Performance-based assessments for AC circuit competencies

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Performance-based assessments for AC circuit competencies This worksheet and all related files are licensed under the Creative Commons Attribution License, version 1.0. To view a copy of this license, visit http://creativecommons.org/licenses/by/1.0/, or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA. The terms and conditions of this license allow for free copying, distribution, and/or modification of all licensed works by the general public. The purpose of these assessments is for instructors to accurately measure the learning of their electronics students, in a way that melds theoretical knowledge with hands-on application. In each assessment, students are asked to predict the behavior of a circuit from a schematic diagram and component values, then they build that circuit and measure its real behavior. If the behavior matches the predictions, the student then simulates the circuit on computer and presents the three sets of values to the instructor. If not, then the student then must correct the error(s) and once again compare measurements to predictions. Grades are based on the number of attempts required before all predictions match their respective measurements. You will notice that no component values are given in this worksheet. The instructor chooses component values suitable for the students parts collections, and ideally chooses different values for each student so that no two students are analyzing and building the exact same circuit. These component values may be hand-written on the assessment sheet, printed on a separate page, or incorporated into the document by editing the graphic image. This is the procedure I envision for managing such assessments: 1. The instructor hands out individualized assessment sheets to each student. 2. Each student predicts their circuit s behavior at their desks using pencil, paper, and calculator (if appropriate). 3. Each student builds their circuit at their desk, under such conditions that it is impossible for them to verify their predictions using test equipment. Usually this will mean the use of a multimeter only (for measuring component values), but in some cases even the use of a multimeter would not be appropriate. 4. When ready, each student brings their predictions and completed circuit up to the instructor s desk, where any necessary test equipment is already set up to operate and test the circuit. There, the student sets up their circuit and takes measurements to compare with predictions. 5. If any measurement fails to match its corresponding prediction, the student goes back to their own desk with their circuit and their predictions in hand. There, the student tries to figure out where the error is and how to correct it. 6. Students repeat these steps as many times as necessary to achieve correlation between all predictions and measurements. The instructor s task is to count the number of attempts necessary to achieve this, which will become the basis for a percentage grade. 7. (OPTIONAL) As a final verification, each student simulates the same circuit on computer, using circuit simulation software (Spice, Multisim, etc.) and presenting the results to the instructor as a final pass/fail check. These assessments more closely mimic real-world work conditions than traditional written exams: Students cannot pass such assessments only knowing circuit theory or only having hands-on construction and testing skills they must be proficient at both. Students do not receive the authoritative answers from the instructor. Rather, they learn to validate their answers through real circuit measurements. Just as on the job, the work isn t complete until all errors are corrected. Students must recognize and correct their own errors, rather than having someone else do it for them. Students must be fully prepared on exam days, bringing not only their calculator and notes, but also their tools, breadboard, and circuit components. Instructors may elect to reveal the assessments before test day, and even use them as preparatory labwork and/or discussion questions. Remember that there is absolutely nothing wrong with teaching to 1

the test so long as the test is valid. Normally, it is bad to reveal test material in detail prior to test day, lest students merely memorize responses in advance. With performance-based assessments, however, there is no way to pass without truly understanding the subject(s). 2

Question 1 Questions Competency: Analog oscilloscope set-up Volts/Div A 0.5 0.2 0.1 1 50 m 2 20 m Position 5 10 m Sec/Div 250 µ 1 m 50 µ10 5 m µ 25 m 2.5 µ 100 m 0.5 µ 10 5 m 500 m 0.1 µ 20 2 m DC Gnd AC 1 2.5 X-Y 0.025 µ off V signal A B Alt Chop Add Volts/Div B 0.5 0.2 0.1 1 50 m Position 2 20 m 5 10 m Invert 10 5 m Intensity Focus Beam find 20 2 m DC Gnd AC Off Cal 1 V Gnd Trace rot. Norm Auto Single Reset Position Triggering Level A B Alt Holdoff Line Ext. Ext. input AC DC LF Rej Slope HF Rej V signal = Set by instructor without student s knowledge f signal = Set by instructor without student s knowledge Between two and five cycles of waveform displayed, without exceeding either top or bottom edge of screen. file 01674 3

Question 2 Competency: Digital oscilloscope set-up Volts/Div A 0.5 0.2 0.1 1 50 m 2 20 m Position 5 10 m Sec/Div 250 µ 1 m 50 µ10 5 m µ 25 m 2.5 µ 100 m 0.5 µ 10 5 m 500 m 0.1 µ 20 2 m DC Gnd AC 1 2.5 X-Y 0.025 µ off V signal A B Alt Chop Add Volts/Div B 0.5 0.2 0.1 1 50 m Position 2 20 m 5 10 m Invert 10 5 m Intensity Focus Beam find 20 2 m DC Gnd AC Off Cal 1 V Gnd Trace rot. Norm Auto Single Reset Position Triggering Level A B Alt Holdoff Line Ext. Ext. input AC DC LF Rej Slope HF Rej V signal = Set by instructor without student s knowledge f signal = Set by instructor without student s knowledge Between two and five cycles of waveform displayed, without exceeding either top or bottom edge of screen. file 03305 4

Question 3 Competency: RMS versus peak measurements Volts/Div A 0.5 0.2 0.1 1 50 m 2 20 m Position 5 10 m Sec/Div 250 µ 1 m 50 µ10 5 m µ 25 m 2.5 µ 100 m 0.5 µ 10 5 m 500 m 0.1 µ V Meter V signal 20 2 m DC Gnd AC A B Alt Chop Add Volts/Div B 0.5 0.2 0.1 1 50 m Position 2 20 m 5 10 m Invert 10 5 m Intensity Focus Beam find 20 2 m DC Gnd AC Off Cal 1 V Gnd Trace rot. Norm Auto Single Reset 1 0.025 µ 2.5 off X-Y Position Triggering Level A B Alt Holdoff Line Ext. Ext. input AC DC LF Rej Slope HF Rej V signal = Set by instructor without student s knowledge V signal(peak) ( with oscilloscope) V signal(rms) ( with true-rms multimeter) file 01693 5

Question 4 Competency: Measuring frequency V signal V signal = f signal = Set by instructor without student s knowledge with oscilloscope t period with counter with oscilloscope f signal file 01660 6

Question 5 Competency: Probe compensation adjustment Volts/Div A 0.5 0.2 0.1 1 50 m 2 20 m Position 5 10 m Sec/Div 250 µ 1 m 50 µ10 5 m µ 25 m 2.5 µ 100 m 0.5 µ 10 5 m 20 2 m DC Gnd AC A B Alt Chop Add Volts/Div B 0.5 0.2 0.1 1 50 m Position 2 20 m 5 10 m Invert 10 5 m Intensity Focus Beam find 20 2 m DC Gnd AC Off Cal 1 V Gnd Trace rot. Norm Auto Single Reset 500 m 0.1 µ 1 0.025 µ 2.5 off X-Y Position Triggering Level A B Alt Holdoff Line Ext. Ext. input AC DC LF Rej Slope HF Rej (Use the built-in square wave signal source in your oscilloscope, and use a 10 probe to measure it) V signal Between two and five cycles of waveform displayed, without exceeding either top or bottom edge of screen. f signal Overcompensated probe waveform Properly compensated probe waveform Undercompensated probe waveform file 01823 7

Question 6 Competency: Inductive reactance and Ohm s Law for AC V supply L 1 V supply = L 1 = f supply = I total V L1 I total I total Qualitative answers only: Increase, decrease, or same as frequency increases as frequency decreases file 01616 8

Question 7 Competency: Series AC inductive reactances V supply L 1 L 2 V supply = L 1 = L 2 = f supply = X L1 X L2 X total Derived (X = V/I) I total X total Analysis Do series inductive reactances add or diminish? Fault analysis Suppose component fails What will happen in the circuit? open other shorted Write "increase", "decrease", or "no change" for each parameter: V L1 I L1 I total V L2 I L2 file 01655 9

Question 8 Competency: Series LR circuit L 1 V supply R 1 V supply = L 1 = R 1 = f supply = V L1 V R1 I total Calculations Fault analysis Suppose component fails What will happen in the circuit? open other shorted Write "increase", "decrease", or "no change" for each parameter: V L1 I L1 V R1 I R1 I total file 01665 10

Question 9 Competency: Parallel LR circuit V supply L 1 R 1 V supply = L 1 = R 1 = f supply = I L1 I R1 I total Calculations Fault analysis Suppose component fails What will happen in the circuit? open other shorted Write "increase", "decrease", or "no change" for each parameter: I L1 V L1 I R1 V R1 I total file 01816 11

Question 10 Competency: Capacitive reactance and Ohm s Law for AC V supply C 1 V supply = C 1 = f supply = I total V C1 I total I total Qualitative answers only: Increase, decrease, or same as frequency increases as frequency decreases file 01617 12

Question 11 Competency: Measuring capacitance by AC reactance A V signal C x V signal = Calculated I C at f = C x I C at f = C x I C at f = C x C x (With C meter) C x (Average) file 01920 13

Question 12 Competency: Series AC capacitive reactances C 1 V supply C 2 V supply = C 1 = C 2 = f supply = X C1 X C2 X total Derived (X = V/I) I total X total Analysis Do series capacitive reactances add or diminish? Fault analysis Suppose component fails What will happen in the circuit? open other shorted Write "increase", "decrease", or "no change" for each parameter: V C1 I C1 I total V C2 I C2 file 01656 14

Question 13 Competency: Capacitive AC voltage divider C 1 V supply C 2 V out V supply = C 1 = C 2 = f supply = V out at specified frequency Qualitative answers only: Increase, decrease, or same V out as frequency increases V out as frequency decreases file 01662 15

Question 14 Competency: Series RC circuit C 1 V supply R 1 V supply = C 1 = R 1 = f supply = V C1 V R1 I total Calculations Fault analysis Suppose component fails What will happen in the circuit? open other shorted Write "increase", "decrease", or "no change" for each parameter: V C1 I C1 V R1 I R1 I total file 01664 16

Question 15 Competency: Parallel RC circuit V supply C 1 R 1 V supply = C 1 = R 1 = f supply = I C1 I R1 I total Calculations Fault analysis Suppose component fails What will happen in the circuit? open other shorted Write "increase", "decrease", or "no change" for each parameter: I C1 V C1 I R1 V R1 I total file 01817 17

Question 16 Competency: Time-domain phase shift measurement Volts/Div A 0.5 0.2 0.1 1 50 m 2 20 m Position 5 10 m Sec/Div 250 µ 1 m 50 µ10 5 m µ 25 m 2.5 µ 100 m 0.5 µ 10 5 m 500 m 0.1 µ C 1 20 2 m DC Gnd AC 1 2.5 X-Y 0.025 µ off R 1 A B Alt Chop Add Volts/Div B 0.5 0.2 0.1 1 50 m Position 2 20 m 5 10 m Invert 10 5 m Intensity Focus Beam find 20 2 m DC Gnd AC Off Cal 1 V Gnd Trace rot. Norm Auto Single Reset Position Triggering Level A B Alt Holdoff Line Ext. Ext. input AC DC LF Rej Slope HF Rej f supply = C 1 = R 1 = Dual oscilloscope trace θ Shift (divisions) Period (divisions) θ Period Shift file 01688 18

Question 17 Competency: Lissajous figures for phase shift measurement Volts/Div A 0.5 0.2 0.1 1 50 m 2 20 m Position 5 10 m Sec/Div 250 µ 1 m 50 µ10 5 m µ 25 m 2.5 µ 100 m 0.5 µ 10 5 m 500 m 0.1 µ C 1 20 2 m DC Gnd AC 1 2.5 X-Y 0.025 µ off R 1 A B Alt Chop Add Volts/Div B 0.5 0.2 0.1 1 50 m Position 2 20 m 5 10 m Invert 10 5 m Intensity Focus Beam find 20 2 m DC Gnd AC Off Cal 1 V Gnd Trace rot. Norm Auto Single Reset Position Triggering Level A B Alt Holdoff Line Ext. Ext. input AC DC LF Rej Slope HF Rej f supply = C 1 = R 1 = Lissajous figure (measured) θ n (Divisions) m (Divisions) θ n m n file 01676 19

Question 18 Competency: Phase shift circuit Description Build an RC circuit to produce the specified phase shift from input to output, and measure this shift with an oscilloscope. θ = f supply = Leading (instructor checks one) Lagging V signal V out Shift (divisions) Dual oscilloscope trace Period (divisions) θ Period C 1 R 1 Shift file 02170 20

Question 19 Competency: Variable phase shift bridge circuit V signal R 1 R pot C 2 C 1 V out R 2 V signal = f signal = R 1 = R 2 = C 1 = C 2 = R pot = Recommendations R 1 2πfC R pot >> R 1, R 2 θ Vout Potentiometer at full-left position θ Vout Potentiometer at full-right position V out Potentiometer at full-left position V out Potentiometer at full-right position file 03468 21

Question 20 Competency: Differential voltage measurement R 1 V supply R 2 R 3 Volts/Div A 0.5 0.2 0.1 50 m 1 Position 20 m 2 5 10 m 10 5 m 20 2 m DC Gnd AC A B Alt Chop Add Volts/Div B 0.5 0.2 0.1 1 50 m Position 2 20 m 5 10 m Invert 10 5 m Intensity Focus Beam find 20 2 m DC Gnd AC Off Cal 1 V Gnd Trace rot. Norm Auto Single Reset Sec/Div 250 µ 1 m 50 µ10 5 m µ 25 m 2.5 µ 100 m 0.5 µ 500 m 0.1 µ 1 0.025 µ 2.5 off X-Y Position Triggering Level A B Alt Holdoff Line Ext. Ext. input AC DC LF Rej Slope HF Rej V supply = R 1 = R 2 = R 3 = V R2 Fault analysis Suppose component fails What will happen in the circuit? open shorted other file 01822 22

Question 21 Competency: Measuring decibels R 1 R 2 R 3 R 4 A B C D E V supply V Volts RMS / dbv V supply = 1 volt RMS = 0 dbv R 1 = R 2 = R 3 = R 4 = (V) (dbv) V A (Establishing reference voltage) V B (V) (dbv) (V) (dbv) V C V D V E file 03456 23

Question 22 Competency: Series-parallel RC circuit R 1 V signal C 1 R 2 V signal = f signal = C 1 = R 1 = R 2 = V R1 I C1 I R2 I total Fault analysis Suppose component fails What will happen in the circuit? open shorted other file 01659 24

Question 23 Competency: Series-parallel RC circuit R 1 V signal C 1 R 2 C 2 V signal = R 1 = R 2 = f signal = C 1 = C 2 = V R1 V R2 V C2 I C1 I total Fault analysis Suppose component fails What will happen in the circuit? open shorted other file 01663 25

Question 24 Competency: Passive filter circuit C 1 V signal R 1 V out V signal = C 1 = R 1 = Filter type (hp, lp, bp, bs) f -3dB θ -3dB Calculations Fault analysis Suppose component fails What will happen in the circuit? open shorted other file 01613 26

Question 25 Competency: Passive filter circuit R 1 V signal C 1 V out V signal = C 1 = R 1 = Filter type (hp, lp, bp, bs) f -3dB θ -3dB Calculations Fault analysis Suppose component fails What will happen in the circuit? open shorted other file 01614 27

Question 26 Competency: Passive RC filter circuit design Description Design and build an RC filter circuit, either high pass or low pass, with the specified cutoff frequency. f -3dB = High-pass (instructor checks one) Low-pass f -3dB θ -3dB V signal V out file 02095 28

Question 27 Competency: Passive twin-tee filter circuit R 1 R 2 C 3 C 1 C 2 V signal R 3 V out V signal = C 1 = C 2 = C 3 = R 1 = R 2 = R 3 = Filter type (hp, lp, bp, bs) f center Fault analysis Suppose component fails What will happen in the circuit? open shorted other file 02580 29

Question 28 Competency: Series resonance R 1 C 1 V signal L 1 R 1 = C 1 = L 1 = f resonant Calculations Fault analysis Suppose component fails What will happen in the circuit? open shorted other file 01690 30

Question 29 Competency: Dampened resonant oscillations Volts/Div A 0.5 0.2 0.1 1 50 m 2 20 m Position 5 10 m Sec/Div 250 µ 1 m 50 µ10 5 m µ 25 m 2.5 µ 100 m 0.5 µ 10 5 m 500 m 0.1 µ V supply C 1 L1 20 2 m DC Gnd AC A B Alt Chop Add Volts/Div B 0.5 0.2 0.1 1 50 m Position 2 20 m 5 10 m Invert 10 5 m Intensity Focus Beam find 20 2 m DC Gnd AC Off Cal 1 V Gnd Trace rot. Norm Auto Single Reset 1 0.025 µ 2.5 off X-Y Position Triggering Level A B Alt Holdoff Line Ext. Ext. input AC DC LF Rej Slope HF Rej L 1 = C 1 = Captured waveform f Typical waveform file 01689 31

Question 30 Competency: Measuring inductance by series resonance C 1 V signal R 1 L x Measure voltage drop with oscilloscope R 1 = C 1 = f resonant Inferred from f resonant with LCR meter L x Calculations file 01691 32

Question 31 Competency: Passive resonant filter circuit C 1 L 1 V signal R load V signal = C 1 = L 1 = R load = Filter type (hp, lp, bp, bs) f resonant θ resonant Calculations Fault analysis Suppose component fails What will happen in the circuit? open shorted other file 01615 33

Question 32 Competency: Passive resonant filter circuit C 1 V signal L 1 R load V signal = C 1 = L 1 = R load = Filter type (hp, lp, bp, bs) f resonant Calculations Fault analysis Suppose component fails What will happen in the circuit? open other shorted Identify changes in filtering type and frequency, if any Filter type (hp, lp, bp, bs) f resonant file 01658 34

Question 33 Competency: Passive LC filter circuit design Description Design and build a passive LC filter circuit with a resonant frequency specified by the instructor. f r = Band-pass (instructor checks one) Band-stop f r V signal V out file 02097 35

Question 34 Competency: Passive differentiator circuit C 1 V signal R 1 V out Required V out waveshape f signal = C 1 R 1 Fault analysis Suppose component fails What will happen in the circuit? open other shorted Identify changes to the output voltage waveshape, if any file 01686 36

Question 35 Competency: Passive integrator circuit R 1 V signal C 1 V out Required waveshapes f signal = V in V out C 1 R 1 Fault analysis Suppose component fails What will happen in the circuit? open other shorted Identify changes to the output voltage waveshape, if any file 01687 37

Question 36 Competency: Passive integrator/differentiator circuit Description Build either a passive integrator or a passive differentiator circuit to produce the desired waveforms given a square-wave input. Integrator f signal = (instructor checks one) Differentiator V out = 1 / 3 V in 5τ in each pulse width V signal V out C 1 R 1 file 02169 38

Question 37 Competency: Tone balance control circuit L 1 R 1 Headphones R pot1 R pot2 C 1 R 2 L 1 = C 1 = R pot1 = R pot2 = (Use audio source for signal, voltage adjusted for ample volume) R 1 = R 2 = Z headphones = Bass control Treble control (Identify which pot is which) Frequency response (maximum bass, minimum treble) Frequency response (minimum bass, maximum treble) V out V out f f Note: when testing the frequency response of the tone control circuit, you may need to replace the headphones with a non-inductive resistor of equivalent impedance, and measure V out across it. file 02022 39

Question 38 Competency: Tone balance control circuit L 1 R pot1 R 1 T 1 Speaker R pot2 C 1 R 2 L 1 = C 1 = R pot1 = R pot2 = R 1 = R 2 = Z speaker = T 1 = (Use audio source for signal, voltage adjusted for ample volume) Bass control Treble control (Identify which pot is which) Frequency response (maximum bass, minimum treble) Frequency response (minimum bass, maximum treble) V out V out f f Note: when testing the frequency response of the tone control circuit, you may need to replace the transformer/speaker assembly with a non-inductive resistor of equivalent impedance, and measure V out across it. file 02023 40

Question 39 Competency: Noise coupling and decoupling Cable length Must be a battery! 1a 2a 3a 4a 1b 2b 3b 4b Mtr C decoupling C decoupling = Cable length = values are qualitative only (least, greatest, little bit less than...) V noise (1b-2b) V noise (2b-3b) Without C decoupling V noise (3b-4b) V noise (1b-2b) V noise (2b-3b) With C decoupling connected across test points V noise (3b-4b) file 01692 41

Question 40 Competency: Transformer voltage ratio V primary V secondary Transformer winding (turns) ratio = V primary = f = V sec Calculations file 03655 42

Question 41 Competency: Transformer voltage/current ratios Power transformer Line voltage R load Terminal block connections There must be no exposed line power conductors! Transformer step-down ratio = V line = R load = Procedure for ensuring no shock hazard when connecting meter: Description (written by student) Observed (written by instructor) V pri V sec I pri I sec file 01675 43

Question 42 Competency: Transformer voltage/current ratios Audio transformer V source R load Terminal block connections 1000:8 Ω Transformer winding (turns) ratio = V source = R load = f source = V pri V sec I pri I sec Calculations file 02122 44

Question 43 Competency: Auto-transformers Description Connect a step-down power transformer in the "buck" auto-transformer configuration, and use it to power a low-resistance load. There must be no exposed line power conductors! V supply = R load = Transformer step-down ratio = I supply V load I load file 01611 45

Question 44 Competency: Auto-transformers Description Connect a step-down power transformer in the "boost" auto-transformer configuration, and use it to power a low-resistance load. There must be no exposed line power conductors! V supply = R load = Transformer step-down ratio = I supply V load I load file 01612 46

Question 45 Competency: Auto-transformers Description Connect a step-down transformer as an autotransformer, to either boost or buck the input voltage. There must be no exposed line power conductors! V supply = Transformer step-down ratio = Boost Buck V out (Be sure to note the transformer s polarity using dot convention) V output file 02131 47

Question 46 Competency: Lissajous figures for frequency ratio measurement Horizontal Volts/Div A Sec/Div 0.5 0.2 250 µ 0.1 1 m 50 µ10 1 50 m 5 m µ 2 20 m Position 25 m 2.5 µ 5 10 m 100 m 0.5 µ 10 5 m 500 m 0.1 µ V signal1 V signal2 Vertical 20 2 m DC Gnd AC A B Alt Chop Add Volts/Div B 0.5 0.2 0.1 1 50 m Position 2 20 m 5 10 m Invert 10 5 m Intensity Focus Beam find 20 2 m DC Gnd AC Off Cal 1 V Gnd Trace rot. Norm Auto Single Reset 1 0.025 µ 2.5 off X-Y Position Triggering Level A B Alt Holdoff Line Ext. Ext. input AC DC LF Rej Slope HF Rej (Voltage specified by instructor, frequencies set by student) V signal1 and V signal2 = Lissajous figure f signal2 f signal1 file 01679 48

Question 47 Competency: Fourier analysis of a square wave Spectrum analyzer V supply = Duty cycle = 50% f fundamental = 1st "overtone" amplitude 1st "overtone" frequency 2nd "overtone" amplitude 2nd "overtone" frequency 3rd "overtone" amplitude 3rd "overtone" frequency file 01684 49

5 1 10 20 5 2 2 1 10 20 0.5 0.2 0.1 50 m 20 m 10 m 5 m 2 m 0.5 0.2 0.1 50 m 20 m 10 m 5 m 2 m Off Cal 1 V Gnd Trace rot. 25 m 100 m 1 m 5 m 500 m 1 2.5 off 2.5 µ 0.5 µ 0.1 µ 0.025 µ Question 48 Competency: Balanced attenuator circuit R 1 Volts/Div A Position DC Gnd AC Sec/Div 250 µ 50 µ10 µ X-Y V signal C 1 C 2 R 2 A B Alt Chop Add Volts/Div B DC Gnd AC Position Invert Intensity Focus Beam find Norm Auto Single Reset Position Triggering Level A B Alt Holdoff Line Ext. Ext. input AC DC LF Rej Slope HF Rej V signal = R 2 = f signal = C 1 = C 2 = R 1 (necessary to achieve undistorted square-wave output) Fault analysis Suppose component fails What will happen in the circuit? open shorted other file 01661 50

Question 49 Competency: Power factor correction for AC motor C 1 Oscilloscope channel 2 here ( 10 probe recommended) AC induction motor Oscilloscope ground here R shunt Oscilloscope channel 1 here Voltage/current display on scope Volts/div = Sec/div = R shunt = (Without C 1 ) (With C 1 connected) θ C 1 I total To correct P.F. to a value of unity file 01685 51

Question 50 Competency: Characteristic cable impedance R 1 A B Long length of cable R pot f supply = R 1 = R pot(max) = Velocity factor = Length of cable Z 0 V AB waveform with R pot set too low V AB waveform with R pot set too high V AB waveform with R pot = Z o file 01915 52

Question 51 (Template) Competency: file 01602 53

Answer 1 Answers You may use circuit simulation software to set up similar oscilloscope display interpretation scenarios, for practice or for verification of what you see in this exercise. Answer 2 You may use circuit simulation software to set up similar oscilloscope display interpretation scenarios, for practice or for verification of what you see in this exercise. Answer 3 You may use circuit simulation software to set up similar oscilloscope display interpretation scenarios, for practice or for verification of what you see in this exercise. Answer 4 You may use circuit simulation software to set up similar oscilloscope display interpretation scenarios, for practice or for verification of what you see in this exercise. Answer 5 There really isn t much you can do to verify your experimental results. That s okay, though, because the results are qualitative anyway. Answer 6 Use circuit simulation software to verify your predicted and measured parameter values. Answer 7 Use circuit simulation software to verify your predicted and measured parameter values. Answer 8 Use circuit simulation software to verify your predicted and measured parameter values. Answer 9 Use circuit simulation software to verify your predicted and measured parameter values. Answer 10 Use circuit simulation software to verify your predicted and measured parameter values. Answer 11 Use circuit simulation software to verify your predicted and measured parameter values. Answer 12 Use circuit simulation software to verify your predicted and measured parameter values. Answer 13 Use circuit simulation software to verify your predicted and measured parameter values. Answer 14 Use circuit simulation software to verify your predicted and measured parameter values. Answer 15 Use circuit simulation software to verify your predicted and measured parameter values. 54

Answer 16 Use circuit simulation software to verify your predicted and measured parameter values. Answer 17 Use circuit simulation software to verify your predicted and measured parameter values. Answer 18 Use circuit simulation software to verify your predicted and measured parameter values. Answer 19 Use circuit simulation software to verify your predicted and measured parameter values. Answer 20 The predicted value should be easy enough to figure out, given this is nothing more than a resistive voltage divider. If you are not configuring the oscilloscope properly, you will not get the correct measurement value! Answer 21 Use circuit simulation software to verify your predicted and measured parameter values. Answer 22 Use circuit simulation software to verify your predicted and measured parameter values. Answer 23 Use circuit simulation software to verify your predicted and measured parameter values. Answer 24 Use circuit simulation software to verify your predicted and measured parameter values. Answer 25 Use circuit simulation software to verify your predicted and measured parameter values. Answer 26 Use circuit simulation software to verify your predicted and measured parameter values. Answer 27 Use circuit simulation software to verify your predicted and measured parameter values. Answer 28 Use circuit simulation software to verify your predicted and measured parameter values. Answer 29 Use circuit simulation software to verify your predicted and measured parameter values. Answer 30 Use circuit simulation software to verify your predicted and measured parameter values. Answer 31 Use circuit simulation software to verify your predicted and measured parameter values. 55

Answer 32 Use circuit simulation software to verify your predicted and measured parameter values. Answer 33 Use circuit simulation software to verify your predicted and measured parameter values. Answer 34 Use circuit simulation software to verify your predicted and measured parameter values. Answer 35 Use circuit simulation software to verify your predicted and measured parameter values. Answer 36 Use circuit simulation software to verify your predicted and measured parameter values. Answer 37 Use circuit simulation software to verify your predicted and measured parameter values. Answer 38 Use circuit simulation software to verify your predicted and measured parameter values. Answer 39 Use circuit simulation software to verify your predicted and measured parameter values, using an AC current source as the motor, and a multi-conductor transmission line as the cable. Note: this may be quite complicated to set up in simulation! Answer 40 Use circuit simulation software to verify your predicted and measured parameter values. Answer 41 Use circuit simulation software to verify your predicted and measured parameter values. Answer 42 Use circuit simulation software to verify your predicted and measured parameter values. Answer 43 Use circuit simulation software to verify your predicted and measured parameter values. Answer 44 Use circuit simulation software to verify your predicted and measured parameter values. Answer 45 Use circuit simulation software to verify your predicted and measured parameter values. Answer 46 Use circuit simulation software to verify your predicted and measured parameter values. Answer 47 Use circuit simulation software to verify your predicted and measured parameter values. 56

Answer 48 Use circuit simulation software to verify your predicted and measured parameter waveforms. Note: this circuit works on the same basic principle as the compensation adjustment on high-quality oscilloscope probes, except that here the resistor is variable and not the capacitor. Answer 49 The meter measurements you take will constitute the final word for validating your predictions. Answer 50 The oscilloscope display will conclusively show when R pot = Z 0. Answer 51 Here, you would indicate where or how to obtain answers for the requested parameters, but not actually give the figures. My stock answer here is use circuit simulation software (Spice, Multisim, etc.). 57

Notes 1 Notes An oscilloscope is nothing more than a graphical voltmeter, having the ability to represent fast-changing voltages over time in the form of a graph. It is very important for all students of electronics to learn how to use an oscilloscope well! Use a sine-wave function generator for the AC voltage source, and be sure set the frequency to some reasonable value (well within the capability of both the oscilloscope and counter to measure). If this is not the first time students have done this, be sure to mess up the oscilloscope controls prior to them making adjustments. Students must learn how to quickly configure an oscilloscope s controls to display any arbitrary waveform, if they are to be proficient in using an oscilloscope as a diagnostic tool. General concepts and principles to emphasize Basic oscilloscope operation (vertical sensitivity, coupling, timebase, triggering) Location of ground connections on both the oscilloscope and signal generator Relationship between the period of a wave and its frequency Suggestions for Socratic discussion and experimentation What is the distinction between a waveform s peak voltage value and its peak-to-peak voltage value? Demonstrate how to make the waveform appear taller (i.e. each cycle extending farther along the vertical axis) on the oscilloscope s display. Demonstrate how to make the waveform appear longer (i.e. each cycle extending farther along the horizontal axis) on the oscilloscope s display. Demonstrate how to set the triggering value for the oscilloscope, and what difference that makes in the displayed waveform. 58

Notes 2 An oscilloscope is nothing more than a graphical voltmeter, having the ability to represent fast-changing voltages over time in the form of a graph. It is very important for all students of electronics to learn how to use an oscilloscope well! Use a sine-wave function generator for the AC voltage source, and be sure set the frequency to some reasonable value (well within the capability of both the oscilloscope and counter to measure). Some digital oscilloscopes have auto set controls which automatically set the vertical, horizontal, and triggering controls to lock in a waveform. Be sure students are learning how to set up these controls on their own rather than just pushing the auto set button! General concepts and principles to emphasize Basic oscilloscope operation (vertical sensitivity, coupling, timebase, triggering) Location of ground connections on both the oscilloscope and signal generator Relationship between the period of a wave and its frequency Suggestions for Socratic discussion and experimentation What is the distinction between a waveform s peak voltage value and its peak-to-peak voltage value? The standard unit of measurement for period is seconds (s). The standard unit of measurement for frequency is Hertz (Hz), which means cycles per second. However, an alternate unit of measurement for frequency is inverse seconds, or s 1. Explain why Hz = s 1. If you change the voltage level of the AC source, does this affect its frequency? Why or why not? Demonstrate how to make the waveform appear taller (i.e. each cycle extending farther along the vertical axis) on the oscilloscope s display. Demonstrate how to make the waveform appear longer (i.e. each cycle extending farther along the horizontal axis) on the oscilloscope s display. Demonstrate how to set the triggering value for the oscilloscope, and what difference that makes in the displayed waveform. 59

Notes 3 Use a sine-wave function generator for the AC voltage source, and be sure set the frequency to some reasonable value (well within the capability of a multimeter to measure). It is very important that students learn to convert between peak and RMS measurements for sine waves, but you might want to mix things up a bit by having them do the same with triangle waves and square waves as well! It is vital that students realize the rule of V RMS = V peak 2 only holds for sinusoidal signals. An important point to note in this exercise is that the variable adjustment on the vertical sensitivity for the input channel must be turned fully so that the knob clicks into its calibrated position. If this fine-adjustment is not set in the calibrated position, your voltage measurements will be inaccurate! If you do choose to challenge students with non-sinusoidal waveshapes, be very sure that they do their voltmeter measurements using true-rms meters! This means no analog voltmeters, which are miscalibrated so their inherently average-responding movements register (sinusoidal) RMS accurately. Your students must use true-rms digital voltmeters in order for their non-sinusoidal RMS measurements to correlate with their calculations. Incidentally, this lab exercise also works well as a demonstration of the importance of true-rms indicating meters, comparing the indications of analog, non-true-rms digital, and true-rms digital on the same non-sinusoidal waveform! General concepts and principles to emphasize The concept of Root-Mean-Square (RMS) measurement Basic oscilloscope operation (vertical sensitivity, coupling, timebase, triggering) Location of ground connections on both the oscilloscope and signal generator Suggestions for Socratic discussion and experimentation Explain what the RMS (Root-Mean-Square) value means for any AC waveform. If you change the frequency of the AC source, does this affect its RMS or peak voltage value? Why or why not? A common misconception among students is that the RMS value of any waveform is always equal to 0.707 times the waveform s peak value. Refute this misconception with a demonstrated example! 60

Notes 4 Use a sine-wave function generator for the AC voltage source, and be sure set the frequency to some reasonable value (well within the capability of both the oscilloscope and counter to measure). An important point to note in this exercise is that the variable adjustment on the horizontal timebase must be turned fully so that the knob clicks into its calibrated position. If this fine-adjustment is not set in the calibrated position, your time measurements (and therefore your frequency calculations) will be inaccurate! General concepts and principles to emphasize Relationship between the period of a wave and its frequency Basic oscilloscope operation (vertical sensitivity, coupling, timebase, triggering) Suggestions for Socratic discussion and experimentation The standard unit of measurement for period is seconds (s). The standard unit of measurement for frequency is Hertz (Hz), which means cycles per second. However, an alternate unit of measurement for frequency is inverse seconds, or s 1. Explain why Hz = s 1. If you change the voltage level of the AC source, does this affect its frequency? Why or why not? Notes 5 If the oscilloscope does not have its own internal square-wave signal source, use a function generator set up to output square waves at 1 volt peak-to-peak at a frequency of 1 khz. If this is not the first time students have done this, be sure to mess up the oscilloscope controls prior to them making adjustments. Students must learn how to quickly configure an oscilloscope s controls to display any arbitrary waveform, if they are to be proficient in using an oscilloscope as a diagnostic tool. General concepts and principles to emphasize Basic oscilloscope operation (vertical sensitivity, coupling, timebase, triggering) Relationship between the period of a wave and its frequency Suggestions for Socratic discussion and experimentation Demonstrate how to make the waveform appear taller (i.e. each cycle extending farther along the vertical axis) on the oscilloscope s display. Demonstrate how to make the waveform appear longer (i.e. each cycle extending farther along the horizontal axis) on the oscilloscope s display. Explain why changes in the triggering level don t seem to affect the displayed waveform at all. 61

Notes 6 In this exercise you will explore the concept of reactance, which is a form of opposition to the flow of electric current that is similar to but not identical to resistance. The distinction between reactance and resistance is a consequence of energy flow in an electrical component: components which transfer electrical energy out of the circuit and into some other form (such as heat in the case of a resistor or mechanical work in the case of an electric motor) exhibit resistance; components which have the ability to absorb energy from the circuit and return energy back into the circuit (e.g. inductors and capacitors) exhibit reactance. In order to achieve results closely approximating ideal, you need to use an inductor that has very little wire resistance (the less, the better!). Your multimeter will be useful here to measure the DC resistance of the inductor which is strictly a function of wire resistance and not inductance and allow you to identify the most suitable inductor for this exercise. The problem with resistance is that it is a fundamentally different kind of opposition to electric current. If your inductor happens to possess a lot of resistance (in addition to inductive reactance), the results will be skewed by the presence of this resistance. Use a sine-wave function generator for the AC voltage source. I recommend against using line-power AC because of strong harmonic frequencies which may be present (due to nonlinear loads operating on the same power circuit). If students are to use a multimeter to make their current and voltage measurements, be sure it is capable of accurate measurement at the circuit frequency! Inexpensive digital multimeters often experience difficulty measuring AC voltage and current toward the high end of the audio-frequency range. General concepts and principles to emphasize Self-induction (i.e. how a single coil is able to experience induction as a consequence of a changing current passing through it) The concept of reactance, and how it differs from resistance The relationship between inductance, frequency, and inductive reactance Suggestions for Socratic discussion and experimentation Identify ways the inductor could be modified so as to possess more inductance. Explain how reactance differs from resistance, even though they are both quantified using the same unit of measurement: ohms (Ω). Explain why current changes as source frequency changes, despite voltage remaining the same. A technique used in making wire-wound resistors is something called a bifilar winding. Bifilar windings have nearly zero inductance. Research how a bifilar winding is made, and then explain how it works to minimize induction and also why this might be important for making resistors. Notes 7 Use a sine-wave function generator for the AC voltage source. I recommend against using line-power AC because of strong harmonic frequencies which may be present (due to nonlinear loads operating on the same power circuit). Specify standard inductor values. If the inductors are not well shielded, be sure to keep them adequately separated so as to avoid mutual inductance! If students are to use a multimeter to make their current and voltage measurements, be sure it is capable of accurate measurement at the circuit frequency! Inexpensive digital multimeters often experience difficulty measuring AC voltage and current toward the high end of the audio-frequency range. 62

Notes 8 In order to achieve results closely approximating ideal, you need to use an inductor that has very little wire resistance (the less, the better!). Your multimeter will be useful here to measure the DC resistance of the inductor which is strictly a function of wire resistance and not inductance and allow you to identify the most suitable inductor for this exercise. The problem with resistance is that it is a fundamentally different kind of opposition to electric current. If your inductor happens to possess a lot of resistance (in addition to inductive reactance), the results will be skewed by the presence of this resistance. Use a sine-wave function generator for the AC voltage source. I recommend against using line-power AC because of strong harmonic frequencies which may be present (due to nonlinear loads operating on the same power circuit). Specify standard resistor and inductor values. If students are to use a multimeter to make their current and voltage measurements, be sure it is capable of accurate measurement at the circuit frequency! Inexpensive digital multimeters often experience difficulty measuring AC voltage and current toward the high end of the audio-frequency range. An extension of this exercise is to incorporate troubleshooting questions. Whether using this exercise as a performance assessment or simply as a concept-building lab, you might want to follow up your students results by asking them to predict the consequences of certain circuit faults. General concepts and principles to emphasize The concept of reactance, and how it differs from resistance The concept of impedance, and how it relates to both resistance and reactance The relationship between inductance, frequency, and inductive reactance Suggestions for Socratic discussion and experimentation Explain how reactance differs from resistance, even though they are both quantified using the same unit of measurement: ohms (Ω). What will happen to the amount of current in this circuit if the source frequency is increased (assuming a constant source voltage)? What will happen to the amount of current in this circuit if the inductor is replaced by another having greater inductance? What will happen to the amount of current in this circuit if the resistor is replaced by another having greater resistance? Explain why the sum of the resistor s voltage and the inductor s voltage does not equal the source voltage. Notes 9 Use a sine-wave function generator for the AC voltage source. I recommend against using line-power AC because of strong harmonic frequencies which may be present (due to nonlinear loads operating on the same power circuit). Specify standard resistor and inductor values, and select a frequency that results in the inductor having a high Q value, so that its parasitic resistance does not become a significant factor in the calculations. If students are to use a multimeter to make their current and voltage measurements, be sure it is capable of accurate measurement at the circuit frequency! Inexpensive digital multimeters often experience difficulty measuring AC voltage and current toward the high end of the audio-frequency range. An extension of this exercise is to incorporate troubleshooting questions. Whether using this exercise as a performance assessment or simply as a concept-building lab, you might want to follow up your students results by asking them to predict the consequences of certain circuit faults. 63

Notes 10 Use a sine-wave function generator for the AC voltage source. I recommend against using line-power AC because of strong harmonic frequencies which may be present (due to nonlinear loads operating on the same power circuit). Specify a standard capacitor value. If students are to use a multimeter to make their current and voltage measurements, be sure it is capable of accurate measurement at the circuit frequency! Inexpensive digital multimeters often experience difficulty measuring AC voltage and current toward the high end of the audio-frequency range. Notes 11 In this exercise you will explore the concept of reactance, which is a form of opposition to the flow of electric current that is similar to but not identical to resistance. The distinction between reactance and resistance is a consequence of energy flow in an electrical component: components which transfer electrical energy out of the circuit and into some other form (such as heat in the case of a resistor or mechanical work in the case of an electric motor) exhibit resistance; components which have the ability to absorb energy from the circuit and return energy back into the circuit (e.g. inductors and capacitors) exhibit reactance. Use a sine-wave function generator for the AC voltage source. I recommend against using line-power AC because of strong harmonic frequencies which may be present (due to nonlinear loads operating on the same power circuit). Specify a standard capacitor value. If students are to use a multimeter to make their current and voltage measurements, be sure it is capable of accurate measurement at the circuit frequency! Inexpensive digital multimeters often experience difficulty measuring AC voltage and current toward the high end of the audio-frequency range. General concepts and principles to emphasize The concept of reactance, and how it differs from resistance The relationship between capacitance, frequency, and capacitive reactance How to interpret numerical capacitor value codes Suggestions for Socratic discussion and experimentation Identify ways the capacitor could be modified so as to possess more capacitance. How well would this experiment work if the source was DC instead of AC? Explain why current changes as source frequency changes, despite voltage remaining the same. If two identical capacitors were connected in series, what would happen to total capacitance in the circuit? What would happen to total capacitive reactance in the circuit? If two identical capacitors were connected in parallel, what would happen to total capacitance in the circuit? What would happen to total capacitive reactance in the circuit? 64

Notes 12 Use a sine-wave function generator for the AC voltage source. I recommend against using line-power AC because of strong harmonic frequencies which may be present (due to nonlinear loads operating on the same power circuit). Specify standard capacitor values. If students are to use a multimeter to make their current and voltage measurements, be sure it is capable of accurate measurement at the circuit frequency! Inexpensive digital multimeters often experience difficulty measuring AC voltage and current toward the high end of the audio-frequency range. Students often become confused when learning about series and parallel capacitive reactances, because many think capacitors are inherently opposite of inductors and resistors in all respects. In other words, they first learn that capacitance (measured in Farads) adds in parallel and diminishes in series, and that this is just the opposite of resistance (Ohms) and inductance (Henrys), and they mistakenly carry this thinking on through capacitive reactance (Ohms) as well. The real lesson of this exercise is that reactances add in series, regardless of their nature. Notes 13 Use a sine-wave function generator for the AC voltage source. I recommend against using line-power AC because of strong harmonic frequencies which may be present (due to nonlinear loads operating on the same power circuit). Specify standard capacitor values. If students are to use a multimeter to make their current and voltage measurements, be sure it is capable of accurate measurement at the circuit frequency! Inexpensive digital multimeters often experience difficulty measuring AC voltage and current toward the high end of the audio-frequency range. An extension of this exercise is to incorporate troubleshooting questions. Whether using this exercise as a performance assessment or simply as a concept-building lab, you might want to follow up your students results by asking them to predict the consequences of certain circuit faults. 65

Notes 14 Use a sine-wave function generator for the AC voltage source. I recommend against using line-power AC because of strong harmonic frequencies which may be present (due to nonlinear loads operating on the same power circuit). Specify standard resistor and capacitor values. If students are to use a multimeter to make their current and voltage measurements, be sure it is capable of accurate measurement at the circuit frequency! Inexpensive digital multimeters often experience difficulty measuring AC voltage and current toward the high end of the audio-frequency range. An extension of this exercise is to incorporate troubleshooting questions. Whether using this exercise as a performance assessment or simply as a concept-building lab, you might want to follow up your students results by asking them to predict the consequences of certain circuit faults. General concepts and principles to emphasize The concept of reactance, and how it differs from resistance The concept of impedance, and how it relates to both resistance and reactance The relationship between capacitance, frequency, and capacitive reactance Suggestions for Socratic discussion and experimentation Explain how reactance differs from resistance, even though they are both quantified using the same unit of measurement: ohms (Ω). What will happen to the amount of current in this circuit if the source frequency is increased (assuming a constant source voltage)? What will happen to the amount of current in this circuit if the capacitor is replaced by another having greater capacitance? What will happen to the amount of current in this circuit if the resistor is replaced by another having greater resistance? Explain why the sum of the resistor s voltage and the capacitor s voltage does not equal the source voltage. 66