Chapter 22 Further Electronics

Transcription

1 hapter 22 Further Electronics washing machine has a delay on the door opening after a cycle of washing. Part of this circuit is shown below. s the cycle ends, switch S closes. t this stage the capacitor is uncharged. The door cannot open until it receives a voltage of 8 V. Question 1 Which of the following best describes how a voltmeter should be connected to measure this voltage? The voltmeter should be connected between and. The voltmeter should be connected between and. The voltmeter should be connected across the battery. The voltmeter should be connected across the switch. Question 2 R 1 is 10 kω. Which of the following is closest to a value for that would give a time delay of around 5 s? 200 µf 300 µf 400 µf 500 µf The following circuit is used for smoothing a half-wave rectified voltage. The voltage time graph before smoothing is shown below. The voltage scale = 2 V/square, and the time scale = 1 ms/square. Question 3 Which of the following is closest to the frequency of the original supply? 400 Hz 300 Hz 200 Hz 100 Hz Question 4 Which of the following is closest to the peak voltage of the rectified but unsmoothed waveform? 4 V 8 V 12 V 16 V

2 2 Physics The circuit shown below is being used to measure the charging rate of a capacitor. t the start of the experiment, the switch is set to position X. efore this is done the capacitor is uncharged. Question 5 Which of the following is closest to the charging time constant? 6.0 ms 60 ms 6.0 s 60 s Question 6 Which of the following is closest to the discharging time constant? 6.0 ms 60 ms 6.0 s 60 s Question 7 (V, 2007 examination) particular piece of apparatus requires an voltage of 6.0 V RMS. transformer is used to provide this output from the 240 V RMS mains. Select the correct one of the following options for the correct value of the ratio: number of turns on the transformer secondary number of turns on the transformer primary Question 8 (V, 2007 examination) full-wave bridge rectifier is to be used in a circuit that produces from an input. Which of the circuits below would be the most suitable?

3 Further Electronics 3 Question 9 (V, 2007 examination) With the rectifier correctly connected and functioning, which of the signals following would be observed if an oscilloscope was connected across the output (V OUT ) of the bridge rectifier? Question 10 (V, 2007 examination) capacitor is charged to 12 V. It is then discharged through a 100 Ω resistor. The voltage across the capacitor, as a function of time, is below. Which of the following is closest to the value of the capacitor, in microfarad? 0.40 µf 4.0 µf 40 µf 400 µf Question 11 (V, 2007 examination) half-wave rectifier is shown on the left below. The voltage across the 100 Ω resistor is shown on the right.

4 4 Physics capacitor is added to the circuit to provide smoothing, as shown. Which of the options below best represents the voltage across the load resistor? Question 12 (V, 2007 examination) Students are constructing a regulated power supply, and propose to use a Zener diode as the voltage regulator. The Zener diode s characteristics are shown below. To test the Zener diode they connect it into a circuit as shown. Which of the following would be closest to the value of V OUT? 2 V 4 V 6 V 12 V

5 Further Electronics 5 Question 13 (V, 2007 examination) Which of the following would be closest to the voltage measured across the resistor? 2 V 3 V 4 V 5 V Question 14 (V, 2007 examination) With no load connected across V OUT, which of the following would be closest to the current through the Zener diode? 20 m 30 m 40 m 50 m student designs, constructs and tests a power supply. She begins by testing some components separately. In the following questions, she studies the operation of a capacitor using an R circuit. To test it, she connects the circuit shown below. Question 15 (V, 2008 examination) is a 100 µf capacitor and R is a 5 kω resistor. Which of the following is closest to the time constant (τ) of this R circuit? 5000 s 5.0 s 0.50 s s Question 16 (V, 2008 examination) Using the same circuit with a different capacitor, the time constant is found to be 6 s. With the capacitor discharged, the switch is moved to position. Which of the graphs following best shows the voltage against time across the capacitor?

6 6 Physics Question 17 (V, 2008 examination) With the capacitor fully charged, the switch is moved to position. Which one of the graphs following best shows the voltage against time across the capacitor? In the questions following, the student considers a bridge rectifier. Each of the diodes of the rectifier is mounted on a separate copper plate. Question 18 (V, 2008 examination) The purpose of these copper plates is best described by: shielding each diode from the electric field of the other diodes shielding the bridge rectifier from interference from outside electric fields dissipating heat generated in the diodes warming the diodes to keep them at a higher temperature in order to operate correctly. Question 19 (V, 2008 examination) Which one of the following circuits best shows how the diodes should be arranged for the bridge rectifier to operate correctly?

7 Further Electronics 7 Question 20 (V, 2008 examination) The diagram below shows the current voltage characteristics of a Zener diode that could be used as the voltage regulator in a power supply. Which one of the following circuits best shows the Zener diode correctly set up to give a 6 V regulated output? Question 21 (V, 2008 examination) transformer is required to give 10 V RMS output from an input of 240 V RMS input. Which one of the options for the ratios of turns N PRIMRY :N SEONRY, below would provide this? 4800 : : : : 141 The student now begins to assemble the power supply. The transformer is set to give 10 V RMS 50 Hz output. s she completes each stage, before connecting the following stage, she tests that stage using a 10 Ω load resistor and an oscilloscope. In Questions 22 27, the oscilloscope is set at 4 V cm 1 on the vertical scale, and 5.0 ms cm 1 on the horizontal scale. She begins with just a transformer, with an output of 10 V RMS at 50 Hz, as shown.

8 8 Physics Question 22 (V, 2008 examination) Which one of the following best shows the display she would see on the oscilloscope? (The squares on the display are all 1 cm sides.) She now connects the full-wave bridge rectifier, as shown. Question 23 (V, 2008 examination) Which one of the following best shows the display she would now see on the oscilloscope? (The squares on the display are all 1 cm sides.) 400 µf smoothing capacitor is now installed in the circuit as shown.

9 Further Electronics 9 Question 24 (V, 2008 examination) Which one of the following graphs best shows the display on the oscilloscope? (The squares on the display are all 1 cm sides.) She now inserts an integrated circuit (I) voltage regulator into the circuit, as shown. Question 25 (V, 2008 examination) Which one of the following best shows the display she would now see on the oscilloscope? (The squares on the display are all of 1 cm side.) Question 26 (V, 2008 examination) With the completed regulated power supply (as shown before Question 25) now operating, which one of the following would best give the approximate power being dissipated in the 10 Ω load resistor? 0.1 W 1.0 W 3.6 W 36 W

10 10 Physics Question 27 (V, 2008 examination) She finds that the ripple voltage is unacceptably high. Which one of the following changes to the power supply would best reduce the ripple voltage at the output, while keeping the power supply operating correctly? Increase the transformer output to 14 V. Replace the voltage regulator with one with an output of 8 V. Replace the 400 µf capacitor with a µf capacitor. Replace the 400 µf capacitor with a 10 µf capacitor. Jenny is building an to regulated smoothed power supply. She sets up the first part of the circuit, as shown in the diagram below. The input of the transformer is 240 V RMS, 50 Hz, and the output 10.5 V RMS. Question 28 (V, 2009 examination) ssuming an ideal transformer, which one of the following best gives the ratio of turns in the primary (N1) to turns in the secondary (N2)? 210 : : : : 210 Jenny connects an oscilloscope across the output terminals, PQ, of the transformer. The vertical scale of the oscilloscope is 5 V/cm, and the horizontal scale is 5 ms/cm. Question 29 (V, 2009 examination) Which one of the displays below will she observe?

11 Further Electronics 11 Jenny next adds a full-wave diode bridge rectifier, a 10 µf capacitor and a 500 Ω load resistor, as shown in the diagram following. Jenny connects the oscilloscope across the load resistor (ST). The output as measured by the oscilloscope is shown below. Question 30 (V, 2009 examination) Which one of the following is the best estimate of the average power dissipated in the 500 Ω load resistor? 0.02 W 0.2 W 0.4 W 5000 W Question 31 (V, 2009 examination) Which one of the following is the best estimate of the peak-to-peak ripple voltage? 0.5 V 5.0 V 8.5 V 13.5 V Jenny now adds a voltage regulator circuit consisting of a resistor, R1, of 100 Ω and a 6 V Zener diode. The circuit is now set up as shown.

12 12 Physics Jenny now connects the oscilloscope across the load resistor (ST). Question 32 (V, 2009 examination) Which one of the following best shows the output she will observe? Jenny finds that the ripple voltage is too great for the application for which she wants to use her power supply. Question 33 (V, 2009 examination) Which one of the following changes would best reduce the ripple voltage? replace R1 with a 50 Ω resistor replace the 6 V Zener diode with a 9 V one replace the capacitor with a 500 µf capacitor replace the transformer with one with a 9.0 VRMS output n R circuit is shown below. With the capacitor discharged, the switch is moved to position P at time t = 0. The voltage across the capacitor,, as measured by the oscilloscope is shown in the diagram below.

13 Further Electronics 13 Question 34 (V, 2009 examination) Which one of the following is the best estimate of the value of the capacitor? 3 µf 1000 µf 3000 µf µf Question 35 (V, 2009 examination) fter 60 s the switch is moved to position Q. Which one of the following best gives the initial current flowing through the milliammeter? 0 m 6.3 m 10 m 10 Question 36 (V, 2009 examination) Which one of the following best gives the voltage measured across the capacitor,, 3 s after the switch is moved to Q? 0 V 3.7 V 6.3 V 10.0 V circuit contains an power source, four diodes and a load resistor as shown in the diagram. Question 37 (V, 2009 examination) Sally connects an oscilloscope across the load resistor, that is, across XY. Which one of the following outputs will she most likely observe?

14 14 Physics Question 38 (V, 2009 examination) Michael has a transformer connected to an oscilloscope. The circuit and the output of the transformer are shown below. Michael now connects a multimeter set on volts across the terminals, JK, of the transformer. Which one of the following best gives the reading he will observe on the multimeter? 7 V 10 V 14 V 20 V John is studying the operation of a Zener diode. The voltage current characteristics of the Zener diode are shown in the diagram below. John sets up the circuit shown. Question 39 (V, 2009 examination) Which one of the following best gives the voltage he will observe across R 2? 0 V 4 V 6 V 10 V Question 40 (V, 2009 examination) Which one of the following best gives the current through the Zener diode? 1 m 2 m 38 m 40 m

15 Further Electronics 15 atherine is designing, building and testing an to voltage-regulated power supply system. The overall circuit design she is using is shown in the diagram below. She removes and adds various components, and changes their values as she tests the system. In each of the following questions, the circuit she is using is shown. In the first question she is testing the transformer by itself. The diagram below shows the transformer by itself. The primary windings are connected to a 240 V RMS 50 Hz supply, and there are 3600 turns in the primary. The output required in the secondary is 12 V RMS. Question 41 (V, 2010 examination) Which one of the following best gives the number of turns in the secondary to provide the required 12 V output? atherine now adds a full-wave bridge rectifier made up of four diodes. The circuit, including a load, is shown in the diagram below. Question 42 (V, 2010 examination) The bridge rectifier consists of four diodes mounted side by side on a heat sink, and connected by wires. Which one of the following circuits shows the diodes correctly connected?

16 16 Physics The characteristic curve for each of the diodes used is shown below. atherine uses a multimeter (set on 0 20 V scale) to measure the voltage (potential difference) across XY. The output current is 20 m. Question 43 (V, 2010 examination) Which one of the following readings is the most likely? 9.2 V 10.6 V 11.3 V 12 V atherine now uses an oscilloscope to test the circuit. She connects the oscilloscope to an signal generator. The vertical scale is set on 2 V/cm, and the horizontal scale on 20 ms/cm. She observes the display shown in the diagram below. Question 44 (V, 2010 examination) Which one of the following best gives the correct value of the peak-to-peak voltage of the signal generator? 7 V 10 V 14 V 20 V Question 45 (V, 2010 examination) Which one of the following best gives the frequency of the signal from the signal generator? 12.5 Hz 50 Hz 63 Hz 125 Hz atherine now intends to add a capacitor to smooth the full-wave rectified signal. First, however, she uses the circuit shown below to test the value of the capacitor.

17 Further Electronics 17 With the capacitor fully charged (to 10 V), she moves the switch to position Q (discharge). From the oscilloscope she plots the graph of the voltage across the capacitor versus time as shown below. Question 46 (V, 2010 examination) Which one of the following best gives the value of the capacitor? 10 µf 50 µf 100 µf 500 µf Question 47 (V, 2010 examination) Which one of the following gives the best estimate of the current through the resistor after 0.03 s? 0 m 5 m 95 m 100 m This capacitor is now to be used to smooth a half-wave rectified, 50 Hz, 10 V peak voltage signal. The unsmoothed signal is shown below. The circuit shown below is set up.

18 18 Physics Question 48 (V, 2010 examination) Which one of the options below and on the next page best shows the signal after the smoothing capacitor is added? atherine now sets up the whole smoothed power supply, incorporating a Zener diode, a suitable small protection resistor R (which may be ignored), and a load resistor of 100 Ω. With the 100 Ω load resistor, she measures the output signal as shown in the graph. She now replaces the 100 Ω load resistor with a 30 Ω resistor.

19 Further Electronics 19 Question 49 (V, 2010 examination) Which one of the options below best shows the effect on the ripple voltage of replacing the 100 Ω load resistor with a 30 Ω resistor? Question 50 (V, 2010 examination) atherine puts back the original load resistor. She selects a different output of the transformer so that the input to the bridge rectifier increases from 12 V to 20 V. Which one of the following best shows the effect on the output signal (with the original 100 Ω load resistor)? atherine resets the transformer so that the input to the bridge rectifier is back to 12 V, and sets up the circuit so that it is as shown in the earlier diagram.

20 20 Physics The multimeter is set on mps, on the 0 20 m range. The oscilloscope is set on 1 V/cm vertically, with the zero line at the bottom of the display. The horizontal scale is 5 ms/cm. The reading on the multimeter is 20 m. The display on the oscilloscope is shown in the diagram below. Question 51 (V, 2010 examination) Which one of the following best gives the power dissipated in the load resistor? 0.01 W 0.08 W 0.1 W 1.2 W Question 52 (V, 2010 examination) In her experiment, atherine has mounted the Zener diode on a heat sink. Which one of the following best describes the purpose of the heat sink? The heat sink shields the Zener diode from outside heat. When the input voltage to the Zener diode is above 6 V, some current passes through the Zener diode, heating it. The heat sink removes this heat. The Zener diode needs to be at a temperature above 20 to operate, and the heat sink maintains this temperature. The whole output current passes through the Zener diode. John is designing, building and testing an to power supply system to provide the high voltage to a cathode-ray tube. The voltage has to be approximately 5000 V and the effective resistance of the cathode-ray tube in the circuit is 5 MΩ ( Ω). The circuit is shown below. The bridge rectifier consists of four diodes connected appropriately, and the smoothing capacitor has a value of 10 µf ( F). Question 53 (V, 2011 examination) Which of the following best gives the time constant, τ, for the circuit consisting of the smoothing capacitor and cathode-ray tube? s 0.5 s 5s 50s

21 Further Electronics 21 Question 54 (V, 2011 examination) Which of the following graphs best shows the signal observed across XY? Question 55 (V, 2011 examination) The next step of the design of the system is to select the power of the transformer. This factor affects the price and the size of the transformer, so John must choose the smallest transformer that delivers the required power. catalogue quoted only the following four available power ratings for transformers: 2 W, 10 W, 100 W, 1000 W. Which of the following will be the minimum power rating transformer to meet the task? 2 W 10 W 100 W 1000 W Question 56 (V, 2011 examination) ill lives in the US. He intends to build the same system as John. ue to the fact that the mains voltage in the US is 110 V RMS and 60 Hz instead of 240 V RMS and 50 Hz as in ustralia, ill has to make some changes to the design. He has to replace the transformer. It is an ideal transformer. Which of the following best gives the reason for this? different transformer is needed to allow for the different input voltage only. different transformer is needed to allow for the different frequency only. different transformer is needed to allow for both the different input voltage and different frequency. different transformer is needed to allow for the different power required. Question 57 (V, 2011 examination) Mary wants to build a regulated power supply powered by a 9 V battery. The required output voltage of the power supply has to be 5.5 V to suit the music player she bought overseas. She built the regulator with the electric circuit depicted below, consisting of a 9 V battery, a 20 kω resistor and a 5.5 V Zener diode. The voltage current characteristics of the Zener diode are shown below.

22 22 Physics She measured the output voltage V XY to be 3.5 V. Which of the following is the most likely reason for the output being lower than she wanted? The Zener diode requires at least 1 m to operate correctly, and the current will be less than this. The 20 kω resistor is to protect the Zener diode from excessive currents, and the value of the resistor is too low. The battery has too high a voltage to allow 5.5 V across XY. The Zener diode is installed incorrectly, with its polarity reversed. Mary decides to use a transformer-rectifier power supply in place of the battery to provide the 5.5 V she needs. It is connected to a 240 V RMS 50 Hz supply. The output of the secondary windings of the transformer is 10 V RMS. The circuit for this is shown below. Question 58 (V, 2011 examination) The primary windings of the transformer have 4800 turns. Which of the following best gives the number of windings required in the secondary windings? Question 59 (V, 2011 examination) Initially the circuit operates correctly, providing 5.5 V smoothed output. However, a fault then develops and the output is as shown below. Which of the following is the most likely cause of this fault? failure of the transformer. failure of the capacitor used for smoothing the signal. failure of all the diodes in the bridge rectifier (blown). failure of one only of the diodes in the bridge rectifier (blown). Question 60 (V, 2011 examination) fter fixing this fault another develops, causing the signal shown below at the output.

23 Further Electronics 23 Which of the following is the most likely cause of this fault? failure of one only of the diodes in the bridge rectifier (blown). failure of all the diodes in the bridge rectifier (blown). failure of the capacitor used for smoothing the signal. failure of the transformer. Tom suggests that a much cheaper circuit could be used to provide the 5.5 V smoothed output required. This circuit is shown below. Question 61 (V, 2011 examination) Which of the following will best show the shape of the output as seen by an oscilloscope connected across XY? Question 62 (modified; V, 2011 examination) While a circuit somewhat similar to this cheaper circuit is sometimes used, it is normally avoided due to safety concerns (if a particular component fails, 240 V can appear across the output). The failure of which of the following components would cause 240 V to appear across the output? (ssume that the failure of the component will cause an open circuit, i.e. a break in the circuit where the component was). Zener diode resistor capacitor diode

24 24 Physics Mickie is attempting to measure the input resistance of a piece of electronic equipment, E. She sets up the following circuit. Question 63 (V, 2011 examination) She uses the oscilloscope to observe the voltage across capacitor. ssume the oscilloscope has a very large input resistance. The capacitor is initially uncharged. Moving the switch to position 1, she observes the following signal on the oscilloscope. Which of the following best gives the value of the capacitor? 5 µf 10 µf 50 µf 1000 µf Question 64 (V, 2011 examination) fter 5 minutes, she moves the switch to position 2 and observes the signal shown below. Which of the following best gives the value of the input resistance of the piece of equipment, E? 0.51 MΩ 1.0 MΩ 1.6 MΩ 2.6 MΩ

25 Further Electronics 25 Question 65 (V, 2012 examination) Maxi has an generator that is powered by batteries. He attaches the output of the generator to the input of a properly adjusted oscilloscope. The voltage signal shown below appears on the oscilloscope. Which of the following is the best estimate of the frequency of the output voltage of the generator? 20Hz 40Hz 50Hz 80Hz Question 66 (V, 2012 examination) Minnie tests a transformer. The primary coil of the transformer has 1200 turns and the secondary coil has 50 turns. The input to the primary coil of the transformer is 240 V RMS. Which of the following is closest to the voltage that will appear across the terminals of the secondary coil of the transformer? (The transformer can be considered ideal) 10 V RMS 14 V RMS 240 V RMS 5760 V RMS Question 67 (V, 2012 examination) Minnie and Maxi connect four identical diodes to form a full-wave bridge rectifier. Minnie and Maxi need to make sure that the connections between the diodes are correct. Which of the following circuits best shows the correct connections for the diodes so that they will act as a full-wave bridge rectifier? Question 68 (V, 2012 examination) Each diode is mounted on a metal plate to provide cooling. With a particular power supply attached, each diode has a potential difference of 0.7 V RMS across it and a current of 140 m RMS flowing through it. Each metal plate is capable of dissipating 100 W per square metre (m 2 ) of plate. Each metal plate has to be large enough to dissipate all the heat produced in each diode. Which of the following is the best estimate of the required size of one of the metal plates? m m m m 2

26 26 Physics Question 69 (V, 2012 examination) Minnie and Maxi are constructing a smoothed and regulated power supply. They test various parts of their power supply. They set up the circuit shown below. The transformer has an output of 15.2 V RMS. The resistor R L is the load resistor. The characteristics of each of the four diodes in the full-wave bridge rectifier are shown below. There is a current of 10 m flowing through the ammeter. They use a multimeter on a setting to measure the voltage across the load resistor R L in the circuit. Which of the following is the best estimate of the value that will be read on the multimeter? 21 V 15V 14V 12V Minnie and Maxi then test a capacitor to use as a smoothing capacitor. They connect the circuit as shown below. Question 70 (V, 2012 examination) With the capacitor initially uncharged and the switch S open, they close the switch and observe the voltage display on the oscilloscope as shown below. Which of the following is the best estimate of the value of the capacitor? 100 µf 150 µf 200 µf 20 mf

27 Further Electronics 27 Question 71 (V, 2012 examination) Minnie and Maxi next connect an extra resistor R as shown below. The capacitor is again initially uncharged and the switch S is open. They then close the switch and observe the voltage display on the oscilloscope as shown in the diagram below. Which of the following is the best estimate of the value of the resistor R? 50 Ω 100Ω 200Ω 400Ω Question 72 (V, 2012 examination) Minnie and Maxi then set up the circuit shown below. This includes a transformer with an output of 15.2 V RMS, a full-wave bridge rectifier, a 100 µf smoothing capacitor and a 100 Ω load resistor. n oscilloscope is connected across points LM. The input voltage to the transformer has a frequency of 50 Hz. Which of the following best shows the signal that will be observed on the oscilloscope (set to )?

28 28 Physics Question 73 (V, 2012 examination) They next test the Zener diode that they intend to use as a voltage regulator. The current voltage characteristics of the Zener diode are shown below. Which of the following circuits best shows the Zener diode correctly connected to test it as a 9 V voltage regulator? Minnie and Maxi now construct the complete circuit for a smoothed regulated 9 V power supply as shown below. The circuit contains a transformer, a full-wave bridge rectifier, a 100 µf smoothing capacitor, a 100 Ω resistor (resistor ), a 9 V Zener diode and a 1000 Ω load resistor (R L ). The output of the circuit is connected to an oscilloscope. Question 74 (V, 2012 examination) Which of the following best describes the function of resistor in the circuit shown above? Resistor limits the total power output. Resistor reduces the voltage across the capacitor. Resistor protects the capacitor from excessive current through it. Resistor protects the Zener diode from excessive current through it. With a load resistor (R L ) of 1000 Ω as shown above question 74, the circuit operates correctly, delivering a smoothed output of 9 V. The voltage display on the oscilloscope is shown below.

29 Further Electronics 29 However, a faulty component causes the circuit to stop operating correctly. Instead of a smoothed 9 V signal, the voltage display shown below appears on the oscilloscope. Question 75 (V, 2012 examination) Which of the following is the best explanation for the incorrect operation of the circuit? One diode in the full-wave bridge rectifier has failed. The failed diode now does not conduct current. ll the diodes in the full-wave bridge rectifier have stopped conducting current. Resistor has stopped conducting current. The capacitor has failed. Question 76 (V, 2012 examination) The faulty component is replaced and the circuit now operates correctly as shown earlier. The 1000 Ω resistor is now replaced with a 100 Ω resistor and the output voltage drops below the required value of 9 V. Which of the following is the most likely cause for this? The voltage drop across the diodes has increased. The voltage drop across the capacitor has increased. The output voltage of the transformer has decreased. The voltage drop across resistor has become too high.