NATIONAL CERTIFICATE (VOCATIONAL) ELECTRONIC CONTROL AND DIGITAL ELECTRONICS NQF LEVEL 2 NOVEMBER 2009

Transcription

1 NATIONAL CERTIFICATE (VOCATIONAL) ELECTRONIC CONTROL AND DIGITAL ELECTRONICS NQF LEVEL 2 NOVEMBER 2009 ( ) 30 October (Y-Paper) 13:00 16:00 Calculators may be used. This question paper consists of 8 pages.

2 ( ) -2- NC770(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. Questions may be answered in any order, but subsections of questions must NOT be separated. Rule off on completion of each question. ALL work you do not want to be marked, must be clearly crossed out. Write neatly and legibly.

3 ( ) -3- NC770(E)(O30)V SECTION A: COMPONENT AND CIRCUIT DIAGRAMS QUESTION 1: COMPONENTS 1.1 Name the IEC electronic symbols marked 1 10 on the given diagram (FIGURE 1). Write only the answer next to the question number ( ) in the ANSWER BOOK A 10 7 FIGURE 1 (10) 1.2 Fixed carbon-composition resistors have their resistance values marked on their casings by means of a colour code. Explain the process that should be followed in determining the value of a 4-band colour-coded resistor. (5) 1.3 Complete the statement given below. Write only the correct word next to the question number ( ) in the ANSWER BOOK. A voltmeter is used to measure and is always connected in with the circuit in question.

4 ( ) -4- NC770(E)(O30)V 1.4 A potentiometer is a resistor that can be adjusted by turning a knob or a spindle Name the TWO different types of potentiometers available in the industry. Explain what it means when a potentiometer is stamped 350 Ω at the bottom. 1.5 The figures below are used to indicate FIVE different types of inductors, also commonly known as coils or chokes. Name the types of these inductors. Write only the answers next to the question numbers ( ) in the ANSWER BOOK (5) How many terminals, including the coil wires, does a single-pole, doublethrow relay have? You are asked to offer your opinion on a broken device that has been retrieved from a burnt-out building by the local fire-fighting unit. The following markings are still visible on the shell of the device: 380/220 AC 250 VA What type of a device is this? Name at least THREE features emanating from the markings. (4) [30]

5 ( ) -5- NC770(E)(O30)V QUESTION 2: CIRCUIT DIAGRAMS 2.1 As is the case of resistors capacitors can also be coupled in series, parallel or a combination of the two. Study the diagram (FIGURE 3) below and determine the following quantities: The total capacitance of the circuit The total charge with respect to the total capacitance determined, when a supply voltage of 20 V is connected across the circuit (3) C 1 = 10 µf C 2 = 20 µf V = 50 V FIGURE What can be said about the magnitude of the total resistance of two individual resistors that are connected in series? A resistive, purely parallel network in a music system consists of three resistors of values 20 Ω, 10 Ω and 5 Ω Draw the parallel network. Calculate the total resistance of the combination. 2.4 Indicate whether the following statements are TRUE or FALSE. Write only 'true' or 'false' next to the question number ( ) in the ANSWER BOOK A long wire has lower inductance than a short wire. Two inductor coils that are connected in parallel will always result in a total inductance value that is lesser than the lower of the two inductor coils. The total capacitance of capacitors that are connected in parallel will always have a value that is lower than the lowest of the individual capacitors. An ammeter should never be connected in parallel in an electronic circuit. [15] TOTAL SECTION A: 45

6 ( ) -6- NC770(E)(O30)V SECTION B: BINARY THEORY AND BASIC COMPUTER COMPONENTS QUESTION 3: BINARY THEORY AND FLIP-FLOPS Use the repeated division-by-2 method to convert decimal 196 to binary. The basic arithmetic principles of addition and subtraction as used in the decimal number system also apply to the binary number system. Add the following binary numbers, and then convert the answer to decimal: and The employer requests you as a technician to facilitate the process of converting from a binary system to a decimal number system (decoding). Complete a truth table that shows decoding from the 8421 BCD code to decimal. The advantage of a JK flip-flop is that it can easily be converted into various other types of flip-flops. (5) (5) (5) Draw a fully labelled IEC symbol of a JK flip-flop. Develop a truth table of the JK flip-flop. (6) (4) Define a decade counter. A shift register is a storage device that is used to store binary data temporarily. Where are shift registers used? With reference to pulse characteristics what does falling time mean? [30] QUESTION 4: BASIC COMPUTER COMPONENTS 4.1 Many different types of electronic equipment need electronic storage known as memory. 4.2 Name FOUR such storage devices that are referred to as secondary memory. When dealing with computers, what is known as an interface? (4)

7 ( ) -7- NC770(E)(O30)V 4.3 The computer makes use of different types of memories to store data or information. One such memory category is the primary memory. Which are the TWO main sub-types that the primary memory can be divided into? 4.4 Choose a word from COLUMN B that matches the description/statement in COLUMN A. Write only the letter (A G) next to the question number ( ) in the ANSWER BOOK. COLUMN A COLUMN B Magnetically-encoded nonvolatile storage device Contains up to seven times more information than a CD A B C firmware volatile memory CPU Software that is embedded in the hardware device Used to enter text and numbers Requires the application of electrical power in order to store data Considered the 'brain' of a computer Basic software that allows the user to interface with the computer D E F G hard disk operating system keyboard DVD (7 x 1) (7) [15] TOTAL SECTION B: 45

8 ( ) -8- NC770(E)(O30)V SECTION C: TRANSDUCERS QUESTION 5: TRANSDUCERS USED IN PROCESS CONTROL What are fibre-optic cables, as applied in digital signals? In your opinion, what are the most important criteria for selecting a good D/A converter? Define a thermistor. List TWO physical phenomena that transducers can sense. All semiconductors fall into one of two categories of photodiodes. Name the TWO photodiode categories. TOTAL SECTION C: GRAND TOTAL: (3) [10]