NATIONAL CERTIFICATE (VOCATIONAL) ELECTRONIC CONTROL AND DIGITAL ELECTRONICS NQF LEVEL 3 NOVEMBER 2010 (12041023) 22 October (Y-Paper) 13:00 16:00 This question paper consists of 6 pages.
(12041023) -2- NC770(E)(O22)V TIME: 3 HOURS MARKS: 100 INSTRUCTIONS AND INFORMATION 1. 2. 3. 4. Answer ALL the questions. Read ALL the questions carefully. Number the answers according to the numbering system used in this question paper. Write neatly and legibly.
(12041023) -3- NC770(E)(O22)V SECTION A: MEASURING INSTRUMENTS, COMPONENTS AND BUILDING CIRCUITS QUESTION 1 1.1 A digital multimeter is a sensitive measuring instrument that can easily be damaged. State FIVE precautions that you should consider when taking care of a digital multimeter. (5) 1.2 A cathode-ray oscilloscope is used extensively in electronic and electrical activities. What are the THREE fundamental (basic) electrical quantities that can be measured with an oscilloscope? (3) 1.3 What is the root-mean-square (rms) value of an alternating current? (2) QUESTION 2 2.1 A tunnel diode is a two-terminal semiconductor junction device which does not show rectification in the usual sense, but exhibits a negative resistance region at very low voltage in the forward-bias characteristic and a shortcircuit in the negative-bias direction. 2.1.1 Draw the IEC symbol of a tunnel diode. (2) 2.1.2 Explain the forward-bias operation of a tunnel diode. (4) 2.2 Electronic components such as transistors sometimes look alike. How would you positively identify transistors? (4) QUESTION 3 3.1 Explain resonance in a series RLC circuit by stating any FOUR prevailing conditions in such a resonating circuit. (4) 3.2 Full-wave rectification can be produced in two ways, one of which is using a centre-tap transformer. 3.2.1 Draw a labelled circuit diagram of a full-wave rectifier using a centre-tap transformer. (6) 3.2.2 Sketch the input and output waveforms of the rectifier in QUESTION 3.2.1 above. (2)
(12041023) -4- NC770(E)(O22)V 3.3 Feedback is normally found in gain-controlled systems. State FIVE characteristics of negative feedback. (5) 3.4 Determine the output voltage in a non-inverting operational amplifier with a 1 V input voltage. The input resistance is 1 kω whilst the feedback resistance is 10 kω. (3) [20] TOTAL SECTION A: 40 SECTION B: BINARY ARITHMETIC AND ASSEMBLY OF A COMPUTER QUESTION 4 4.1 Convert each of the following numbers (where applicable) to their BINARY EQUIVALENTS and then carry out the indicated arithmetic operations in the binary system. 4.1.1 27 10 23 10 (Use 2 s complement method) (6) 4.1.2 11011 2 + 13 8 + 10 10 + 2F 16 (Give the final answer in octal) (5) 4.1.3 101 2 x 110 2 (Give the final answer in octal) (4) 4.1.4 24 10 4 10 (5) [20] QUESTION 5 5.1 Static electricity can easily damage and destroy complementary metal oxide silicon (CMOS) devices contained in a computer board. State FOUR checks and precautions to be taken before assembling a computer with the above observation in mind. (4)
(12041023) -5- NC770(E)(O22)V 5.2 Choose a cause from COLUMN B that matches the error in COLUMN A. Write only the letter (A F) next to the question number (5.2.1 5.2.6) in the ANSWER BOOK. COLUMN A COLUMN B 5.2.1 TYPE OF ERROR Memory error A POSSIBLE CAUSE Problem with hard disk 5.2.2 5.2.3 Non-system disk Operating system not found B C Keyboard or keyboard connector defective Power supply 5.2.4 5.2.5 5.2.6 Keyboard does not respond CPU fan not running No display on the screen D E F Faulty memory chip Stiffy disk left in drive VGA card, VGA slot or monitor defective (6) TOTAL SECTION B: 30 SECTION C: INTRODUCTION TO LADDER LOGIC AS USED IN PLCs QUESTION 6 6.1 Transducers that convert mechanical energy into electrical energy can be classified according to economical and practical methods. List THREE such methods. (3) 6.2 Draw a neat, labelled circuit diagram of an electronic pressure gauge. (7)
(12041023) -6- NC770(E)(O22)V QUESTION 7 7.1 Name TWO input devices and TWO output devices as used in a ladder diagram. (4) 7.2 Study the ladder diagram below and answer the questions that follow. + X 1 Y - X 2 FIGURE 1: LADDER DIAGRAM 7.2.1 What do the symbols X 1 and Y represent? (2) 7.2.2 How many rungs does the ladder diagram contain? (1) 7.2.3 Describe how the ladder diagram works. (5) 7.2.4 Predict the end-result of the ladder diagram by developing a truth table for all possible combinations of the inputs. (4) 7.2.5 Draw an equivalent logic gate schematic diagram of the ladder diagram in FIGURE 1 above, by using switches, a lamp and a battery. (4) [20] TOTAL SECTION C: GRAND TOTAL: 30 100