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TABLE OF CONTENTS

LAB 1

ELECTRONIC CIRCUIT TESTING AND TROUBLESHOOTING

Page

LEARNING OBJECTIVES 3

INTRODUCTION 3

CONTENTS OF THIS LAB 3

WHAT YOU WILL ALSO NEED FOR THIS LAB 4

PREREQUISITES 4

REFERENCES 4

EXPERIMENT 1 5

EXPERIMENT 2 6

EXPERIMENT 3 8

EXPERIMENT 4 10

EXPERIMENT 5 12

EXAMINATION 14

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LAB 2W

ELECTRONIC CIRCUIT TESTING AND TROUBLESHOOTING

LEARNING OBJECTIVES

When you complete this lab, you will be able to:

1. Name the measurements you can make with a digital multimeter.

2. Use a digital multimeter to measure voltage, current and resistance.

3. Construct a power supply circuit and demonstrate its operation.

4. Troubleshoot the power supply with a digital multimeter.

INTRODUCTION

This lab introduces you to the digital multimeter. It is by far the most useful

of all electronic test instruments. You can measure DC and AC voltages,

DC current, and resistance. It is the primary test instrument in almost all

electronic testing, troubleshooting and servicing. In this lab you will learn to

use the meter.

This lab also includes a power supply kit. All electronic equipment contains

some kind of power supply that takes the AC power line input and converts

it to one or more DC voltages that power the electronic circuits. A great

deal of troubleshooting involves power supplies. Of all the circuits in

electronic equipment, the power supply is one of the most likely to fail. In

this lab, you will build a typical power supply then learn how to test and

troubleshoot it with the multimeter. You will also be able to use the power

supply later in the program and for your own projects.

CONTENTS OF THIS LAB

With this lab you should have received the Elenco M-1000C digital

multimeter and the Elenco XP-15K power supply kit. You will also receive

an envelope of additional resistors: 2.5 ohms, 5 watt and 75 ohms, 5 watts.

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WHAT YOU WILL ALSO NEED FOR THIS LAB

You will also need some common hand tools and other items to complete the

project. These items are listed below.

Soldering iron (small tip, low wattage, 15-35 watts)Side cutters (small size)

Needle nose pliers (small size)

Small standard screwdriver (flat blade and Philips types)

Wire strippers or knife

Safety Goggles (highly recommended when soldering)

9 volt battery

Typical alkaline cell (size AA, C or D)

You can purchase the tools and batteries at any hardware store. Most RadioShack stores also have these items. You should have received the soldering

iron as part of Lab 1 in this program.

The experiments in this Lab will use the multimeter and some of the

components from the power supply kit before you build the kit.

PREREQUISITES

You should have completed course EET 104 before completing this Lab.

REFERENCESIf you need to refresh your memory on any of the subject matter in this Lab,

refer back to the text Crash Course in Electronics Technology. Unit 12 on

power supplies and Unit 15 on Test Equipment and Troubleshooting will be

especially helpful.

EXPERIMENT 1

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MULTIMETER FAMILIARIZATION

OBJECTIVE

Become familiar with the capabilities of the digital multimeter.

PROCEDURE

1. Unpack the M-1000C multimeter. Set the test leads aside for the

moment and find the operator’s instruction manual.

2. Read all of the operator’s instruction manual.

3. Answer the following questions.

a. What is the power supply for this multimeter?

b. List the four main characteristics the meter will measure.

c. Can the meter measure AC current?

d. What is the maximum AC voltage the meter can measure?e. What is the lowest DC voltage range?

f. What is the maximum DC current range?

g. Give the resistance measuring range of the meter.

h. What two semiconductor devices can be tested with this meter?

i. What is a continuity test?

j. The red test lead is plugged into the ________ jack on the

meter and the black lead is plugged into the _______ jack.

Check your answers in the next section.

FOLLOW UP

Here are the answers to the questions above.

a. A 9 volt battery.

b. DC voltage, AC voltage, DC current, and resistance.

c. No, AC current cannot be measured.

d. The maximum AC voltage that can be measured is 750 volts.

e. 200 mV

f. 10 amperesg. Less than 200 ohms up to 2 Megohms (2000K ohms).

h. Diodes and bipolar transistors.

i. A continuity test is a resistance measurement to see if a low

resistance electrical path (continuity) exists between two points.

j.

k. V ΏmA, COM

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EXPERIMENT 2

USING THE MULTIMETER

OBJECTIVE

To use the multimeter to measure DC and AC voltages, resistance and

continuity.

PROCEDURE

1. Connect the test leads to the meter: red to V ΏmA and black to

COM.

2. Set the meter to read DC voltage. Use the 20 volt range. Note:

Always select the range just above the expected voltage. 3. Locate a 9 volt battery and test it with the meter. Put the red lead

on the positive terminal and the black lead on the negativeterminal. Record the voltage. ________________

4. Reverse the test leads on the battery terminals, red to negative and

black to positive. What does the display read? ______________

5. Repeat step 3 using an alkaline cell (size D, C, AA or AAA).

______________________

6. Repeat step 4 using the same alkaline cell. ___________________

7. Set the meter to read AC voltage on the 200 volt range.

8. Insert the probes into a convenient AC wall outlet.

BE EXTREMELY CAREFUL NOT TO SHORT THE LEADS

OR TOUCH THE METAL PART OF THE PROBES AS A

SHOCK WILL RESULT!! Measure and record the AC voltage. _____________

9. Locate the parts for the XP-15K power supply. In one of the

plastic bags, find the two color-coded resistors.

10. Read the color codes for the two resistors and record their values.

___________________, ___________________

11. Set the multimeter for measuring resistance. Use the 200 or 2000

ohm ranges as needed.

12. Measure the two resistors with the meter and record the values. _____________________, ______________________

How do your color code values compare to the measured values?

Are the resistors within tolerance?

13. Locate the AC power cord for the power supply kit. Use the

multimeter to check for continuity between the AC plug prongs

and the unconnected wires at the other end of the cable.

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14. Set the mulitmeter to measure continuity. (2000 ohm range)

15. Connect one test lead to an AC plug prong then check both of the

bare wires for an indication. Repeat with the other prong and wire.

What resistance value do you measure? ___________________

16. Find the two 1N4001 diodes in the power supply parts bag.

17. Test both diodes with the multimeter. Record the forward voltages

for both diodes. _________________, ___________________

18. What reading does the meter give if the diode is not connected in

the forward biased condition?

FOLLOW UP

In testing the 9 volt battery, you should have measured 9 volts or a bit

more. A new fresh 9 volt battery may read as high as 9.25 volts. An

alkaline cell, regardless of its size, should measure 1.54 volts if new.

When you reversed the leads, you should have measured the samevoltage but a negative sign was displayed indicating that the leads are

reversed.

The AC voltage measurement should have produced a voltage of about

120 volts. The actual value will vary in your area from about 110 to 125

volts depending upon the time and your distance from the power station.

The two resistors from the power supply kits are 130 ohms and 1500

ohms (1.5K) with a tolerance of ± 5%. The range on the 130 ohm

resistor is ± 6.5 ohms. The resistor is within tolerance if it is in the 123.5

to 136.5 ohm range. The range on the 1500 ohm resistor is ± 75 ohms.

The OK range is 1425 to 1575 ohms. Your readings on the meter should

have been within these ranges.

A continuity test should have produced a buzz when you found the wire

connected the AC plug prong. If the meter does not buzz, a resistance

reading of zero provides the correct indication. No connection or an

open circuit produces a 1 in the left hand position on the LCD display.

Your testing of the diodes should have produced readings in the 450 mV

range. The negative (black) lead should have been connected to the

diode cathode (end of the diode with the band). Reversing the diode

produces a meter reading of 1 indicating an open circuit or very high

resistance.

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EXPERIMENT 3

BUILDING THE POWER SUPPLY KIT

OBJECTIVE

To assemble the power supply kit and verify its operation.

PROCEDURE

Constructing the Kit

1. Unpack the Elenco XP-15K. Make sure you have all the parts.

2. Read the entire Assembly and Instruction Manual to be sure of what you are going to do. Gather all of your tools and materials.

3. If you have not soldered for a while, be sure to read the Soldering

Techniques section on page 2.

4. Follow the directions given in the manual to assemble the kit. Stop

on page 6 of the manual after you have completed the line cord

assembly. Do not put the power supply in its box yet as you will

be making some measurements on the circuit.

5. Before testing the unit, go back and double check your wiring and

component installation. Be sure all connections are soldered and

all the parts are in the right place.

Testing the Power Supply

1. Plug in the AC line cord.

2. Set your multimeter for DC voltage measurements on the 20 volt

range. Touch the probes to the power supply output terminals.

You should read a voltage between about 1 and 15 volts.

3. Vary the voltage control to see that the output voltage varies from

1.25 volts to 15 volts. If you can vary the voltage control on thefront panel, the power supply is operating properly.

4. Go on to the section called Testing the XP-15 Power Supply on

page 7 of the manual. Perform the tests indicated. If you get the

results specified, the unit is working satisfactorily.

5. In the test procedure on page 7, notice that you are looking for a

0.1 volt decrease in output voltage when you connect the load.

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What is the reason for this test and what name is given to this

characteristic of the power supply?

6. If the unit does not work as specified, go to the troubleshooting

section in the manual on page 9. Troubleshoot the power supply

given the details on page 9. Some things to look for are:

a. Unsoldered or poorly soldered connections.

b. Diodes installed backwards.

c. Electrolytic capacitors installed backward.

d. Regulator IC wired incorrectly.

e. Wires not connected to anything.

7. If you find a problem, correct it then repeat the tests in the steps

above.

8. If you cannot find the problem, contact AII with the details on your

problem.

FOLLOW UP

When you connect a load to a power supply, the output voltage will

naturally drop. This is caused by an increased voltage drop across the

regulator series pass transistor, the transformer windings, diodes and a

decreased capacitor charge. This output deviation is what the regulator IC

corrects for. Without the regulator, the output could drop up to several volts

when a load is applied. The regulator compensates for any output variations.

With the regulator the drop is only 0.1 volt maximum which is very good.

This characteristic of the power supply is referred to as its regulation. If you

will look at the specifications of the power supply on page 2 of the manual,

you will see a load specification of 0.1 volt which is the maximum amount

the output will change with maximum load applied.

Note the line regulation specification on page 2 also. If the AC input line

voltage should change, and it does by several volts during the day, the power

supply output voltage will also change. The regulator IC corrects for this as

well and there will be a maximum variation of 0.1 volt.

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EXPERIMENT 4

POWER SUPPLY ANALYSIS

OBJECTIVE

Study the power supply circuit and its operation.

PROCEDURE

1. Read the sections Circuit Description and Regulator Circuit on

pages 7 and 8 of the power supply manual.

2. If you have difficulty understanding any of this material, review

Unit 12 in your text Crash Course in Electronics Technology.

3. Answer the following questions about the circuit. Refer to the

schematic diagram on page 10 of the manual.

a. What type of rectifier is used in this circuit? b. Which part is the filter capacitor?

c. Which two components determine the output voltage?

d. The IC input voltage from the rectifier diodes is higher than the

regulator output voltage. What happens to the difference

between the two voltages?

e. Where would you put an off/on switch in this circuit?

f. Does the regulator IC reduce the amount of ripple at the output?

4. Take the Quiz on page 9 of the manual and check your answers.

FOLLOW UP

As a quick review of the power supply, the transformer steps the 120

volts AC line down to about 36 volts that is center tapped to provide two

sources of 18 volts to the diodes. The diodes form a typical full wave

rectifier. C1 is an electrolytic capacitor that filters the pulsating DC into

a more constant DC. There is about 2.5 volts of AC ripple at this point in

the circuit.

The IC regulator compensates for AC line voltage or load changes tomaintain a constant output voltage. This voltage is determined by the

values of R1 and the pot VR1. C2 is an additional filter capacitor. The

regulator IC does reduce the ripple because the circuit compensates for

output voltage variations and ripple is such a variation.

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The specific answers to the questions in step 3 are:

a. Full wave

b. C1

c. R1 and VR1

d. The difference between the IC input and output voltages

appears across the regulator series pass transistor. This is the

circuit operating voltage. The IC input is always about 20 volts

while the output varies depending upon how the output voltage

is set. If the output is set to 12 volts there will be 20 – 12 = 8

volts across the transistor. This difference voltage is lost and

causes high power dissipation in the series pass transistor. This

power is dissipated by attaching the IC to a metal “heat sink”

that helps to absorb the excess heat and protect the IC.

e. The most common place for an on/off switch is in series with

the AC line and the transformer primary winding.f. Yes, the regulator does reduce the output ripple considerably.

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EXPERIMENT 5

POWER SUPPLY TROUBLESHOOTING

OBJECTIVE

To learn a method of locating a problem in a power supply.

PROCEDURE

1. At this point, your power supply should be working but not yet

installed in its box.

2. Check to remove any resistors from the output terminals.

3. Assume that the power supply does not work. That means that you

do not get the desired DC voltage at the output. You would

determine that by a voltage measurement with your multimeter.

Assume the output is zero. 4. Look at the schematic diagram of the power supply on page 10 of

the manual. There are about a half dozen or so reasons why the

output is zero. But as a starting point, note that if the output is

shorted, the output would be zero. So be sure to remove any load

before doing any troubleshooting.

5. If the output is still zero, the capacitor C2 may be shorted. You

could remove one lead of this capacitor and remeasure the output.

If you do get a reading, C2 was shorted and you can replace it.

6. Next, the IC may be bad. One quick way to check this is to

measure the voltage at the input to the regulator at pin 1 on the

terminal strip. It should be about 20 volts DC. If you do measure

20 volts but zero output, then the IC is bad. Or resistors R1 or

VR1 may be open.

7. Turn on the power supply and measure this voltage yourself.

8. If there is no DC at the output of the rectifier, the LED will be off.

This could be caused by a shorted C1 or open or shorted diodes.

You can remove one lead of C1 and then remeasure. If you get a

DC voltage, C1 was shorted and you can replace it.

9. If you do not get a DC voltage, then one or both of the diodes may be bad. You can remove them and test them with your meter then

replace them if defective.

10. One remote possibility is a bad transformer. Most transformers are

pretty reliable and rarely fail. But occasionally one of the

windings may open. You can measure the AC voltage across the

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secondary winding yellow wires. Make that measurement now. It

should be about 36 volts.

11. If you still do not have voltage, measure the AC line input. It

should be about 120 volts. If you do not get this, then the line cord

or its plug is defective. If these are OK then there may be no

voltage at the wall outlet. Check for a blown breaker.

12. One measurement you may need to make is output current.

Assume an output voltage of 8 volts and a load resistor of 75 ohms.

Calculate the load current.

IL = _________________________

13. Turn on the power supply and set the output voltage to 8 volts.

14. Set your multimeter to measure current on the 200 mA range.

15. Connect one lead of the 75 ohm resistor to the black output

terminal. Then touch the meter probes to the red output terminal

and the free lead of the resistor. Note the current. IL = ____________________________

16. Is the meter connected in series with or in parallel with the load?

FOLLOW UP

The troubleshooting procedure described here works from the output back to

the input. You can also work from the input to the output but it is usually

easier to go in the reverse direction as it usually leads to a faster solution.

Keep in mind, that there could be multiple problems. For example, if C1

shorts, it could also damage the diodes.

The load current for 8 volts across 75 ohms is I = 8/75 = 0.1067 amperes or

about 106 mA.

To measure the current, you have to put the meter in series with the load and

output voltage. Your measured value should have been close to 106 mA.

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EXAMINATION

LAB 2W

Place a check beside the letter corresponding to your answer

for each question. Then mark your choices on the Answer

Sheet. Send only the Answer Sheet to AII.

1. If you wish to measure a voltage of 50 volts with the digital

multimeter, what is the optimum range to use?

a. 2 volt

b. 20 volt

c. 200 volt

d. 1000 volt

2. A continuity test is performed with the digital multimeter set to

measure

a. Voltage

b. Current

c. Resistance

d. Diode forward voltage

3. To measure current with the multimeter, the leads must be

connected how?

a. In series with the circuit. b. In parallel with the circuit.

4. Looking at the schematic diagram of the power supply on page 10

of the manual, select the components below that help minimize

ripple.

a. C1

b. C2

c. IC1

d. All of the above.

5. The output of the power supply may be shorted and not cause

damage.

a. True

b. False

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6. Which component in the power supply dissipates the most power?

a. C1

b. D1 and D2

c. IC1

d. T1

7. An infinite resistance reading or an open circuit is indicated on the

LCD multimeter screen as

a. A 1 in the left-most position

b. An infinity symbol.

c. A blank screen

d. 9999

8. Refer to the power supply schematic on page 10 of the manual.

You measure a DC voltage of 20 volts at pin 3 of the IC and avoltage or 6 volts at the power supply output. The voltage drop

across the IC is

a. 6 volts

b. 14 volts

c. 20 volts

d. 26 volts

9. Assume that diode D2 in the power supply opened. What would

be the effect on the power supply?

a. The output would be zero.

b. The output would be half of the voltage produced if both diodes

are working.

c. The output would be about 20 volts AC.

d. The power supply would still work but the ripple output would

be higher.

10. Why is the regulator IC mounted to the aluminum panel?

a. There is no where else to put it.

b. The panel helps dissipate the heat produced in the IC.c. The panel is the circuit ground.

d. It makes the IC close to the transformer.

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