ENGR221_Lab _1_Spring_2024_Measurement of Resistance

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Example: ----- 1111111 -I - /I RedBlack OrangeSilver 20000±10% Resistor is 20,000 n ± 10% 1 3 Measurement of Resistance Name Date Class READING Text, Sections 2-1 through 2-5 OBJECTIVES After perfonning this experiment, you will be able to: I. Determine the listed value of a resistor using the resistor color code. 2. Use the DMM (or VOM) to measure the value of a resistor. 3. Detennine the percent difference between the measured and listed values of a resistor. 4. Measure the resistance of a potentiometer and explain its operation. MATERIALS NEEDED Resistors: Ten assorted values One potentiometer (any value) SUMMARY OF THEORY Resistance is the opposition a substance offers to current. The unit for resistance is the ohm, symbolized with the Greek letter capital omega (fl). A resistor is a component designed to have a specific resistance and wattage rating. Resistors limit current but, in doing so, produce heat. The physical size of a resistor is related to its ability to dissipate heat, not to its resistance. A physically large resistor can dissipate more heat than a small resistor, hence the larger one would have a higher wattage rating than the smaller one. Resistors are either fixed (constant resistance) or variable. Fixed resistors are usually color coded with a four-band code that indicates the specific resistance and tolerance. Each color stands for a number, as described in the text and reprinted in Table 3-1 for convenience. Figure 3-1 shows how to read the resistance and tolerance of a four-band resistor. The resistance of resistors is measured using a DMM or VOM, as described in Experiment 2. If you are using a VOM, the zero reading should be checked whenever you change ranges on the meter by Table 3-1 ,I 111 II , I- /I I st digit 2nd digit Multiplier Tolerance Nole: In the rnuhiplier band, Gold"' X 0.1 Silver"' XO.DI Figure3-1 I ' ' Digit Color 0 Black l Brown 2 Red 3 Orange Resistance value, 4 Yellow first three bands 5 Green 6 Blue 7 Violet 8 9 Gray White Tolerance, fourth band 5 % IO% 20% Gold Silver No band

2 touching the test leads together. If you are using a nonautoranging DMM, a suitable range needs to be selected. Resistance normally should not be measured in a circuit as other resistors in the circuit wi11 affect the reading. The resistor to be measured is removed from the circuit, and the test leads are connected across the resistance. The resistor under test should not be held between the fingers as body resistance can affect the reading, particularly with high-value resistors. (It is okay to hold one end of the resistor under test.) The most common form of variable resistor is the potentiometer. The potentiometer is a three-terminal device with the outer terminals having a fixed resistance between them and the center terminal connected to a moving wiper. The moving wiper is connected to a shaft that is used to vary the resistance between it and the outer terminals. Potentiometers are commonly found in applications such as volume controls. Another type of variable resistor is the rheostat. A rheostat consists of two terminals. The control varies the resistance between the two terminals. A potentiometer can be connected as a rheostat by connecting the moving wiper and one of the outer terminals. PROCEDURE l. Obtain IO four-band fixed resistors. Record the colors of each resistor in Table 3 - 2. Use the resistor color code to determine the color - code resistance of each resistor. Then measure the resistance of each resistor and record the measured value in Table 3 - 2. The first line has been completed as an example. 2. Compute the percent difference between the measured and color-coded values using the equation: difference = IR me.. uied -R color code I X I 00 Rcolorcode The percent difference is shown as an absolute (positive) value for a11 resistors. Complete Table 3-2. Table3-2 Resistor Color of Band Color-Code Value Measured Value % Difference Ist 2nd 3rd 4th 0 brown green red silver 1.5 kil + 10% 1.46 k!l 2.7% I 2 3 4 5 6 7 8 9 10

3 3. Obtain a potentiometer. Number the terminals 1, 2, and 3 as illustrated in Figure 3-2. m--L S che m at i c)' mbol Figure 3-2 Measure and record the resistance between terminals 1 and 3 of the potentiometer (the outside terminals). R1.3 = Vary the potentiometer's shaft and monitor the resistance between terminals 1 and 3. Does the resistance change? Explain. - 4. Turn the potentiometer completely counterclockwise (CCW). Measure the resistance between terminals 1 and 2. Then measure the resistance between terminals 2 and 3. Record the measured resistance in Table 3-3. Compute the sum of the two readings and enter it into Table 3 - 3. 5. Turn the shaft 1/3 turn clockwise (CW) and repeat the measurements in step 4. 6. Turn the shaft 2/3 tum CW and repeat the measurements in step 4. What did you find about the sum of the resistance in steps 4, 5, and 6? Table 3-3 Step Shaft Position Resistance Measured Between Sum of Resistance Readings Tenninals 1 - 2 Tenninals 2 - 3 4 ccw 5 ½ CW 6 cw CONCLUSION Terminal I

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4 EVALUATION AND REVIEW QUESTIONS 1. Predict the resistance between terminals 1 - 2 and 2 - 3 for the potentiometer if the shaft is rotated fully CW. 2. (a) Are any of the resistors measured in Table 3 - 2 out of tolerance? _ (b) You suspect that the percent difference between color-code and measured values could be due to error in the meter. How could you find out if you are correct? 3. Determine the resistor color code for the following resistors. The tolerance is 10 % . (a) 12 f! (b) 6.8 kn (c) 910 n (d) 4.7 Mn (e) 1.0 n 4. Determine the expected value and tolerance for resistors with the following color codes: (a) red-red-black - gold (b) violet-green - brown - silver (c) green-brown-brown-gold (d) white-brown-gold-gold (e) gray-red-yellow-silver 5. A resistor is color coded: red - violet-orange-gold. (a) What is the largest value the resistor can be and still be in tolerance? (b) What is the smallest value? 6. What is the difference between a potentiometer and a rheostat? FOR F U RTHER INVESTIGATION Obtain 20 resistors marked with the same color code on each resistor. Carefully measure each resistor. Determine the average value of resistance. Then find the deviation of each resistor from the average by computing the amount each value differs from the average. The average deviation can then be found by computing the average of the deviations. Summarize your findings. Are any resistors outside the marked tolerance? Is there any evidence that all resistors are either higher or lower than their marked values? What do differing values of resistors tell you about actual measurements as opposed to calculated or color - code values?

5 4 Voltage Measurement and Reference Ground Name Date Class READING Text, Sections 2-6 and 2-7. (Optional: See Section 4--9 for additional information on reference ground.) OBJECTIVES After performing this experiment, you will be able to: 1. Connect a circuit from a schematic diagram. 2. Use voltages measured with respect to ground to compute the voltage drop across a resistor. 3. Explain the meaning of reference ground and subscripts used in voltage definitions. MATERIALS NEEDED Resistors: One 330 fl, one 680 fl, and 1.0 kfl SUMMARY OF THEORY Energy is required to move a charge from a point of lower potential to one of higher potential. Voltage is a measure of this energy per charge. Energy is given up when a charge moves from a point of higher potential to one of lower potential. Voltage is always measured with respect to some point in the circuit. For this reason, only potential differences have meaning. We can refer to the voltage across a component, in which case the reference is one side of the component. Alternatively, we can refer to the voltage at some point in the circuit. In this case the reference point is assumed to be "ground." Circuit ground is usually called reference ground to differentiate it from the potential of the earth, which is called earth ground. Various symbols for ground are shown in Figure 2-49 of the text. This manual uses the symbol in Figure 2-49(a) throughout. An analogy can clarify the meaning of reference ground. Assume a building has two floors below ground level. The floors in the building could be numbered from the ground floor, by numbering the lower floors with negative numbers. Alternatively, the reference for numbering the floors could be made the lowest floor in the basement. Then all floors would have a positive floor number. The choice of the numbering system does not change the height of the building, but it does change each floor number. Likewise, the ground reference is used in circuits as a point of reference for voltage measurements. The circuit is not changed by the ground reference chosen.

6 B 1 8 8 V " Figure 4-1 illustrates the same circuit with two different ground reference points. The circuit in Figure 4-1 (a) has as its reference point B. Positive and negative voltages are shown. If the reference point is moved to point C, the circuit voltages are all positive, as shown in Figure 4-l(b). Voltage is always measured between two points. To define the two points, subscripts are used. The voltage difference (or simply voltage) between points A and B is written as V AB where the second letter in the subscript identifies the reference point. If a single subscripted letter is shown, the voltage is defined between the lettered point and the circuit's reference ground. I Forho1h c1rcoi 1 s; VA 8 4 V VAc= 12 VI 4V R 1 A l ' A = 4 v 1 1 , A v , 1 .. 12 v 4V R 1 +12 V B l ' g = O V +12 V +··+- - 1 8V R2 C l ' c = - 8 V , .... . _. C I ,. 0 V PROCEDURE (.a) (b) Figure 4-1 l. Measure three resistors with the listed values given in Table 4-1. Record the measured values in Table 4-1. You should always use the measured value in experimental work. 2. Construct the circuit shown in Figure 4-2. An example of how a series circuit like this can be built on a protoboard (solderless breadboard) is shown in the Introduction to the Student section, Figure 1-2. Set the power supply to +IO V. Measure the voltage across each resistor in the circuit. Enter the measured values in Table 4-2. Table 4-1 Table 4-2 ,\ I( I 1 · , = +10 V j >R 6§on . R W D/ 1.0kQ ' C Figure 4-2 -1 8 V R 2 VB<. .- 8 v /B Measured Value Vs VAB Vac Vco Component Listed Value Measured Value R, 3300 R2 6800 R3 1.0 k!l

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Measured Voltage Voltage Difference Calculation VA Ve VAe = VA Ve = Vee = Ve - Ve = Ve 0.0 V (ref) Veo = Ve - Vo = VD 7 3. Assign point D as the reference ground. Measure the voltage at points A, B, and C with respect to point D. The voltage readings are made with the reference probe connected to point D. Enter the measured values in Table 4-3. Then use the measured voltages to compute the voltage differences VAB• Vsc, and Ven· Table4-3 Measured Voltage Voltage Difference Calculation VA VAe = VA - Ve = Ve Vee = Ve - Ve = Ve Veo = Ve - Vo = Vo 0.0V (ref) 4. Now measure the voltages in the circuit with respect to point C. The circuit is not changed. Only the reference point changes. Move the reference probe of the voltmeter to point C. This point will now represent ground. The voltage at point D now has a negative value. Enter the measured voltages in Table 4-4. Compute the voltage differences as before and enter them in Table 4 - 4. Table4-4 5. Move the circuit reference point to point B. Again, there is no change to the circuit other than the reference ground. Repeat the measurements of the voltages with respect to circuit ground. Compute the voltage differences and enter the data in Table 4-5. Table 4-5 Measured Voltage Voltage Difference Calculation VA VAe c: VA - Ve = Ve 0.0 V (ref) Vee = Va - Ve = Ve Veo = Ve - Vo = VD

Measured VoltageVoltage Difference Calculation VA 0.0 V (ref) Va V e Vo VAB = VA Yse = Va - Yeo = Ve Ve "" V e .,. Vo = 8 6. Now make point A the reference point and repeat the measurements. Enter the data in Table 4-6. Table4-6 CONCLUSION EVALUATION AND REVIEW QUESTIONS 1. Compare the voltage difference calculation in Table 4-3 through Table 4-6. Does the circuit's reference point have any effect on the voltage differences across any of the resistors? Explain your answer. 2. Define the term reference ground. 3. If you measured V A 8 as 12.0 V, what is the V 8 A? 4. Assume VM - - 220 V and VN = - 150 V. What is VMN? 5. If a test point in a circuit is marked + 5.0 V and a second test point is marked 3.3 V, what voltage reading would you expect on a voltmeter connected between the two test points? Assume the reference lead on the meter is at the lowest potential. FOR F U RTHER INVESTIGATION Warnin g : The power supplies used in this procedure must have floating output terminals. If you are not s ure, check with your instructor before proceeding with this investigation. Replace the + 10 V supply used in this experiment with two + 5 V supplies in series. Attach the 5 V output of one supply to the common of the second supply. Call this point the reference ground for the circuit. Measure the voltages throughout the circuit. Summarize your results.

ENGR221_Lab _1_Spring_2024_Measurement of Resistance

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