NATIONAL CERTIFICATE (VOCATIONAL) ELECTRONIC CONTROL AND DIGITAL ELECTRONICS NQF LEVEL 4 NOVEMBER 2009

Transcription

1 NATIONAL CERTIFICATE (VOCATIONAL) ELECTRONIC CONTROL AND DIGITAL ELECTRONICS NQF LEVEL 4 NOVEMBER 2009 ( ) 30 October (Y-Paper) 13:00 16:00 This question paper consists of 7 pages.

2 ( ) -2- NC790(E)(O30)V TIME: 3 HOURS MARKS: 100 INSTRUCTIONS AND INFORMATION 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.

3 ( ) -3- NC790(E)(O30)V SECTION A: REPLACING FAULTY COMPONENTS ON A PC BOARD AND BASIC DESIGN PROCEDURES QUESTION 1 You are given a faulty PC board and asked to remove and replace some components. 1.1 The component you need to replace is a MOSFET. State FIVE safety precautions you would take when handling MOSFET components. (5) 1.2 Briefly explain how you would go about removing the component from the printed circuit board (PCB) in QUESTION 1.1. (4) 1.3 You need to find a suitable replacement for the component in QUESTION 1.1. Assume you could not find an exact replacement for this component. Explain how you would use the relevant technical manuals to find a suitable replacement. (5) 1.4 Make use of the data sheet given in TABLE 1 to answer the following questions What does the abbreviation V CEO stand for? (1) What is the polarity of the 2N2905 transistor? (1) What is the current gain of the 2N2905 transistor? (1) What is the maximum collector current of the 2N2905 transistor? (1) From the list of transistors provided on the data sheet, select the most suitable replacement transistor for the 2N2905. (2) Device Pkg or casing Polarity V CEO (min) I C (max) P D h FE h I C & V CE V CE I C F T (min) F T I C 2N2218 TO-39 NPN N2222 TO-18 NPN N2905 TO-39 PNP BC109 TO-18 NPN BC237A TO-92 NPN BC327A TO-92 PNP N6038 TO-126 PNP N6123 TO-220 NPN TABLE 1 [20]

4 ( ) -4- NC790(E)(O30)V QUESTION The circuit given in FIGURE 1 is that of a simple operational amplifier delay timer. 9V R R A Reset R L LED C FIGURE 1 0V R A FIGURE Calculate the value of the voltage at pin 2 of the 741 operational amplifier in the circuit in FIGURE 1. (2) Determine the range of voltage values at pin 3 of the 741 operational amplifier that will keep the output LED off. (1) At what value of the voltage at pin 3 of the 741 operational amplifier will the output LED switch on? (1) Briefly explain how the output of the operational amplifier will change from a low voltage value to a high voltage value. (3) 2.2 Filter circuits are used extensively in electronic circuits. These circuits are used to pass certain frequencies and to remove unwanted frequency signals. Study the filter circuit given in FIGURE 2 below and answer the questions that follow. C V in R FIGURE Identify the type of filter circuit in FIGURE 2. (1) Draw the frequency response curve for the circuit in FIGURE 2. (3) Explain the operation of the filter circuit of FIGURE 2. (2)

5 ( ) -5- NC790(E)(O30)V Name an application where any filter circuit is used. (1) 2.3 Oscillators are used in a number of electronic circuits. They can be found in computers, radio transmitters and receivers, audio equipment and in many other areas Name the requirements for oscillations to occur. (2) The circuit in FIGURE 3 is that of an astable multivibrator used as an oscillator to switch two LEDs on and off continuously (flasher unit). Once the circuit is switched on the LEDs alternatively switch on and then off. Assume that LED 2 is on. Briefly explain how LED 2 is switched off and how it will switch on again. R 1 R 2 R 3 R 4 V cc C 1 x y C 2 TR 1 TR 2 LED 1 LED 2 FIGURE 3 (6) 2.4 Draw a block diagram of an inverting power supply with a built-in battery charger. (8) [30] SECTION B: BINARY DECODING AND LOADING SOFTWARE ONTO A COMPUTER QUESTION The following is a list of BCD codes: 2421, 8421, 642-3, 7421, Excess 3, 5421, 4221, , 5211, , From the list select a weighted, self-complementing BCD code. (1) Write your selected code in table format. (3)

6 ( ) -6- NC790(E)(O30)V 3.2 Use the EBCDIC table below to code the statement x=5 to binary NUL DLE DS SP & - { } \ SOH DC1 SOS / a j ~ A J STX DC2 FS SYN b k s B K S ETX DC3 c l t C L T PF RES BYP PN d m u D M U HT NL LF RS e n v E N V LC BS EOB YC f o w F O W DEL IL PRE EOT g p x G P X CAN h q y H Q Y EM i r z I R Z SMM CC SM [ ] : 1011 VT $ ' # 1100 FF IFS DC4 < * 1101 CR IGS ENQ NAK ( ) _ 1110 SO IRS ACK + ; > = 1111 SI IUS BEL SUB! ^? (3) 3.3 Express the following 16-bit floating point binary number in its decimal equivalent: Mantissa Exponent (3) [10] QUESTION The software currently loaded on your computer needs to be replaced/ upgraded with a newer version. You have received the newer version on CD-ROM and you need to install it onto your computer. Explain step by step how you are going to install the software. (5) 4.2 Several steps can be taken to protect computer systems from malicious damage like viruses and worms. Discuss how one can guard against virus infections and other malicious software. Use the following topics in your discussion: Anti-virus software Self education (2) (3) [10]

7 ( ) -7- NC790(E)(O30)V SECTION C: SYNCHRO-SERVER MOTORS AND OPERATION OF PLC'S QUESTION Explain your understanding of the term synchro system as used in motioncontrol technology. (2) 5.2 Make use of circuit symbols to draw a circuit diagram of any synchro system to illustrate the coupling between the transmitter and receiver. State the phase difference between the transmitter and receiver for the circuit you have drawn. (9) 5.3 Design a ladder logic diagram such that when a push button switch is pressed a light is switched on. The light stays on for 5 seconds and then switches off. (7) 5.4 Explain the operation of the ladder logic diagram given in FIGURE 4. A CR1 B CR1 CR2 END FIGURE 4 (7) 5.5 Predict how the operation of the ladder logic circuit of FIGURE 5 will be affected as a result of the following faults: 'Stop' push button switch fails to close (2) The lamp switch fails to close Consider each fault independently (that is one at a time, no multiple faults). Start Stop Lamp Lamp END FIGURE 5 (3) [30] TOTAL: 100

8 ( ) -8- NC790(E)(O30)V