University of Portland EE 271 Electrical Circuits Laboratory. Experiment: Kirchhoff's Laws and Voltage and Current Division

Transcription

1 University of Portland EE 271 Electrical Circuits Laboratory Experiment: Kirchhoff's Laws and Voltage and Current Division I. Objective The objective of this experiment is to determine the relationship between voltages around a closed loop (known as Kirchhoff s Voltage Law or KVL) and the relationship between currents in a node (known as Kirchhoff s Current Law or KCL). II. List of Components The following components will be used in this experiment: Six 1 kω resistors One 10 kω resistor One 100 kω potentiometer III. Prelab Assignment 1. Draw the circuit in Figure 8 and show how the DMM would be connected in order to measure the voltage V2. Show which DMM ports are connected to which parts of the circuit. 2. Draw the circuit in Figure 8 and show how the DMM would be connected in order to measure the current I2. Show which DMM ports are connected to which parts of the circuit. IV. Procedure Part 1: Measure the resistor values Measure and record the resistor values. Are they all within the 5% tolerance? University of Portland - p. 1 of 7 - Exp - Kirchhoffs Laws.docx

2 Part 2: Determine the relationship of voltages around a closed loop (KVL) Part 2a: Construct the circuit shown in Figure 1 with Vs = 6V and R1 = 1 kω. Measure the voltages around the closed loop formed by Vs and V1. Determine the relationship between Vs and V1. Figure 1: Simple Circuit Part 2b: Construct the circuit shown in Figure 2 with Vs = 6V, R1 = 1 kω and R2 = 2 kω. Measure the voltages around the closed loop formed by Vs, V1, and V2. Determine the relationship between Vs, V1, and V2. Figure 2: Pretty Simple Circuit University of Portland - p. 2 of 7 - Exp - Kirchhoffs Laws.docx

3 Part 2c: Construct the circuit shown in Figure 3 with Vs = 6V, R1 = 1 kω, R2 = 2 kω, R3 = 2 kω, R4 = 1 kω, and R5 = 10 kω. Measure the voltages Vs, V1, V2, V3, V4, and V5. Determine the relationship around the closed loop formed by V3, V4, and V5. Figure 3: Not So Simple Circuit Part 2d: Determine the relationship around any other loop of your choice in the circuit shown in Figure 3. Part 2e: Does the relationship hold for the loop formed by Vs, R2, R5, and R3? Explain. Part 2f: Does the relationship hold for the voltages that do not form a closed loop, such as Vs, V1, and V5? Explain. Part 3: Determine the relationship of currents into a node (KCL) Part 3a: Measure the currents Is, I1, I2, I3, I4, and I5 in the circuit in Figure 3. Part 3b: Determine the relationship of the currents for the node labeled A. Part 3c: Determine the relationship of the currents for any of the other nodes of your choice in the circuit in Figure 3. University of Portland - p. 3 of 7 - Exp - Kirchhoffs Laws.docx

4 Part 4: Determine the relationship of voltages for series resistors (voltage division). Part 4a: Construct the circuit in Figure 4 with Vs = 6V, R1 = 1 kω, and R2 = 2 kω. Measure Vs, V1, and V2. Determine a formula for V1 and V2 in terms of Vs, R1, and R2. This formula is called voltage division. Figure 4: Resistive Circuit Part 4b: Construct the circuit in Figure 5 with Vs = 6V, R1 = 1 kω, R2 = 2 kω, and R3 = 10 kω. Measure Vs, V1, V2, and V3. Determine a formula for V1, V2, and V3 (voltage division). Figure 5: Resistive Circuit University of Portland - p. 4 of 7 - Exp - Kirchhoffs Laws.docx

5 Part 5: Potentiometer A potentiometer, which is called a pot for short, is a variable resistor that is often used as a variable voltage divider. Among its many uses is to control the volume in audio electronics because it can be used to divide a voltage down to a lower level, which makes an audio signal quieter. A potentiometer has three pins. The center pin is called the wiper because it is a movable contact that slides across a resistor when you turn the knob. A potentiometer can be modeled as shown in Figure 6. Model for Potentiometer R 1 R 2 Symbol for Potentiometer k POT Figure 6: Electrical Model for Potentiometer Part 5a: Determine how a potentiometer works Measure the total resistance between pins 1 and 3 of the 100K pot. Does the resistance between pins 1 and 3 change when the knob is turned? Find the maximum and minimum resistance for R1. Find the maximum and minimum resistance for R2. Note that when R1 is at the maximum resistance, R2 is at the minimum resistance. University of Portland - p. 5 of 7 - Exp - Kirchhoffs Laws.docx

6 Part 5b: Setup the power supply to deliver 1V, and set the Current Control knob to limit the current to a low level (see Power Supply tutorial for details). Connect the circuit shown in Figure 7 being careful to connect the two outside potentiometer pins (pins 1 and 3) to the power supply, NOT the center pin (pin 2). NOTE: If the center pin (pin 2) of the potentiometer is accidentally connected to the power supply, it can short out the power supply and destroy the potentiometer. Measure the voltage V2 while turning the knob. What are the minimum and maximum values of V2? Explain how the voltage V2 changes as the knob is turned. 1 Vs 100k POT Figure 7: Potentiometer Circuit V 2 - Part 5c: Analyze the potentiometer circuit Model the potentiometer in the circuit in Figure 7 as shown in Figure 6, and find an equation that describes V2 in terms of Vs, R1, and R2. What are the maximum and minimum values of V2 according to your equation? Part 6: Determine the relationship of current in parallel resistors (current division). Part 6a: Construct the circuit in Figure 8 with Vs = 6V, R1 = 1 kω, R2 = 2 kω, and R4 = 10 kω. Measure IS, I1, and I2. Determine a formula for I1 in terms of IS, R1, and R2. This formula is called current division. Also determine a formula for I2. Figure 8: Resistive Circuit University of Portland - p. 6 of 7 - Exp - Kirchhoffs Laws.docx

7 Part 6b: There is an old saying that current takes the path of least resistance. Is that saying true? Does all of the current take the path of least resistance? Explain. V. Conclusion Write a paragraph that summarizes what you have learned in this lab. Describe the relationship between the voltages around a closed loop (KVL). Describe the relationship between the currents in a node (KCL). Describe how the voltage across a resistor that is in series with other resistors can be computed (voltage division), what a potentiometer is, and how the current through a resistor that is in parallel with other resistors can be computed (current division). University of Portland - p. 7 of 7 - Exp - Kirchhoffs Laws.docx