettle draws a current of 10 A and the toaster 3 A, find thepower used by each of the appliances. The two sockets arewired in parallel to a 230 V supply.An electric drill draws a current of 1.5 amps from a 110 voltsupply. Calculate the power rating of the drill.
54Pupil ProblemsAn emergency power generator has to drive 80 lamps. Eachlamp takes 60 W at 230 V. Calculate the current through eachbulb 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?
55Pupil ProblemsThe 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.
56Voltage DividersInput 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.
57Voltage 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.
58Voltage 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).
60Pupil ProblemsCalculate Vo in the following exercises
61Pupil ProblemsCalculate Vo in the following exercises
62Switches 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)
63Switch Contacts Types of switch contacts: SPST (Single Pole Single Throw)SPDT (Single Pole Double Throw)
64Pupil ActivityCopy the circuit into your note book. Simulate the circuit using Croc Clips, then describe in your own words how the circuit operates.
65Analogue TransducersA thermistor is a device whose resistance varies withtemperature. It is a temperature-dependent resistor. Thereare two main types.Negative temperature coefficient (t or NTC) – whereresistance decreases as temperature increases.Positive temperature coefficient (+t or PTC) – whereresistance increases as temperature increases.The circuit symbols for and typical characteristics of the two types of resistor are shown on the next slide.
67Data ChartsIn 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.
68Strain GaugesStrain 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.
69Light 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).
70Pupil ActivityUse your Data Book to find the resistance of an ORP12 LDR for the following light conditions:10 Lux40 Lux100 Lux
71Pupil Activity Copy the circuit shown below into your note book. Using the Electrical Modular Boards, construct the voltage divider circuit.Using a multimeter measure Vo.Warm the thermistor up with your fingers and re measure Vo.Describe the operation of the voltage divider.Reverse the position of the thermistor and resistor. Repeat 3,4 & 5.
72Pupil Activity Copy the circuit shown below into your note book. 2) Using the Electrical ModularBoards, construct the voltagedivider circuit.3) Using a multimeter measure Vo.Cover the LDR up with yourhand and re measure Vo.4) Describe the operation of thevoltage divider.5) Reverse the position of theLDR and resistor. Repeat 3,4 &5. Describe what is happening.
73Pupil ActivityA 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.
74PotentiometersSome more examples of potentiometers.
75Voltage 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.
76Pupil ActivityTo 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
77Pupil ProblemsCalculate the voltages that would appear across each of the resistors marked ‘X’ in the circuits below.6v0v
78Pupil ProblemsIn each of the following voltage divider circuits determine the unknown quantity.
79Pupil ProblemsIn each of the following voltage divider circuits determine the unknown quantity.
80Pupil ProblemsAn 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: 80C OR 20C.
81Pupil 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?
82Pupil ProblemsA 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
83Voltage DividersWe 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.
84TransistorsThe 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.
85Transistors A transistor is an electronic switch Transistors amplify current which enables them to drive heavy loads such as motorsA voltage of 0.7V will switch on a NPN transistorCollectorBaseEmitterNPN Bipolar Transistor
86Transistors ActivitySimulate the circuit using Croc Clips. Fill in the table shown below.Base resistorvalue (K)Base/EmitterVoltage (mV)Base current (A)Lampon/off22001000470220100473322101
87TransistorsYour teacher will work through a number of problems which will look at calculating:Base currentVoltage drop over RBase – Emitter voltageMRVin
88Transistors Activity 5V (B) 5V (A) 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?10KBuzzer1k
89RelaysAlthough 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.
90RelaysThe 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.
91Relays – 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.
92SolenoidA solenoid is another output transducer that has a coil inside. Circuits containing a solenoid require a protective diode as well.
93Pupil Activity - RelayBuild the following circuit using modular boardsChange the resistance of the thermistor by heating it up with your handListen carefully, you should hear the relay contacts opening and closingNow 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.
94DPDT RelayAs 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 dividerAdd a transistor driving circuit and a DPDT relayConnect the relay up so as the motor drives clockwise and anticlockwise depending on the amount of light hitting the LDR
96CapacitorsCapacitors 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.
97CapacitorsPolyester 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).
98Pupil Activity 9V Construct the following circuit Allow the capacitor to charge upConnect the end of the LED to 0VThe LED should light up for a short period of time10K+100uF0V
99555 Integrated CircuitAn 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.
100Pupil 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.