EET140/3 ELECTRIC CIRCUIT I

Transcription

1 SCHOOL OF ELECTRICAL SYSTEM ENGINEERING UNIVERSITI MALAYSIA PERLIS EET140/3 ELECTRIC CIRCUIT I MODULE 1 PART I: INTRODUCTION TO BASIC LABORATORY EQUIPMENT PART II: OHM S LAW PART III: SERIES PARALEL CIRCUIT LECTURER S NAME PROGRAMME/GROUP NAME MATRIC NO DATE MARKS PART I PART II PART III TOTAL

2 PART I - INTRODUCTION TO BASIC LABORATORY EQUIPMENT OBJECTIVE 1. To acquaint students with the skill to read resistor and capacitor values based on color code and digital/alphabet code. 2. To familiarize students in using multimeter to measure resistance, voltage and current as a basic tool in measurement. 3. To make students understand how to do real connections or wiring in the laboratory based on the given schematic diagram using breadboard to easily connect components together to build circuits. INTRODUCTION a) Resistor Coding The color code technique is used to show resistance values of carbon resistors without having to measure it. In this technique color bands are printed on the resistor. The procedure for determining the resistance of a color-coded resistance is described in Table 1. The first two bands determine the first two digits of the resistor value, while the third band determines the power of 10-multiplier. For the resistor with value less than 10 the third band is either silver or gold. The fourth band is the per cent tolerance for the chosen resistor. If resistors have only three bands, it means the fourth band has no color. Sometimes a fifth band is employed for some high precision resistor where the first three bands represent the significant digit. The fourth band is the multiplier while the fifth band is the tolerance. In the other case, for some standard 4-band code, a fifth band may indicate the manufacturer's special code for some physical characteristic or failure rate of the component. For increasing wattage, the size of a resistor will increase accordingly. The larger sized resistors from about 5 W and up or wire winding resistors are not color-coded but are using digital and alphabet code printed on its body. In writing the value of resistors: k stands for multiplier "kilo" and M for multiplier "mega". The alphabet written after the resistor value shows the tolerance: F = 1%, G = 2%, J = 5%, K= 10% and M = 20%. Resistance should never be measured in a live network due to the possibility of damaging the meter with excessively high currents and obtaining readings that have no meaning. In a constructed circuit, to measure a single resistance value, just take off one end of its terminal to avoid the effect of other resistances in the circuit. This applies in the same manner to the other components such as capacitor and inductor.

3 The standard code is adopted by the manufacturer through their trade association, the Electronic Industries Association (EIA). Table 1: Resistor standard code Color 1 st Band (1 st Significant Digit) 2 nd Band (2 nd Significant Digit) 3 rd Band (Multiplier) 4 th Band (Tolerance) Black Brown % Red % Orange % Yellow % Green Blue Violet Grey White Gold % Silver % No Color % Reading from left to right Example 1: The value of this resistor is 25 x 10 1 ± 10% = 250± 10% ohms Minimum value = 225 Maximum value = 275

4 Example 2: R33F = 0.33 ±1% 4k7 = 4.7 x R0 = R = M = 50 x k8J = 6.8 x 10 3 ±5% R39 = k2M = 2.2 x 10 3 ±20% 1R0 = 1 b) Breadboard When building a "permanent circuit" the components can be "grown" together (as in an integrated circuit), soldered together (as on a printed circuit board), or held together by screws and clamps (as in-house wiring). In the lab, we want something that is easy to assemble and easy to change. We also want something that can be used with the same components that "real" circuits use. Most of these components have pieces of wire or metal tabs sticking out of them to form their terminals. A breadboard is used to make up temporary circuits for testing or to try out an idea. No soldering is required so it is easy to change connections and replace components. Parts will not be damaged so they will be available to re-use afterwards. Figure 1 shows a front look of a typical small breadboard used in the laboratory. Figure 1: The Front look of breadboard The breadboard has many strips of metal (copper usually) which run underneath the board. The metal strips are laid out as shown in Figure 2 below.

5 Figure 2: Metal strips When wiring, it is important to keep your work neat! This will save time in debugging when your circuit doesn t work. Here are some tips: Keep your wires short, do not loop wires over the chip, use the bus lines for Ground or a DC supply voltage (e.g. VCC) and sometimes to get cleaner signals, short the metal base of the breadboard to the circuit s ground. c) Multimeter A Multimeter is a basic tool in electric and electronic fields. It is a multipurpose device to measure voltage, current and resistance. Basically, there are two types of multimeter used either in the education or industrial field based on the electronic circuits inside them: analog and digital meters. The analog meter, broadly known as VOM (volt-ohm-milliammeters) uses a mechanical moving pointer which indicates the measured quantity on a calibrated scale. It requires the user a little practice to interpret the location of the pointer. The digital meter broadly known as DMM (digital multimeter) used a number or numerical display to represent the measured quantity. It has a high degree of accuracy and can eliminate usual reading errors compared to the analog meters. Students should be adept at using both meters throughout their studies. Resistance Measurement: For VOM always reset the zero-adjust whenever you change scales. In addition, always choose the range setting that will give the best reading of the pointer location. As an example, to measure a 500- resistance, choose function switch resistance with a range setting of X 1k. Finally do not forget to multiply the reading by the proper multiplication factor. If you are not sure about the value always starts with the highest range and going downwards until the appropriate scale is chosen. For DMM remember that any scale marked "k will be reading in kilo-ohms and any with M " scale in mega-ohms and so on. There is no zero-adjust on a DMM meter but make sure that the resistance reads zero when shunting both leads. Polarity does not concern in resistance measurement. Either lead of the meter can be placed on either terminal end of the component, it will be the same. Voltage Measurement: When measuring voltage levels, make sure the meter is connected in parallel with the element whose voltage is to be measured. Polarity is important because the

6 reading will indicate up-scale or positive reading for correct connection and down-scale or negative reading if the reverse connection of the meter test leads to the resistor's terminals. Therefore a voltmeter is not only excellent for measuring voltage but also for polarity determination. Choose the correct function switch, for example, DCV to measure dc voltage and turn to the range switch that has slightly bigger value than the voltage to be measured. Current Measurement: When measuring current levels, make a series connection between the meter and the component whose current is to be measured. In other words, disconnect the particular branch and insert the ammeter. The ammeter also has a polarity marking to indicate the manner they should be hooked-up in the circuit to obtain an up-scale or positive measurement. For analog meter pay attention that reversing the polarity of the meter may cause damage to the pointer. Again always start with the higher range going downwards to avoid damaging the instrument. The connection of the multimeter to measure different electrical quantities is shown in both a schematic diagram and real wiring illustration in the laboratory in Figure 3 and 4. R V s V/ A Figure 3: Schematic Diagram. Figure 4: Real wiring diagram for illustration

7 EQUIPMENT/COMPONENT Multimeter (1) DC power supply (1) Resistor (1/4 W) 1 k, 2.2 k, 4.7 k, 10 k, 6.8k, Breadboard (1) PROCEDURE Part A: Reading resistor by colour coding **PRE-LAB: Determine the nominal value or colour bands of a resistor based on colour coding technique for each case given in Table 2. Check your answers with the measured values in the laboratory by using a multimeter. COLOUR BAND No. Band 1 Band 2 Band 3 Band 4 1 Yellow Violet Orange Gold 2 Brown Black Red Gold 3 Brown Black Orange Gold 4 Red Red Red Gold 5 Blue Grey Red Gold PRE LAB: Nominal Value(W) Table 2 Measured Value (W) Within Tolerance: YES/NO Verified by: Part B: Resistance, Voltage and Current Measurements Using the supplied equipment/components in the laboratory, hook-up the series resistive circuit as in Figure 5. Measured values: R 1 = R 2 = R 3 = Figure 5: Series resistive circuit Verified by:

8 As a common practice, always measure the actual value of the resistors used in the circuit and set the source value using a multimeter to reduce errors from the expected results. Perform the following instructions: 1. Measure currents I1 and IT. Should they be the same? Give your reason. Answer: I1 = ma IT = ma Comment: 2. Measure voltage drop across resistor R1. Answer: VR1 = V 3. Measure voltage drop across the combination resistors R2 and R3. Answer: VR2R3 = V 4. Disconnect the power supply and measure the total resistance in the circuit, Req. Answer: Req = Verified by:

9 PART II - OHM S LAW OBJECTIVES 1. To learn the relationship among R, V and I. 2. To experimentally prove the mathematical relationship among R, V and I. INTRODUCTION Ohm s law defines that voltage is proportional to the current and vice versa. The circuit current is inversely proportional to the resistance R. Both current and voltage have a linear relationship with resistance remain constant. The three forms of Ohm s Law are, EQUIPMENT AND PARTS Digital Multimeter DC Power Supply Resistor: 1 kω - 2 units V V = IR, I =, and R = R **PRE-LAB: Determine the calculate value of current using Ohm's Law in Table 1. PROCEDURE 1. Determine the nominal value resistance for the resistor in Figure 1 by referring the resistor standard code table in PART I. 2. Measure the resistance of the resistor by using the digital multimeter and record the value in Table Connect the circuit as shown in Figure 1. V I Figure 1

10 4. Set the power supply output for 4 V as measured by the digital multimeter. Record this value in Table Using the digital multimeter, measure and record the current flows through the circuit in Table Repeat steps 4 and 5 by varying the power supply from 8 V to 20 V. Record all measurements in Table Repeat steps 1 to 5, for a configuration circuit in Figure 2. Record all the measurements in Table 2. When you are measuring the voltage, please measure across both resistors. 8. Plot a I versus V graph for theoretical and measurement values for data in Table 2 and discuss. RESULTS Figure 2 PRE-LAB: THEORETICAL MEASUREMENT V (V) R ( ) I (A) V (V) R ( ) I (A) Table 1 Verified by:

11 PRE-LAB: THEORETICAL MEASUREMENT V (V) R ( ) I (A) V (V) R ( ) I (A) Table 2 Disscussion: Verified by:

12 PART III - SERIES PARALLEL CIRCUIT OBJECTIVES 1. Test the theoretical analysis of the series-parallel circuit through direct measurements. 2. Improve the skills of identifying series and parallel elements. 3. Measure properly the resistance, voltages and currents of a series-parallel circuit. INTRODUCTION Two components which are connected to a node are said to have a series connection. The same amount of current will flow across all components in the series connected circuit. The total resistance is the sum of all resistances in the circuit. The total voltage drop across all resistors is the same as the voltage source. R = R + R + R + R eq R N Two or more components which are connected to common nodes are said to have a parallel connection. The voltages across all resistors in the circuit are equal. The total current is the algebraic sum of all currents for each node in the parallel connection circuit. The total resistance is the sum of all resistance in terms of conductance. R eq = 1 R T = 1 R R R T EQUIPMENT AND PARTS Digital Multimeter DC Power Supply Resistor: 1kΩ-2units, 2.2kΩ-1unit Breadboard **PRE-LAB: Calculate the circuit variable listed in Table 2. (Theoretical Value) PROCEDURE 1. Measure the actual value of the resistor and record in Table Construct the circuit as shown in Figure 1.

13 Figure 1 3. Measure the actual resistance between terminals AB (RAB). 4. Connect the DC source 10 V as shown in Figure 2. Figure 2 5. Measure the voltage across the resistor 1 kω (V1), 10 kω (V2), 2.2 kω (V3) and total current flows through the circuit (IT) as shown in Figure 2, then record in Table Construct the circuit as shown in Figure 3. Figure 3 7. Measure actual resistance between terminals CD (RCD). 8. Connect the DC source 10 V as shown in Figure 4. Figure 4

14 9. Measure the voltage across the resistor 1 kω (V4), 10 kω (V5), 2.2 kω (V6) and the current flows through the resistor I1, I2, I3 and I total as shown in Figure 4, then record in Table Complete Table 2. RESULT Theoretical Value Measurement Value 1 kω 10 kω 2.2 kω Table 1 Circuit Variables PRE-LAB: Calculated Value Measured Value % Difference RAB V1 V2 V3 IT RCD V4 V5 V6 I1 I2 I3 Itotal Table 2 Discussion: Verified by: