NATIONAL CERTIFICATE (VOCATIONAL) ELECTRONIC CONTROL AND DIGITAL ELECTRONICS NQF LEVEL 3 NOVEMBER 2009

Transcription

1 NTIONL ERTIFITE (VOTIONL) ELETRONI ONTROL ND DIGITL ELETRONIS NQF LEVEL 3 NOVEMER 2009 ( ) 27 November (X-Paper) 09:00 12:00 This question paper consists of 10 pages.

2 ( ) -2- N780(E)(N27)V TIME: 3 HOURS MRKS: 100 INSTRUTIONS ND INFORMTION nswer LL the questions. Read LL the questions carefully. Number the answers according to the numbering system used in this question paper. Write neatly and legibly.

3 ( ) -3- N780(E)(N27)V SETION QUESTION 1: USE OF MESURING INSTRUMENTS 1.1 n important function of an oscilloscope is to measure voltage. This instrument can measure both and D voltages. s a technician who is required to measure the voltage of an waveform: Describe the procedure required to make an voltage measurement using an oscilloscope with a front panel as given in FIGURE 1. (The voltage that needs to be measured has a peak-topeak value of 30 V as shown in FIGURE 1.) (7) Determine the waveform s period. (1) alculate the frequency of the waveform. Focus Vertical position Horizontal position Intensity 5.0 volts/div ms time/div 1ms 100µs On Off D GND Signal input 10ms Trigger level 10µs FIGURE 1 (2) [10]

4 ( ) -4- N780(E)(N27)V QUESTION 2: OMPONENTS, RETIFIERS, MPLIFIERS ND TUNED RL IRUITS 2.1 Use FIGURE 2 (circuit of an power control) below to answer the questions that follow. D F E FIGURE Identify the symbols indicated by D and F in FIGURE 2. (2) riefly explain the operation of the part indicated by the symbol labelled F What is the main function of the part indicated by the symbol labelled D in FIGURE 2 above? 2.2 Use FIGURE 3 to answer the questions that follow. (5) (1) 1 2 V P V S D 4 D D 3 D R L FIGURE riefly explain the function of the circuit in FIGURE 3. (3) Redraw the voltage waveform given in FIGURE 3 in the NSWER OOK and then draw the corresponding waveform you would expect across points 7 and 8: (a) Without the capacitor connected (1) (b) With the capacitor connected (1)

5 ( ) -5- N780(E)(N27)V The purpose of the capacitor is to smooth the output voltage. riefly explain how this is accomplished. (2) 2.3 FIGURE 4 shows the circuit of a single-stage transistor amplifier. V R OUT R IN V OUT V IN FIGURE Is this a common base, common emitter or common collector amplifier? (1) Explain how negative feedback is achieved in FIGURE 4. (4) 2.4 You are a newly qualified artisan employed by Eskom and given the following circuit diagram: R f R i - + V in V out nswer the following questions: FIGURE What kind of an operational amplifier does the circuit in FIGURE 5 represent? (2) Write the formula for closed loop gain () in terms of Vo and Vi. (1) alculate the value of Vo if = -20 and Vin = 50 V. (2)

6 ( ) -6- N780(E)(N27)V 2.5 FIGURE 6 is the characteristic (response) curve of a medium Q for a series resonant circuit. I Medium Q f r Medium R f FIGURE riefly explain how the Q-factor is determined in a series resonant circuit. (2) Redraw FIGURE 6 in the NSWER OOK and show in your drawing the response curves for a high Q and a low Q on the same axes to show how the value of the resistor affects the value of Q. (3) [30] TOTL SETION : 40 SETION : INRY RITHMETI ND OMPUTER ND HRDWRE QUESTION 3: INRY RITHMETI 3.1 onvert each of the following numbers into their INRY equivalents and then complete the calculations in the binary number system: , ,1 2 (convert the answer to octal) (4) ,52 8 x E, 16 (convert the answer to hexadecimal) (6) (convert the answer to octal) (6) 3.2 Use the 1s complement to perform 10101, (4) [20]

7 ( ) -7- N780(E)(N27)V QUESTION 4: OMPUTER ND HRDWRE 4.1 Identify the computer ports given in FIGURE 7 and state their functions. Port 1 FIGURE 7 Port 2 (4) 4.2 The customer brings a P to your workshop and indicates that the unit has no power and that the power light does not illuminate, the fan inside the power supply does not turn on and that the indicator light on the keyboard does not turn on. Write down THREE possible causes of these problems and explain how each problem can be diagnosed. (6) [10] TOTL SETION : 30 SETION QUESTION 5 (TRNSDUERS ND LDDER LOGI S USED IN PL) 5.1 Use the block diagram (FIGURE 8) below to answer the following questions What is the name of the transducer represented by FIGURE 8? (2) Explain the basic operating principle of this block diagram. Op- mp Pulse generator Pulse counter Display unit FIGURE 8 (6)

8 ( ) -8- N780(E)(N27)V 5.2 The following diagram (FIGURE 9) shows the ladder diagram of a PL. END FIGURE Identify the symbols labelled: (a) (b) (c) (1) (1) (1) How many rungs are there in FIGURE 9? (1) How many inputs are there in FIGURE 9? (1) omplete the table below by referring to FIGURE 9. INPUT INPUT INPUT OUTPUT D Off(0) Off(0) Off(0) Off(0) Off(0) Off(0) On(1) On(1) On(1) On(1) On(1) Off(1) (4) 5.3 FIGURE 9 represents the switching circuit of the ND logic function. Make use of ladder logic symbols to draw the equivalent ladder diagram that would represent this function. attery L FIGURE 9 (4)

9 ( ) -9- N780(E)(N27)V 5.4 Various options are provided as possible answers to the following questions. hoose the answer and write only the letter ( E) next to the question number ( ) in the NSWER OOK Which ONE of the statements below describes the functioning of the ladder diagram in FIGURE 10 below? Y END FIGURE 10 D E If Input is true then Output Y will not be energised. If Output Y is false then Input will be energised. If Input is false then Output Y will be energised. If Output Y is true then Input will be energised. If Input is true then Output Y will be energised. (2) Which ONE of the statements below describes the functioning of the ladder diagram in FIGURE 10 below? Y1 END FIGURE 10

10 ( ) -10- N780(E)(N27)V D E If Input is open ND Input is closed, then Output Y1 will be energised. If Input is open OR Input is open, then Output Y1 will be energised. If Input is closed OR Input is open, then Output Y1 will be energised. If Input is closed OR Input is open, then Output Y1 will not be energised. If Input is closed ND Input is open, then Output Y1 will not be energised. (3) Which ONE of the statements below describes the functioning of the ladder diagram on FIGURE 10 below? Y2 END FIGURE 10 D E If Input is open OR Input is open OR Input is open, then Output Y2 will be energised. If Input is open OR Input is open OR Input is open, then Output Y2 will not be energised. If Input is closed OR Input is closed OR Input is closed, then Output Y2 will be energised. If Input is closed OR Input is closed ND Input is closed, then Output Y2 will be energised. If Input is closed ND Input is open OR Input is open, then Output Y2 will be energised. (4) TOTL SETION : 30 GRND TOTL: 100