Lab 1 Resistance Voltage and Current Measurements

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California State University, Sacramento *

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117L

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Electrical Engineering

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Dec 6, 2023

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117L, Laboratory 1 RESISTANCE, VOLTAGE AND CURRENT MEASUREMENTS Pre-Lab : Prior to the laboratory meetings, make a data sheet that can be used to record all of the data that will be taken during the experiment. That way you can write your lab report as you proceed with the experiments. Use a word processor or spreadsheet so that your data is collected in a digital format. The lab report must have your name, your lab partner’s name (if working in in a team), and the date at the top. Purpose : The purpose of this experiment is to study DC measurements of resistance, voltage and current. Voltage and current divisions as well as Kirchhoff’s laws will be verified. We will used Digital Multi-Meters (DMM) and/or Analog Discovery 2 kits to make resistance, voltage and current measurements. It is impossible to insert a meter in an electrical circuit without altering the circuit, thus the quantity being measured. This experiment will investigate how the impedance of the test probes can impact the measurements. Analog Discovery 2, Probes and Meters This lab will use both a Digital Multi-Meter (DMM) and an Analog Discovery 2 kit (AD2). When documenting your work, it is best to identify the make and model of the test equipment being used for your measurements. For example, for this lab a “Fluke 77 DMM and Digilent Analog Discovery Kit 2 was used” In the lab, we will use breadboard to build our circuits. Figure 1 shows a set-up with the AD2 and a diagram of the AD2 connector interface. Figure 1, Analog Discovery 2 and breadboad set-up. When making measurements with an oscilloscope, it is important to be aware of the probe capabilities. Figure 1 shows the label on the oscilloscope probes that come with the AD2 kit. This particular probe is rated for 100MHz operation and has a 1x and 10x multiplication factor that can be set by switching the

117L, Laboratory 1 RESISTANCE, VOLTAGE AND CURRENT MEASUREMENTS red button on the probe tip. In this lab, we will generally use the 1x setting . When using an oscilloscope, you always want to be aware of the probes specifications. When measuring high frequency circuits, it may be necessary to use oscilloscope probes that have higher bandwidth. If measuring low voltage signals, one may select a probe with a different multiplication factor (for example 10x or 100x). In the 1x mode, the DC resistance of the probe is ≈ 1MΩ, while in 10x more the resistance is ≈ 10MΩ. We will check this later in the lab. Figure 2. Oscilloscope probe that comes with the Analog Discovery 2 kit. When making voltage measurements, we connect the meter in parallel with the circuit element we want to test. When using the DMM, make sure the probes are plugged into the proper sockets. Most DMMs have separate connections for voltage measurements and current measurements. When in resistance and voltmeter mode, the DMM will present a very high parallel resistance (>10MΩ) and should apply minimal load to the circuit. When making current measurements, ammeter is connected in series with the circuit element where we want to measure current. When configured for current measurements, the DMM will have a very low series resistance (<<1Ω) . Figure 3, DMM Connections

117L, Laboratory 1 RESISTANCE, VOLTAGE AND CURRENT MEASUREMENTS Figure 3, Voltage and Current Measurements PART I. Resistance Measurements In Part II and III of this lab, we will use the resistor values identified in table 1. Select the resistors, measure their resistance with the DMM and calculate the percent error from the nominal value. Before making any voltage or current measurements, the resistance of all of the resistors used in this experiment should be measured and recorded in format shown in table 1. As part of the laboratory report the percent error (between measured and specified values) for each resistor should be calculated. Table 1: Resistance Measurements Reference Designator Resistor Value, Ω Measured Value Percent Error R1 100K R2 390K R3 1.0K R4 2.0K R5 3.3K DMM = (Brand and model number) PART II Voltage Measurements Resistance Voltage and Current Measurements : DMM Voltmeters typically have very high input resistance (around 10GΩ) when the voltage is below 3V. This is so they induce minimal additional load on the circuit. With such high input resistance, the voltmeter may be considered an open circuit for most readings. A. Unloaded Voltage Readings Construct the voltage divider circuit shown in figure 4.

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117L, Laboratory 1 RESISTANCE, VOLTAGE AND CURRENT MEASUREMENTS Figure 4, Voltage Divider On your breadboard, build the circuit similar as shown in figure 1. Use connections to the V+ and GND pins on the AD2 to provide the 2VDC voltage. Plug the AD2 USB connector into your computer, open the Waveforms software, select the device manager form the menu selection and activate your AD2 module. Figure 5, Activate the AD2 Module, Device Manager Menu Figure 6, Activate the DC power Supply , Voltage Source Menu?

117L, Laboratory 1 RESISTANCE, VOLTAGE AND CURRENT MEASUREMENTS Three voltages (the input voltage and two resistor voltages) should be measured for the circuit. You should be able to show that the three voltages verify Kirchhoff’s voltage divider law. In addition, voltage division may be used to verify the two resistor voltages. You may assume that the resistor values are as measured in Part I. The voltmeter uses the same terminals as the Ohmmeter in the DMM. Record your data in a table like the following: Table 2: Voltage Measurements Voltage DMM Volts, V Figure 4 AD2 Volts Figure 4 1x probe setting AD2 Volts Figure 4 10x probe setting V4 VR1 VR2 DMM = xxxx? B. Loaded Voltage Readings This part considers how the internal resistance of the oscilloscope probes might affect the measurement. Select the Voltmeter from the menu bar in the Waveforms program. Note that for DC measurements, the jumper on the AD2 probe interface board should be moved from AC to DC as shown in figure 7. Figure 7, Jumper for AD2 DC voltage measurements With the AD2 probe, repeat the 3 voltage measurements in table 2. First collect the measurements with the channel 1 probe set for 1x multiplier. Then repeat the measurements with the probes set for 10x multiplier. With the voltages measured on R2, you should be able to calculate the resistance seen looking into the oscilloscope probe. Assume that the DMM provides the highest parallel resistive load and the resistance measured in Part 1 is accurate. The resistance measured across R2 is now the parallel combination of R2 and the resistance oscilloscope probe. Use the parallel resistor equation to calculate the probe resistance when the AD2 probe is in the 1x and 10x modes

117L, Laboratory 1 RESISTANCE, VOLTAGE AND CURRENT MEASUREMENTS PART III Current Measurements The next group of measurements uses the circuit in Figure 5. Before building the circuit on your breadboard with the resistors that were measured in Part 1. Three currents (the three resistor currents) should be measured for this circuit. You should be able to show that the three currents verify Kirchhoff’s current law. In addition, current division should be used to verify the currents in the two parallel resistors. As shown figure 3, the ammeter function on most DMMs uses different plug ports and dial settings. When in ammeter mode, the DMM has a very internal resistance and can be considered a short circuit. Since current is a "through" (series connection), the circuit must be "broken" to insert the ammeter. The data sheet for part III could look like the following: Table 3: Current Measurements Figure 8, Current Divider Circuit DISCUSSION TOPICS At the end of every laboratory report, list the discussion questions and your specific answers. It is expected that most students will be able to provide additional insights, as well as connections between experimental and theoretical analysis. These topics are to be discussed in sentences and are not to be answered with one or two word answers. 1. How close to the color code values were the resistors and were they within the specified tolerances. (refer to color code chart ) 2. Were Kirchhoff’s laws verified? 3. Were current and voltage division verified? 4. Did the oscilloscope probed produce different voltage measurements when in 1x and 10x mode? Which setting provided higher parallel resistance? 5. Did the measured input voltage V4 match that shown on the AD2 display? 6. Would you rather use a 1x or a 10x oscilloscope probe if you wanted to disturb the circuit as little as possible? Reference Designator Resistor Value, Ω Measured Value Percent Error Series Current R3 2K R4 1K R5 3k

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