Electric Circuit I Lab Manual Session # 2

Transcription

1 Electric Circuit I Lab Manual Session # 2 Name: Group:

2 Breadboard and Wiring Objective: The objective of this experiment is to be familiar with breadboard and connection made on it. Introduction: In this lab you ll use a breadboard to implement simple circuits consisting of resistors, capacitors, inductors, diodes, and various integrated components. Breadboards provide you with a general wiring fabric in which to make connections among circuit components. Learning how to use this wiring fabric effectively takes time and, more vital, practice. Figure 1 shows you the basic layout of the breadboards you ll be using. Figure1: Basic Breadboard As you can see, the surface of board is covered in pinholes into which you can insert wires or electric component leads. Beneath the plastic surface, the holes are connected with a network of metal tabs. But not all pinholes are connected together. Every row in each of the two main columns is wired together. [Note: The rows are NOT connected across the column.] You can 2

3 also see a series of pinhole groups arranged in columns along the left and right sides of the board. These are useful for wiring global signals and power supply voltages. Figure2: Special Sections for Global Signals Making a connection between two components is simple once you understand the internal wiring of the board. Let s take a simple example of connected two resistors in series or in parallel. The general wiring fabric of the breadboard provides an unlimited number of possibilities, but we want to focus on the efficient implementations. Never use more jumper wire than necessary! Let the breadboard do the work for you. Figure3: Basic Wiring Combinations 3

4 Figure 4 shows 3 examples of series wiring and 2 examples of parallel wiring. The pinholes where the component leads connect to the board are exaggerated with big circles. Figure 4: Example Uses of the Breadboard These few examples show you the basics and give you enough knowledge to do the labs. A few general points to remember when you begin wiring a circuit: Try to use the breadboard to make connections, i.e. avoid the use of extra jumper wire. Murphy s law tells us that the more wire you use, the more likely you are to make an error by shorting two wires or incorrectly wiring the circuit. When you use extra jumper wire, keep it as short as possible. The lab kits provide jumper wire of various lengths. Use them all! A mess of long wires creates the St. Louis Arch syndrome, where all your wires make glorious arches over each other. Now you can t be expected to appreciate why these arches are electrically undesirable (They act as antennas), 4

5 but you can appreciate how easily they complicate your life. If your circuit looks like a bush of wires, imagine how hard it will be to debug it when [Note: not if] it doesn t work correctly. Keep related components together. Make the circuit on the breadboard look as much as possible like the schematic you re implementing. The visual correspondence will help you differentiate the various nodes; plus, your TA can do more to help you debug if he can easily make sense of your circuit. All dots in each line are electrically connected 5

6 Power Supply The power supply used in the lab contains two 0-20V Dc supplies and one 5V Dc supply as shown in Figure 5 Figure 5 6

7 OHM S LAW Introduction: The relation between the voltage across a certain resistor and the current flowing through it is governed by Ohm s Law, which states that The potential difference (voltage) across an ideal conductor is proportional to the current passing through it. The constant of proportionality is called the "resistance", R. Ohm's Law is given by: V = I R Where V is the potential difference between the two terminals of the resistor,r.i is the current flowing through the resistance. It is often expressed as: Where g is the conductance, g = 1/R. I = g V Materials that obey Ohm's Law are called "ohmic" or "linear", because the potential difference across it varies linearly with the current. Ohm's Law can be used to solve simple circuits. A complete circuit is a closed loop, which contains at least one source of voltage and at least one potential drop. The sum of the voltages around a complete circuit is zero. 7

8 Procedures: 1. Use the Ohmmeter to measure two of the resistors on your bench. 2. Connect the following circuit as shown in the figure. 3. Set the DC voltage source in the Com3lab master Unit to be 5V. 4. Adjust multimeter 1 of the Com3lab unit to measure the current (Is) in the resistors. 5. Adjust multimeter 2 of the Com3lab unit to measure the voltage drop (V) across the resistor R1. 6. Record the measured current and voltage in the following table. Change the voltage source to be 6, 7, 8,9,10 volts. For each value, record (Is) and (V). 7. Plot the graph between Is and V on the shown graph with the proper scale. 8. Calculate the slope of the line drawn. 9. Compare the measured value of R1 using the ohmmeter and the reciprocal of the slope of the line. Comment. 8

9 Serial and Parallel Configurations Objective: The objective of this experiment is to the serial and parallel configurations of resistors. Introduction: I. Serial Resistors Consider a circuit in which three resistors R1, R2 and R3 are connected in Series as shown in the following figure: The total resistance of this set of resistors connected in series (in this case three), is the sum of the resistances of the individual resistors: The current, I, flows through each of the resistors. The total resistance between the points A and B, call it R, will produce a potential drop V between the test points, where V = IR. 9

10 II. Parallel Resistors Consider a circuit in which three resistors R1, R2 and R3 are connected in Parallel as shown in the following figure: Thus the total resistance of a set of resistors connected in parallel (in this case three) is given by the equation: The potential difference across the points A and B will be V, and will be equal to the current I multiplied by the total resistance of the parallel resistors, R. (V = IR). 10

11 Procedures: 1. In the box in front of you choose three types of resistors. 2. Read the bands and record the values of the resistors. 3. Use the ohmmeter to measure the values of the three resistors. 4. Connect the three resistors in series on the breadboard, use the ohmmeter to measure the equivalent resistance. Compare the measured resistance with the calculated one. 5. Connect the 3 resistors in series as in Figure 7.Set the voltage source to a fixed value. 6. Use the Com3lab multimeter to measure the loop current. 7. Calculate the equivalent resistance by dividing the source voltage by the loop current and compare R with the calculated value. 8. Take any two resistors of known values to you and connect the three resistors in parallel on the breadboard, use the ohmmeter to measure the equivalent resistance. Compare the measured resistance with the calculated one. 9. Connect them in parallel as shown in Figure 8. Set the voltage source to a fixed value. 10. Measure the total current. 11. Calculate the equivalent resistance by dividing the source voltage by the loop current and compare R with the calculated value. 11