DET: Technological Studies Applied Electronics Intermediate 2

Transcription

1 DET: Technological Studies Applied Electronics Intermediate

2 Spring 1999 HIGHER STILL DET: Technological Studies Applied Electronics Intermediate 2 Support Materials *+,-./

3 CONTENTS Teacher s guide Students materials Outcome 1 Outcome 2 Outcome 3 Technological Studies Support Materials: Applied Electronics (Intermediate 2) 1

4 TECHNOLOGICAL STUDIES INTERMEDIATE 2 APPLIED ELECTRONICS TEACHER S GUIDE Technological Studies Support Materials: Applied Electronics (Intermediate 2) Teacher s Guide

5 TEACHERS GUIDE Support Materials - Overview The support materials for Technological Studies courses in Higher Still have been created to specifically address the outcomes and PC in each unit at the appropriate level. These materials contain a mixture of formal didactic teaching and practical activities. The support materials for each unit have been divided into outcomes. This will facilitate assessment as well as promoting good teaching practice. The materials are intended to be non-consumable, however it is at the discretion of each centre how to use these materials. Each package of support materials follows a common format: 1. Statement of the outcome. 2. Statement of what the student should be able to do on completion of the outcome. 3. Learning and teaching activities. 4. Sequence of structured activities and assignments. 5. Formal Assessment NAB - assessing knowledge PC. Computer simulation - assessing simulation PC. Practical assignments - assessing practical PC. It is important to note that the National Assessments have been designed to allow assessment either after each outcome has been completed or as an end of unit assessment when all outcomes have been completed depending on the needs of the centre. The use of SQA past external paper questions has been used throughout the materials and the further use of these questions is encouraged. Using past questions provides the opportunity for students to: Work at the appropriate level and rigour Prepare for external assessment. Consolidate teaching and learning. Integrate across units. Homework is a key factor in effective teaching and learning. The use of resources such as P & N practice questions in Technological Studies is very useful for homework activities and also in preparation for external assessment. The use of integrated questions across units is essential in preparation of students for External Assessment. Technological Studies Support Materials: Applied Electronics (Intermediate 2) Teacher s Guide 1

6 Support Materials - Content Outcome 1 - Demonstrate knowledge and understanding of the relationship between current and voltage in simple resistive d.c. networks. The purpose of this unit of work is to investigate and analyse resistive d.c. networks. Student activities cover calculations on Ohm's Law, Kirchoff's 1 st and 2 nd Laws as well as practical activities relating to d.c networks. When students have completed this unit of work they should be able to: Determine the relationship between current and voltage in a d.c. network Determine the relationship between supply current and branch currents in a combined series-parallel resistive d.c. network Determine the relationship between applied voltage and the series voltage drops in a combined series-parallel resistive d.c. network Perform calculations to determine equivalent resistance of a network. Construct specified resistive networks Test specified resistive networks. Outcome 2 - Design and construct a simple electronic system to meet a given specification. The purpose of this unit of work is to introduce student to simple electronic control systems based on voltage dividers and bi-polar transistors. The circuits should be able to control output devices and component protection is introduced. Student activities include calculations relating to voltage dividers and transistor gain and construction of simple control systems. When students have completed this unit of work they should be able to: Use manufacturers data sheets to aid selection of appropriate input and output transducers for a given purpose Recognise that changes in the resistance of an input transducer can be converted to changes in voltage using a voltage divider network Carry out calculations involving voltage divider networks Perform calculations using transistor current gain equation Recognise that the transistor can be used as a switch Describe the operational characteristics of various electronic components Perform calculations to verify the specified operation of a circuit Construct specified control systems Test specified control systems. Technological Studies Support Materials: Applied Electronics (Intermediate 2) Teacher s Guide 2

7 Outcome 3 - Design and construct a simple combinational logic system to meet given specifications. The purpose of this unit of work is to introduce students to simple combinational logic systems. Student activities include interpretation of data sheets and construction of simple combinational logic systems. When students have completed this unit of work they should be able to: Identify single logic gate symbols Complete truth tables for single logic gates Analyse combinational logic circuits Complete truth tables for combinational logic circuits Write Boolean expressions for simple combinational logic systems Identify differences between the TTL and CMOS families of IC's Identify types of logic gates, given pin layout diagrams or IC number (logic gate IC's) Use manufactures data sheets/cd-rom to aid selection of appropriate integrated circuits for a given purpose. Correctly 'power up' an IC for use - on breadboard, circuit simulation and diagrammatically Technological Studies Support Materials: Applied Electronics (Intermediate 2) Teacher s Guide 3

8 Resources The resources listed below are items that the centre should provide for each student. It may be possible on some occasions for student to share resources, such as multimeters during practical activities, however any activity that will be used to satisfy an assessment requirement must be undertaken individually. It is expected that centres already presenting Technological Studies at Standard Grade or Higher Level will have the majority of these resources for the current courses. Circuit Simulation Software ( Optional at Intermediate 2) Crocodile Clips Oak Logic Electronic workbench PC/Mac Acorn PC Any circuit simulation software that will enable student to create and test d.c. networks, simple electronic systems and combinational logic systems would be satisfactory. General equipment - required for all units Power supply Breadboards Wire for links - 0.6mm solid core insulated wire. Wire strippers Digital multimeter Outcome 1 Resistors R, 470 R, 1K Outcome 2 Resistors - 220R, 1K, 10K Variable resistor - 20K LDR - ORP12 Thermistors - Types 1, 2, 3, 4, 5 Transistors (NPN various) - BC 108, BFY 51, 2N 3053 Access to manufacturer data sheets/catalogue/cd - ROM LED's Outcome 3 Various IC's (TTL or CMOS) containing common arrangements of two and three input logic gates LED's Resistor values around 370 R. Logic probe. Technological Studies Support Materials: Applied Electronics (Intermediate 2) Teacher s Guide 4

9 Assessment In most Higher Still courses there are two types of assessment, internal and external Internal Assessment - this can be conducted in a number of ways: 1. Knowledge based - tested through NAB 2. Practical - tested in class under appropriate conditions. 3. Software simulation (only used in some courses and units) Internally assessment is subject to central moderation. External Assessment - Assessed by means of an external examination The external examination will provide the basis for grading attainment in course awards and is marked externally. To gain the award of the course, the student must pass all unit assessments as well as the external assessment. Recording and retention of evidence All evidence of performance should be retained by the centre for moderation purposes. NAB - Test A record of the candidate's performance must be kept which shows: The score achieved if a cut-off score is used When a candidate has achieved an outcome Practical assessment A record of the candidate's performance must be kept which shows: When circuit simulation is used - a brief description of the circuit being evaluated. Whether the candidate has evaluated the circuit correctly. Where a circuit is required to be constructed - a brief description of the circuit being constructed. Whether the candidate has constructed the circuit to the given specification. Technological Studies Support Materials: Applied Electronics (Intermediate 2) Teacher s Guide 5

10 Assessment Summary of each Unit The following is a summary of the assessment requirements for each outcome. Outcome 1 - Applied Electronics (Int 2) 1. National Assessment Bank item (Test) - Providing written and graphical evidence for PC a and b. 2. Practical activity - providing performance evidence for PC c. The practical activities contained in the support materials will satisfy the assessment requirements for this aspect. Centres should ensure that when candidates are carrying out the practical activity for assessment purposes, appropriate conditions are in place. Outcome 2 - Applied Electronics (Int 2) 1. National Assessment Bank item (Test) - Providing written and graphical evidence for PC a, b, c and d. 2. Practical activity - providing performance evidence for PC e and f. The practical activities contained in the support materials will satisfy the assessment requirements for this aspect. Centres should ensure that when candidates are carrying out the practical activity for assessment purposes, appropriate conditions are in place. Assessment of the computer simulation aspect can be done using the assignments provided in the support materials. Students should be able to evaluate the circuits effectively to satisfy the assessment requirements; this can be done either in writing or as a verbal report to the teacher/lecturer. Outcome 3 - Applied Electronics (Int 2) 1. National Assessment Bank item (Test) - Providing written and graphical evidence for PC a, b, c and d. 2. Practical activity - providing performance evidence for PC e and f. The practical activities contained in the support materials will satisfy the assessment requirements for this aspect. Centres should ensure that when candidates are carrying out the practical activity for assessment purposes, appropriate conditions are in place. Assessment of the computer simulation aspect can be done using the assignments provided in the support materials. Students should be able to evaluate the circuits effectively to satisfy the assessment requirements; this can be done either in writing or as a verbal report to the teacher. Technological Studies Support Materials: Applied Electronics (Intermediate 2) Teacher s Guide 6

11 TECHNOLOGICAL STUDIES INTERMEDIATE 2 APPLIED ELECTRONICS SECTION 1 OUTCOME 1 Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1

12 OUTCOME 1 Demonstrate knowledge and understanding of the relationship between current and voltage in simple resistive d.c. networks. When you have completed this unit you should be able to: determine the relationship between current and voltage in a d.c. network determine the relationship between supply current and branch currents in a combined series-parallel resistive d.c. network determine the relationship between applied voltage and the series voltage drops in a combined series-parallel resistive d.c. network perform calculations to determine equivalent resistance of a network construct specified resistive networks test specified resistive networks. Before you start this unit you should have a basic understanding of: resistor colour codes electrical circuit symbols Ohm s law use of breadboards use of circuit test equipment: multimeter and/or logic probe. Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 1

13 ELECTRIC NETWORKS An electric circuit is a closed loop made up from electrical components such as batteries or voltage sources, bulbs, switches and wires. AE.Int2. O1 fig 1 Voltage, Resistance and Current Voltage In most electric networks a battery or a voltage supply provides the energy source for the circuit. Voltage is measured in volts (V). Resistance All materials conduct some electricity. Some are good and some are poor. Materials that are good at conducting are called conductors. Those which are poor, are called insulators. Examples of good conductors are silver and copper. Examples of good insulators are glass and rubber. A good conductor is one that offers very little resistance to the flow of electric current. In other words, it lets current flow with very little voltage being applied. Connecting wire used in the construction of electric circuits usually has a very low resistance - it allows electricity to flow freely. Resistors can come in the form of purpose made resistors that have fixed or variable values or in the form of any electrical component that offers resistance to the flow of current in the circuit. Resistance is therefore a measure of how much voltage is required to let a current flow. Resistance is measured in ohms (Ω). Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 2

14 Current Current (I) is the rate of flow of electricity in a circuit and is measured in Amperes (A) Ohm's Law If you apply a voltage to a resistor and make a current flow through it you should find that doubling the voltage difference across the resistor doubles the current flowing through it. Thus we can say that current is proportional to the voltage difference across a resistor. The rule that current is proportional to the voltage difference is an important rule in electronics and is known as Ohm s Law. The relationship between the voltage, resistance and current in a circuit gives rise to Ohm s Law formula: Voltage = Current x Resistance V = I x R This triangle is designed to help you remember how to find V, R and I. AE.Int2. O1 fig 2 To find R, cover R to give R = V/I To find V, cover V to give V = I x R To find I, cover I to give I = V/R Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 3

15 Assignments: Using Ohm s Law Use Ohm s Law in the following examples. 1. A voltage of 6 V is applied across a 1K5 resistor. Find the current that will flow through the resistor. 2. An electric lamp has a resistance of 470 R and is connected to a supply voltage of 110 V. Calculate the current the lamp will draw from the supply. 3. Find the potential difference across a 47 K resistor that takes a current of 450 ma. 4. A current of 650 ma passes through a component that has a resistance of 2 K. Find the potential difference across the component. 5. Calculate for the circuit shown: a) the value of the resistor AE.Int2. O1 fig 3 6. Using the values from the circuit, determine: a) the reading on the ammeter. b) how much current would flow if the value of the resistor were halved AE.Int2. O1 fig 4 Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 4

16 7. Calculate the current flowing through this circuit. AE.Int2. O1 fig 5 Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 5

17 Series and Parallel Circuits It is possible to connect resistive components in two ways, in series where components are connected end to end and in parallel where each component receives the supply voltage. AE.Int2. O1 fig 5a Series circuits When components are connected end to end (in series) to form a closed loop the current is common to all the components and the voltage is divided up amongst them. That is, the sum of the voltage drops across the circuit components must equal the total voltage input to the circuit. This is known as Kirchoff s 2nd Law, which states: the sum of the emf s (voltage supplies) in a closed circuit is equal to the sum of the potential drops round that circuit. V T = V 1 + V 2 + V 3... In a series circuit the Voltage Drop across each resistor is found in the following way: (remember, as there are no branches for the current to flow into, the supply current must flow through each resistor). V 1 = I T x R 1 and V 2 = I T x R 2 and V 3 = I T x R 3 and so on for the number of resistors. Study the example below which shows Kirchoff s 2nd Law in practice. Each bulb is rated at 6 V, and the supply voltage is 18 V. AE.Int2. O1 fig 6 Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 6

18 Disadvantages of Series circuits are: if one component fails, all components go off because the circuit is broken. the supply voltage is shared out amongst the components, this means that a component may not get the required voltage. Resistors in Series According to Kirchoff s 2nd Law, when resistors are connected in series the supply voltage is shared out amongst them. When you add up the individual voltages dropped over the resistors they should equal the supply voltage. As there are no branches for the current to flow into, the supply current must flow through each resistor. When resistors or resistive components are connected in series, the effect is to add more resistance to that circuit. The total resistance can be found by simply adding up all the resistance values in the circuit. There are two important points to remember about resistors in series: a) the same current flows through each resistor. b) the sum of the voltages across each resistor is equal to the voltage across the combination. The total resistance of the circuit is given by R T = R 1 + R 2 + R 3... AE.Int2. O1 fig 7 Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 7

19 Parallel Circuits When components or resistive materials are connected in parallel, each component receives the supply voltage from the voltage source and the current is shared out amongst the components. Study the example shown below: AE.Int2. O1 fig 8 If you look at junction X, we can see that current I 1 + I 2 + I 3 must be equal to the total current I T being supplied from the voltage source. This is known as Kirchoff s 1st Law, in other words, the sum of the current flowing towards a single junction in a circuit is equal to the sum of the currents leaving the junction. I T = I 1 + I 2 + I 3... The current used in each branch of the circuit is found in the following way: (remember, in a parallel circuit the voltage drop across each resistor is the same and is the value of the supply voltage). I 1 = V S / R 1 and I 2 = V S / R 2 and I 3 = V S / R 3 and so on for the number of branches. An advantage of a parallel circuit over a series circuit is that if one of the components fail, it is only that one that is effected. A disadvantage is that parallel circuits draw more current than series circuits. Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 8

20 Resistors in Parallel A parallel circuit has a number of branches, each one having a load connected to it. Each branch receives the supply voltage, which is useful if you are trying to run 2 or 3 devices from one supply voltage. Each branch will have it s own current, and the total supply current is found by adding up all the branch currents. When resistors or resistive components are connected in parallel, the effect is to reduce the resistance in the circuit. There are two important points to remember about resistors in parallel: a) the same voltage acts across each resistor. b) the sum of the currents through each resistor is equal to the current flowing from the voltage source. AE.Int2. O1 fig 9 The total resistance in a parallel circuit is given by: 1/R T = 1/R 1 + 1/R 2 + 1/R 3... Special Case: 2 Resistors in Parallel There is a special rule that can apply for adding 2 resistors in parallel. Total Resistance (R T ) = Product/Sum R R R 1 = T R + 1 R 2 2 Note: This special formula only works for circuits with 2 resistors in parallel. Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 9

21 Worked examples: Series Circuit 1. For the series circuit shown calculate: a) The total resistance ( R T ) b) The circuit current (I C ) c) The voltage drop across both resistors (V 1 ), (V 2 ). C AE.Int2. O1 fig 10 a) R T = R 1 + R 2 = R T = 24 Ω b) V S = I C x R T I C = V S / R T = 12/24 I C = 0.5 A c) V 1 = I C x R 1 = 0.5 x 6 V 1 = 3 V V 2 = I C x R 2 = 0.5 x 18 V 2 = 9 V Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 10

22 We can use Kirchoff s 2nd Law to check the answers calculated for the voltage drops. V T = V 1 + V 2 = V T = 12V Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 11

23 Worked examples: Parallel Circuit 1. For the parallel circuit shown calculate a) The total resistance ( R T ) b) The circuit current (I C ) c) The current through each resistor (I 1 ), (I 2 ). C AE.Int2. O1 fig 11 a) 1/R T = 1/R 1 + 1/R 2 or it is possible to use the special case formula for 2 resistors in parallel R T = R 1 x R 2 / R 1 + R 2 = 8 x 12/ = 96/20 R T = 4.8 Ω b) V S = I C x R T I C = V S / R T = 12/4.8 I C = 2.5 A c) I 1 = V S / R 1 = 12/ 8 I 1 = 1.5 A Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 12

24 I 2 = V S / R 2 = 12/ 12 I 2 = 1 A We can use Kirchoff s 1st Law to check the answers calculated for the current in each branch. I C = I 1 + I 2 = I C = 2.5 A Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 13

25 Assignments: Series and Parallel circuits 1. For the circuit shown, calculate a) The total resistance of the circuit b) The circuit current AE.Int2.O1 fig 11a 2. For the circuit shown, calculate: a) The total resistance b) The circuit current c) The voltage drop across each resistor d) Use Kirchoff s 2nd law to verify your answers in part c) AE.Int2.O1 fig 11b 3. For the circuit shown, calculate a) The total resistance of the circuit b) The circuit current AE.Int2.O1 fig 11c Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 14

26 4. A circuit has three resistors connected in series. Their values are 15 R, 24 R and 60 R. Calculate the total resistance of the circuit. 5. 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 V, determine the circuit current and the supply voltage. 6. For the circuit shown, calculate a) The total resistance of the circuit b) The circuit current AE.Int2.O1 fig 11d 7. For the circuit shown, calculate a) The total resistance of the circuit b) The circuit current c) The current flowing through R1 (10R) d) The current flowing through R2 (24R) AE.Int2.O1 fig 11e Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 15

27 8. For the circuit shown, calculate a) The total resistance of the circuit b) The circuit current c) The current flowing through R1 660R) d) The current flowing through R2 (470R) e) Use Kirchoff s 1st law to verify your answers in parts c) and d) AE.Int2.O1 fig 11f 9. A 66 R resistor and a 75 R resistor are connected in parallel across a voltage supply of 12 V. Calculate the circuit current. 10. 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. Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 16

28 Combined Series and Parallel Circuits Until now we have been looking at Series or Parallel circuits individually. It is possible and quite common to have series and parallel connections in the same circuit. Consider the combined series and parallel circuit shown in the figure. AE.Int2.O1.fig 11g You can see that R 2 and R 3 are connected in parallel and R 1 is connected in series with the parallel combination. Some points to remember when you are dealing with combined series and parallel circuits. 1. The voltage drop across R 2 is the same as the voltage drop across R The current through R 2 added to the current through R 3 is the same as the current through R The voltage drop across R 1 added to the voltage drop across R 2 (which is the same as across R 3 ) would equal the supply voltage Vs. Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 17

29 Worked Example: Combined Series and Parallel circuits 1. For the combined series and parallel circuit shown, calculate: a) The total circuit resistance (R T ). b) The circuit current (I C ). c) The voltage drop across resistor R 1 (V R1 ). d) The current through resistor R 2 (I 2 ). = 10 R = 24 R = 48 R = 12 V AE.Int2. O1.fig 11h a) In the first instance you must calculate the equivalent resistance of the parallel arrangement (R P ) of R 2 and R 3. It is possible to use the special case formula for 2 resistors in parallel. R P R = R 2 2 R + R 3 3 R R R P P P = = = 8.28Ω The total circuit resistance (R T ) is then found by adding R P to R 1. R T = R 1 + R P R T = R T = 32.28Ω Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 18

30 a) It is now possible to calculate the circuit current. V S = I C R T I C V = R S T I I C C = = 0.37A c) The voltage drop across R 1 is found by using the resistance across R 1 and the current through R 1. V = I R V R1 = I C R 1 V R1 = V R1 = 8.88V d) The current through R 2 is found by using the resistance of R 2 and the voltage drop across R 2. By using Kirchoff's 2 nd Law we know that the voltage drop across the parallel arrangement must be: V S = V R1 + V P V P = V S V R1 V P = V P = 3.12V By using Kirchoff's 1 st Law we know that the circuit current I C will 'split' or divide between the two resistors R 2 and R 3. Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 19

31 In order to find the current through R2 V = I R V P = I 2 R 2 I 2 V = R P 2 I I 2 2 = = 0.312A Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 20

32 Assignments: Combined Series and Parallel Circuits 1. For the circuit shown, calculate a) The resistance of the parallel combination b) the total circuit resistance AE.Int2.O1 fig 11h 2. For the circuit shown, calculate: a) The total resistance b) The circuit current c) The voltage drop across each resistor AE.Int2.O1 fig 11j 3. For the circuit shown, calculate a) The total resistance of the circuit b) The circuit current c) The current through each resistor d) The voltage drop across each resistor AE.Int2.O1 fig 11k Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 21

33 Measuring current, voltage and resistance The most commonly used device for measuring voltage, current and resistance is the multimeter, most of which are now digital. Most digital multimeters have the facility to measure and display values of voltage, current and resistance. Other useful additional features are on some of the more advanced meters is the ability to measure capacitance, continuity and transistor gain. There are some important points you need to know before attempting to measure direct current and voltage. Measuring Current The ammeter setting is used to measure the flow of current in a circuit. In this mode the meter must be connected in series with the circuit components, that is, you must break the circuit at a convenient point as shown in the figure below. In this way, the current flowing in the circuit also flows through the meter and is recorded in Amps. AE.Int2. O1 fig 12 A meter set in the ammeter mode has a very low resistance, so that it does not reduce the current that it is designed to measure. Measuring Voltage The voltmeter setting is used to measure the voltage across a component. In this mode the meter must be connected in parallel with the circuit components as shown in the figure below. AE.Int2. O1 fig 13 Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 22

34 A meter set in the voltmeter mode has a very high resistance, so that when it is connected across the component very little current is diverted from the component. When measuring unknown voltages and currents, always set the meter to the highest range then work down to increase the sensitivity of your measurements. Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 23

35 Assignments: Using a digital multimeter Use the equipment provided to construct and investigate the following circuits. Equipment: Breadboard Digital multimeter Power supply Resistors R, 470 R, 1K Wire for links Wire strippers Connect the multimeter in the correct mode and measure the stated values in each circuit. For each circuit you should verify your measured results by calculation. 1. Measure and record the values of voltage and current using the positions shown. AE.Int2. O1 fig Measure and record the current flowing around this circuit. AE.Int2. O1 fig 15 Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 24

36 3. Measure and record the voltage across the resistor in this circuit. AE.Int2. O1 fig Measure and record the circuit current and the voltage across each resistor. AE.Int2. O1 fig Measure and record the circuit current, the current through each resistor and the voltage across the resistors. AE.Int2. O1 fig 18 Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 25

37 6. Measure and record the circuit current, the current through each resistor and the voltage across each resistor. AE.Int2. O1 fig Measure and record the circuit current, the current through each resistor and the voltage across each resistor. AE.Int2. O1 fig 20 Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 26

38 Circuit Simulation Software It is possible to use circuit simulation software such as Crocodile Clips to investigate electric and electronic circuits. Circuit simulation is widely used in industry as a means of investigating complex and costly circuits as well as basic circuits. Circuit simulators make the modelling and testing of complex circuits very simple. The simulators make use of libraries of standard components along with common test equipment such as voltmeters, ammeters and oscilloscopes. Using Crocodile Clips or another similar software package construct and test the following circuits. 1. AE.Int2. O1 fig AE.Int2. O1 fig 22 Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 27

39 3. AE.Int2. O1 fig AE.Int2. O1 fig AE.Int2. O1 fig 25 Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 1 28

40 TECHNOLOGICAL STUDIES INTERMEDIATE 2 APPLIED ELECTRONICS SECTION 2 OUTCOME 2 Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2

41 OUTCOME 2 Design and construct a simple electronic system to meet a given specification. When you have completed this unit you should be able to: Use manufacturers data sheets to aid selection of appropriate input and output transducers for a given purpose Recognise that changes in the resistance of an input transducer can be converted to changes in voltage using a voltage divider network Carry out calculations involving voltage divider networks Perform calculations using transistor current gain equation Recognise that the transistor can be used as a switch Describe the operational characteristics of various electronic components Perform calculations to verify the specified operation of a circuit Construct specified control systems Test specified control systems. Before you start this unit you should have a basic understanding of: Resistor colour codes Electrical circuit symbols Ohm s law Kirchoff s 1st and 2nd laws Use of breadboards Use of circuit test equipment: multimeter and/or logic probe. Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2 1

42 TRANSDUCERS Any electronic system can be broken down into three distinct parts AE.Int2.O2.fig1 The input and output parts must interface with the real world A transducer is a device that converts one form of energy into another e.g. a microphone is a transducer that changes Sound Energy into Electrical Energy. Output Transducers Output transducers in electronic systems are used to convert Electrical Energy into another form that can be detected by the user or used in some other way. Common Output Transducers The table gives some examples of common output transducers that you may have met before. Output Transducer Output Energy Bulb Lamp LED Buzzer Loudspeaker Earphone Motor Pump Solenoid Heating element Light Sound Movement Heat Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2 2

43 Examples of Common Output Transducers At the output of an electronic system the output transducer converts the electrical signal in to some other useful form of energy such as heat, light, sound or mechanical energy. Electric Motors Electric motors convert the electrical signal into rotational kinetic energy. Before a motor is connected to a circuit it is necessary to know the characteristics of the motor in terms of working voltage and the maximum current to be drawn by it in order to determine the correct choice of driver. The most common and likely choice to drive a motor from an electronic circuit would be the relay. Solenoids The solenoid consists of a magnetic core that is free to move position inside a coil. When current flows through the coil and it is energised, the magnetic core is pulled into the centre of the coil (along the coil axis). This converts the electrical signal into linear motion. A solenoid is used when in and out motion is required. Solenoids require very large currents in order to produce meaningful force and they are usually switched on and off by using relays. AE.Int2.O2.fig1c - Circuit symbol for a solenoid Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2 3

44 Input Transducers Input transducers convert a change in physical conditions (e.g. temperature) into a change in an electrical property (e.g. voltage) which can then be processed electronically to produce either a direct measurement of the physical condition (Temperature in o C) or to allow something to happen at a predetermined level (e.g. switching on the central heating at 20 o C). Common Input Transducers The table gives some examples of common input transducers that you may have met before. Physical condition to be monitored Input Transducer Electrical property that changes Temperature Thermistor Thermocouple Platinum Film Resistance Voltage Resistance Light LDR Selenium Cell Photo Diode Resistance Voltage Resistance Displacement Slide Potentiometer Variable Transformer Variable Capacitor Resistance Inductance Capacitance Force Piezo electric crystal Voltage Angle Rotary Potentiometer Resistance It can be seen that electrical properties that change fall into three groups 1. Transducers that produce voltage. 2. Transducers that change the value of resistance. 3. Transducers that change either the value of inductance or capacitance. Changes in the resistance of an input transducer are usually converted to changes in voltage before the signal can be processed. This is normally done using a voltage divider circuit. Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2 4

45 Examples of common Input Transducers The switch We all make use of switches every day. We use them to turn on lights, personal stereos, hairdryers and numerous other devices. A switch in its simplest form is used for making and breaking an electrical circuit. It usually contains metal contacts which when touching allow current to flow. Switch types There are several ways in which the contacts in mechanical switches can be operated. Some are push button, toggle, slide or magnetic (reed), tilt and electromagnetic relay. Switches are wired up to suit their application. A switch with it s contacts apart when it is not operated is called a normally open switch. The simplest type of switch is represented by the symbol shown below AE.Int2.O2.fig2 Notice that the switch consists of two parts, a pole and a contact. This switch is called a single pole single throw switch (SPST). It is given this name because its single pole can be thrown into contact in one position only. Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2 5

46 Three further commonly used switch layouts are given below. AE.Int2.O2.fig3 AE.Int2.O2.fig4 AE.Int2.O2.fig5 Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2 6

47 Relay The relay is not strictly speaking an output device but a switch that can be driven by the output from an electric or electronic circuit. It is an electromechanical device consisting of two main parts - the operating coil (which is essentially a solenoid) and the contacts. AE.Int2.O2.fig1a AE.Int2.O2.fig1b - Circuit symbol for a relay An electric current is sent through the coil that energises it. The coil becomes magnetic and it attracts a spring-loaded armature that moves the contacts together (energised position). Switching the supply off to the coil causes the relay to re-set to the normal (de-energised) position. These contacts can then be used to switch on a very powerful circuit or a number of circuits. The relay is a very useful device and is particularly useful for energising devices that require substantial amounts of current. It is perhaps the most commonly used switch for driving devices that demand large currents. Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2 7

48 Variable Resistor (Potentiometer) A potentiometer or variable resistor can be used in a circuit either as a voltage or current control device. AE.Int2.O2.fig5a Potentiometers normally have three tags, the outer ones being connected to the ends of the resistive material and the centre one the wiper. The spindle of the potentiometer is connected to the wiper, which is able to traverse from one end of the resistance to the other when the spindle is rotated. As the spindle rotates a sliding contact puts more or less resistive material in series with the circuit. Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2 8

49 Light Dependent Resistor (LDR) The LDR (sometimes called a photoresistor) is a component whose resistance depends on the amount of light falling on it. It s resistance changes with light level. In bright light its resistance is low (typically around 1K). In darkness its resistance is high (typically around 1M). The circuit symbol and typical characteristics are shown below. AE.Int2.O2.fig6 AE.Int2.O2.fig50 - Graph of Illumination / Resistance Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2 9

50 Thermistors A Thermistor is a device whose resistance varies with temperature. It is a temperature dependent resistor. There are two main types: 1. Positive temperature coefficient (+t) or (ptc) - where resistance increases as temperature increases. 2. Negative temperature coefficient (- t) or (ntc) - where resistance decreases as temperature increases. The circuit symbol and typical characteristics are shown below. AE.Int2.O2.fig7 AE.Int2.O2.fig8 Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2 10

51 AE.Int2.O2.fig51 - Graph of Temperature / Resistance Strain Gauges These 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 and as they are loaded you can obtain a reading on a voltmeter. AE.Int2.O2.fig9 Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2 11

52 Criteria for selecting transducers Once the physical condition to be monitored and the output requirements of the system have been identified, a choice of transducers has to be made. A number of different criteria may need to be taken into account, some of which are listed in the table below. Response Time Most transducers are required to respond to the change in conditions. If changes occur quickly a transducer with a fast response time may be required. Linearity This is especially important when using transducers in measuring instruments Sensitivity If changes in physical conditions produce only small changes in the electrical properties of an input transducer, then a differential amplifier (covered in later units and at Higher) may be required to amplify the small changes before further processing can take place. Physical Size This may be an important criterion dependent on the system the transducer is to be placed in. (e.g. a loudspeaker may be inappropriate as an output transducer for a personal stereo). Robustness This may be dependent on the environment that the transducer is exposed to ( or the users will be exposed to) Accuracy Accuracy of the transducer could be of the utmost importance in some situations. Repeatability The ability of a transducer to consistently reproduce the reading for the same conditions. Cost Given all the above, is the transducer cost effective for the application? Full technical details of transducers and all electronic components are contained in manufactures data sheets and increasingly in catalogues. (RS Components supply a range of data sheets plus the RS Catalogue on CD-ROM. The CD-ROM contains product technical details as well as data files in pdf. format that can be easily accessed and printed off if hard copies are required.) Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2 12

53 Assignments: Use of manufacturer s data sheets or CD-ROM. Use manufacturers data sheets to answer the following questions 1. A kiln used in a brick making process has to heat the bricks to a temperature of C. Which temperature input transducer would be most suitable for monitoring the kiln temperature and why? 2. A photographer wants to time how long his flash light bulb comes on for when he takes a photograph. To do this, he connects a light sensor to a timer as shown below. AE.Int2.O2.fig10 With reference to appropriate manufacturers data sheets, decide which of the following light input transducers would be most suitable and why: - LDR, Selenium cell, and photo diode. 3. Most computers use LEDs as indicators to show various conditions. Why are LEDs used in preference to normal filament bulbs? Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2 13

54 INPUT SIGNALS Voltage divider Circuits If an input transducer changes it s resistance as the physical conditions change, then the resistance change has to be first converted into a voltage change before the signal can be processed. This is normally done by using a voltage divider circuit. If two or more resistors are connected in series (see figure 11 below), the voltage over each resistor will depend on the supply voltage and the ratio of the resistances. In Outcome 1, you have already investigated circuits that have two or more resistors in series in them and you will recall that changing the value of one of the resistors will have the effect of changing the voltage dropped across that resistor. In other words they were voltage divider circuits. Voltage divider circuits work on the basic electrical principle that if two resistors are connected in series across a supply, the voltage load across each of the resistors will be proportional to the value of the resistors. AE.Int2.O2.fig11 The layouts of voltage divider circuits are conventionally represented as shown above in fig 11. There are a number of different ways that a voltage divider circuit can be represented. Some of these are shown in fig 12. Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2 14

55 AE.Int2.O2.fig12 These three diagrams each represent the same circuit, in slightly different form. This should not be a cause for concern since all that has occurred is that the diagram has been rotated around on it s side. The circuit diagram shown on the left is of the type used in Outcome 1. The reason for the change to the style on the right is simply so that inputs and outputs can easily be added to the circuit. As we progress through this section and onto more advanced circuits, it will become apparent to you why these circuits are positioned as they are. Consider fig 11 Increasing the value of one of the resistors will increase the voltage drop across it. (You can use Ohm s Law to confirm this if you wish). When monitoring physical conditions, one of the resistors in the circuit is an input transducer, the resistance of which will change depending on the physical conditions. In general to calculate the voltage across any resistor in a series circuit, we can use the equation. Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2 15

56 Voltage across resistor R = Supply Voltage x (size of resistor R/ Total resistance) For fig 11 R2 V2= Vcc R1+ R2 Worked Example Calculate the voltage signal, V2 across the resistor R2, in the voltage divider circuit shown. AE.Int2.O2.fig 13 Applying the voltage proportion formula R2 V2= Vcc R1+ R2 40 V2= V2= 4V The voltage over the 80K resistor could be calculated in the same way, but this is unnecessary for this circuit since we can use Kirchoff s 2nd Law to confirm the answer. i.e. the voltages over each of the components in a series circuit must add up to the supply voltage, hence the voltage over the 80K resistor is 12V - 4V = 8V. It is also possible to continue to use Ohm s Law to solve these voltage divider questions. You may choose whichever method you are most comfortable with. Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2 16

57 Obtaining a signal voltage from a voltage divider circuit If we were to replace one of the fixed resistors in a voltage divider circuit with an analogue sensor, e.g. a Thermistor, we would now have a system which generates a signal voltage which is proportional to the change in the physical environment, in this case temperature. If you look at the E & L or Alpha analogue input boards you will find that this is the method used to generate signal voltages. AE.Int2.O2.fig 13a V sig changes in proportion with the resistance of the Thermistor R th. The Thermistor in fig 13a can be replaced by any analogue sensor, e.g. the LDR and will generate a signal voltage proportional to the resistance of the sensor. Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2 17

58 Assignments: Voltage divider equation 1. Using the formula described above, calculate the voltages that would appear across each of the resistors marked X in the circuits below. 2. In each of the following voltage divider circuits determine the unknown quantity. AE.Int2.O2.fig16 Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2 18

59 3. A ntc (negative temperature coefficient) Thermistor is used in a voltage divider circuit as shown in fig 17. 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 a) 80 0 C b) 20 0 C AE.Int2.O2.fig17 4. What would happen to the voltage across the Thermistor in the circuit shown in fig 17 as the temperature is increased? 5. What would happen to the voltage across the resistor in the circuit shown in fig 17 as the temperature increases? 6. A Thermistor (type 5) is used in a voltage divider circuit as shown in fig 18. The characteristics of the Thermistor are shown in the graph. If the voltage V 2 is to be 4.5V at C, determine a suitable value for R 1. State whether the V 2 will increase or decrease as the temperature drops. Explain your answer. AE.Int2.O2.fig18 Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2 19

60 Sensing circuits Light sensors Obtain the relevant components and equipment then construct the light sensing circuit shown in fig 19. AE.Int2.O2.fig 19 In normal light conditions, measure and record the voltage across the LDR and the fixed 10 K resistor. Cover the LDR, repeat the measurements and record them. You should have found in this circuit configuration that the resistance of the LDR increases as the light level decreases, so in this case the signal level (V out ) will rise as it gets dark. Change the position of the LDR and the fixed resistor as shown in fig.20. AE.Int2.O2.fig 20 Repeat the measurements taken on the first circuit record these. You should have found that changing the position of the LDR and the fixed resistor allows the signal to change in the opposite direction i.e. the signal level will rise as it gets light. Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2 20

61 Temperature sensors This is a very similar arrangement to light sensing and like the light sensing circuits, temperature sensing circuits can be arranged to produce a signal to move in the opposite direction when the same temperature change is applied. Obtain the relevant components and equipment then construct the light sensing circuit shown in fig 21. Use a type 3 Thermistor (TH3). AE.Int2.O2.fig 21 In normal room temperature conditions, measure and record the voltage across the Thermistor and the fixed resistor. Apply heat to the Thermistor and repeat the measurements and record them. Try reversing the positions of the Thermistor and the fixed resistor. Record what happens. Technological Studies Support Materials: Applied Electronics (Intermediate 2) Outcome 2 21