be.01.12 voltage & current dividers instructor lab

UNITED STATES MARINE CORPSMARINE CORPS COMMUNICATION-ELECTRONICS SCHOOL

TRAINING COMMANDBOX 788251

TWENTYNINE PALMS, CALIFORNIA 92278

INSTRUCTOR LAB HANDOUT

VOLTAGE & CURRENT DIVIDERS

BE.01.12

BASIC ELECTRONICS COURSE

M092721

REVISED 08/15/2008

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TERMINAL LEARNING OBJECTIVES:

1. Provided cables, connectors, Test Measurement and Diagnostic Equipment, tools and a unit under test; test ground electronics equipment, per the references. (2800-ACT-1303)

2. Given a faulty electronic device, Test Measurement and Diagnostic Equipment and tools, diagnose basic electronic circuits, per the references. (2800-ACT-1305)

ENABLING LEARNING OBJECTIVES:

1. Without the aid of references, select the statement that identifies the characteristics of a voltage/current divider, per the references. (2800-ACT-1303m)

2. Without the aid of references, given a voltage or current divider electronic circuit, TMDE, and references, connect the TMDE to the circuit to be tested, per the references. (2800-ACT-1303y)

3. Without the aid of references, given a voltage or current divider test circuit, TMDE, and references, adjust the TMDE to display a recorded measurement, per the references. (2800-ACT-1303z)

4. Without the aid of references, given a voltage or current divider test circuit, TMDE, and references, record specified circuit parameters, per the references. (2800-ACT-1303aa)

STUDENT INFORMATION:

1. Ensure that you have the primary references and Student Handout SH BE.01.

2. You will be responsible for the learning objectives. Ensure that you understand all the information contained in the Student Handout. Studying your notes, reading the references, and completing the assigned homework will accomplish this. The BE.01 Performance Examination and the BE.01 Written Examination will test the learning objectives.

3. Complete the practical application portion of this handout.

4. If you need extra help, ask!

STUDENT HANDOUT PRESENTATION:

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During this lab you will be using the DC FUNDAMENTALS BOARD also known as board 1.

I. VOLTAGE DIVIDER:

A. Ensure that the Base Unit Power Source is turned on. Ensure that the DC FUNDAMENTALS Circuit Board is inserted into the base unit. Do not energize the Power Supply to the circuit at this time.

B. Locate the VOLTAGE DIVIDER circuit block (refer to Figure 12-01).

Figure 12-01

C. Setup the multimeter to measure resistance. Measure and record the resistive value of R1 by placing the red test lead into the test point that is closest to the top of R1 and the black test lead into the test point that is closest to the bottom of R1.

1. R1 = _910 _____

D. Measure and record the resistive value of R2 by placing the red test lead into the test point that is closest to the top of R2 and the black test lead into the test point that is closest to the bottom of R2.

2. R2 = _1.6k ____

E. Measure and record the value of total resistance by placing the red test lead into the test point closest

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to the top or R1 and the black test lead into the test point that closest to the bottom or R2.

3. RT = _2.51k ___

F. Remove the Ohmmeter from the circuit and set it up to measure DC Voltage. Measure and record the applied voltage by applying the red test lead into the test point that is closest to the most positive part of the power supply (top) and the black test lead into the test point that closest to the most negative part of the power supply (bottom).

4. VA = _15VDC____

G. Remove the Voltmeter from the circuit. Insert a two-post connector into the two-post test points between the top of the power supply and R1. Measure and record the voltages indicated by placing the Voltmeter across the indicated components.

5. VR1 = _5.44VDC__

6. VR2 = _9.56VDC__

H. Using the equation for the proportionality method, calculate the voltage across the resistors.

7. VR1 = _5.44VDC__

8. VR2 = _9.56VDC__

9. Does the measured and calculated values for the voltage drops approximate each other?

Yes / No

I. Insert a two-post connector into the two-post test points between R1 and R2 in order to connect the load resistors into the circuit. Place switch S1 into position ‘A’ (up) to place R4 as the load. Measure and record the voltage of R1 by placing the red test lead into the test point that is closest to the top or R1

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and the black test lead into the test point that is closest to the bottom or R1.

10. VR1 = _5.78VDC__

J. Measure and record the voltage across the parallel branches of R2 and R4 by placing the red test lead into the test point closest to the top of the two resistors and the black test lead into the test point that is closest to the bottom of the two resistors.

11. VR2||R4 = _9.22VDC__

K. Move switch S1 to position ‘B’ (down) and repeat the readings as before. Note this time that the load resistor R3 instead of R4 is in parallel with resistor R2.

12. VR1 = _12.79VDC_

13. VR2||R4 = _2.07VDC__

L. Remove the two-post connectors and the test leads from the circuit. Click on the right arrow to advance to the next screen. Note that the CM light has turned on which indicates that CM-8 has been activated.

M. Setup the multi-meter to measure resistance. Measure and record the indicated values.

14. R1 = _1.59k ___

15. R2 = _1.6k ____

16. RT = _3.19k ___

N. Remove the test leads from the circuit and setup the multimeter to measure DC Voltage. Measure and record the applied voltage to the circuit.

17. VA = _15VDC____

O. Remove the test leads from the circuit. Insert two-post connectors into the two-post test points at the top of the power supply and between R1 and R2. Make

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sure that switch S1 is in position ‘A’. Measure and record the indicated voltages.

18. VR1 = _7.85VDC__

19. VR2||R4 = _7.15VDC__

P. Place switch S1 into position ‘B’ (down) and perform the indicated measurements as before.

20. VR1 = _13.74VDC_

21. VR2||R3 = _1.26VDC__

22. What effect does changing the value of the load resistor from 10 times greater than R2 to 10 times less than R2 have on the voltage drop across the load?

Increases / Decreases

Click on the right arrow to advance the screen to the review questions. Answer these questions on the screen as well as in this Lab Workbook.

1. An unloaded resistive voltage divider is a __________.

a. series circuit

b. parallel circuit

c. series/parallel circuit

d. combination of series and parallel circuits

2. The voltage drops in a series voltage divider are __________.

a. inversely proportional to parts of the applied voltage

b. equal to the current through each resistor divided by the resistance

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c. proportional to the series resistance

d. proportional to the applied voltage3. If R2 in this figure changed in value to 2.6k, the

voltage drop across R2 would be __________.

a. 3.86VDC

b. 7.5VDC

c. 8.86VDC

d. 11.1VDC

4. The value of R1 appears to have changed to 604 in the circuit depicted. What is the approximate value of RCM9?

a. R1 is in parallel with a 1k resistor

b. R 1 is in parallel with a 1.8k resistor

c. R1 is in series with a 1k

resistor

d. R2 is in parallel with a 1.8k resistor

5. When a load is added to a voltage divider, the resistance of the load is __________.

a. placed in parallel with the voltage divider

b. combined with the voltage divider

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c. combined with the resistor it parallels

d. placed in series with the voltage divider

II. CURRENT DIVIDER:

A. On the VOLTAGE DIVIDER Circuit Block depicted in Figure 12-01 place the switch S1 into position ‘A’ (up). Insert a two-post connector into the two-post test points between resistors R1 and R2.

B. Setup the multi-meter to measure DC Current. Measure and record total circuit current by placing the red test lead into the test point that is closest to the top of the power supply and the black test lead into the test point that is closest to the top of R1 (observe polarity).

1. IT = _6.36mA___

C. Using the nominal resistive values and the following proportionality equation, calculate the current flowing through R2.

2. IR2 = _5.76mA___

D. Using the nominal resistive values and the following proportionality equation, calculate the current flowing through load resistor R4.

3. IR4 = _600 A ____

4. Does the sum of the values of IR2 and IR4 equal IT.

Yes / No

E. Click on the right arrow to advance to the next screen. Note that the CM light has turned on which indicates that CM-3 has been activated.

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F. Ensure that the switch is still in position ‘A’ and that the multimeter is still configured to measure DC Current. Measure and record the indicated values as before.

5. IT = _4.93mA___

6. IR2 = _4.47mA___

G. Using the nominal resistive values and the proportionality equations used earlier, calculate the following current values.

7. IR2 = _4.47mA___

8. IR4 = _460 A ____

9. Does the sum of IR2 and IR4 equal the measured value of IT?

Yes / No

Click on the right arrow to advance the screen to the review questions. Answer the questions on the screen as well as in this Lab Workbook.

1. A current divider requires __________.

a. two or more resistors in series

b. two or more resistors in parallel

c. a 10:1 voltage ratio

d. a 10:1 current ratio

2. To determine a branch current in a circuit with two parallel resistors, which formula would be used?

a. I R1 = (IT x R2) / (R1 + R2)

b. IR1 = (R1 / R1) + (R2 x IT)

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c. IR1 = (R2 / R1) + (R2 x VA)

d. IR1 = R1 / (RT x VA)

3. With switch S1 in position ‘A’ as depicted in the figure, branch current for R2 is __________.

a. 0.57A

b. .057A

c. 5.7mA

d. 577A

4. With switch S1 in position ‘B’ as depicted in the figure, the current through R3 is __________.

a. 14.2mA

b. 13.1mA

c. 1.25mA

d. .598mA

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5. If the current through R1 decreased in the circuit depicted due to a change in R1, that change to the resistor would be __________.

a. a short

b. nothing

c. a decrease in resistive value

d. an increase in resistive value

III. LOADING VOLTAGE/CURRENT DIVIDERS:

A. Locate the VOLTAGE DIVIDER Circuit Block as depicted in Figure 12-01. Do not energize the circuit at this time.

B. Setup the multimeter to measure resistance. Measure and record the indicated values as before.

1. R1 = _910 _____

2. R2 = _1.6k ____

3. RT = _2.51k ___

C. Remove the multimeter from the circuit and set it up to measure DC Voltage. Measure and record the value of voltage dropped across resistor R2.

4. VR2 = _9.56VDC__

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D. Remove the multimeter from the circuit and configure it to measure DC Current. Measure and record total current flow in the circuit.

5. IT = _5.98mA___

E. Insert two (2) two-post connectors into the circuit at the junction between R1 and R2 and at the top of the power supply. Move switch S1 to position ‘B’. Setup the multimeter to measure DC Voltage. Measure and record the voltage drop on the load resistor by placing the red test lead into the test point closest to the top of the load resistor and the black test lead into the test point closest to the bottom of the load resistor.

6. VRL = _2.07VDC__

F. Remove the test leads from the circuit and setup the multimeter to read DC Current. Measure and record total current by replacing the two-post connector located at the top of the power supply with the test leads (observe polarity).

7. IT = _14.21mA__

G. Remove the test leads from the circuit and replace them with a two-post connecter. Move switch S1 to position ‘A’. Setup the multimeter to measure DC Voltage. Measure and record the load voltage as before.

8. VRL = _9.22VDC__

H. Remove the test leads and the two-post connector at the top of the power supply from the circuit. Setup the multimeter to measure DC Current. Measure and record the current through the circuit by placing the test leads into the two-post test points at the top of the power supply.

9. IT = _6.36mA___

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10. Compare the total currents from questions 5 and 7 with the value from question 9. To add a resistor in parallel with the circuit (to load it down) causes IT to __________.

Increase / Decrease / Remain The Same

11. What can be done to prevent a drastic increase in IT from circuit loading?

a. Make VA smaller.

b. Nothing.

c. Make the load resistor 10 times larger than the resistor it parallels.

d. Place the load in series instead of parallel.

Click on the right arrow to advance to the review questions. Answer these questions on the screen as well as in this Lab Workbook.

1. When a series voltage divider is loaded with a parallel resistor, the divider voltage decreases because __________.

a. total circuit current decreases

b. total circuit current increases

c. total circuit resistance increases

d. branch current decreases

2. Voltage dividers are used in electronic equipment to __________.

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a. provide circuits with different voltages

b. load down the power supply

c. increase the cost of the equipment

d. increase the voltage of the power supply

3. In order to change the value of resistor R2 from 1.6k to 1.08k, a resistor is added __________.

a. in parallel with R1

b. in series with R1

c. in series with R2

d. in parallel with R 2

4. If a 3.3k resistor is placed in parallel with R2, the voltage drop across R2 would be __________.

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a. 6.86VDC

b. 7.5VDC

c. 8.13VDC

d. 8.98VDC

5. As the load resistance across R2 increases, the voltage drop across R1 __________.

a. decreases

b. increases

c. stays the same

d. cannot be determined

CONCLUSIONS:

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1. A voltage divider is a string of resistors designed to produce various individual voltages less than the source voltage.

2. The voltage drop across resistors in a voltage divider is directly proportional to the resistance.

3. A current divider is two or more resistors in parallel which each other, designed to provide various currents which are less than the total line current.

4. The current through each branch in a current divider is inversely proportional to the resistance of the branch.

5. A load is that part of a circuit that absorbs (uses) power.

6. Loading a circuit is the effect that a device has on the overall circuit. A larger load produces a larger equivalent resistance which develops a larger current.

REFERENCES:

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1. Operator Manual: Fluke 77/BN2. Basic Electronics (9th Edition) 3. Introductory Electric Circuits (Electron Flow Version)4. WinFACET, Lab-Volt Systems Inc.

PREPARED BY: ____________________________________________TITLE: __________________________________________________SIGNATURE: ________________________ DATE: ______________MODIFIED BY: _P.A.DEDEAUX__________ DATE: _03/17/2008___APPROVED BY: ____________________________________________TITLE: __________________________________________________SIGNATURE: ________________________ DATE: ______________

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be.01.12 voltage & current dividers instructor lab

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