Four Components of a circuit Power supply Conductor Load device Controlling device

Power Supply Generator Line source (wall outlet) Transformer secondary Multimeter (when used as an ohmeter)

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Conductors Wires Solder runs (see on circuit board) Circuit protection devices (fuses, circuit breakers)

Load devices Heating elements Relay coils Solenoids Resistors

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Multimeter (when used as a voltmeter) Motors Lamps NOTE: Not All Load Devices Demonstrate Their Use

Of Power (Resistors And Transformers) Controlling devices

Switches Relay

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TROUBLESHOOTINGFigure 1. Basic DC Circuit

NOTE: S1 is optional

Circuit Simplification How circuits are identified: • By what mfg or engineer states • By load device • By load device and controlling device (a

switch), ie (S3) • By load device and lock-in path• Load device and locked in an energized

state by controlling device (relay points) • Lock in path is an alternate path of current • By load device and mode of operation

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Developing an operational chart and sequence of events Mechanical - what the operator does to get the

circuit to perform (flip switches, set timer, control settings, etc)

Electrical - what happens electrically in the circuit in response to the operators actions (lamp lights, motor runs, etc)

State of the load device Load device energized Load device is not energized

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Troubleshooting Procedure Perform sequence of events

Write down all symptoms - things which you should see, hear, or feel happen but do not

Terminology used to identify symptom DNE - device fails to energize at any time DNLI - device energizes but fails to lock in DNDE - device does not de-energize at designated time

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Circuit analysis Rule out all component(s) which do not cause all

symptoms shown Identify all components which could cause all

operational symptoms Check current path of faulty component(s) for good

voltage Choose a half way point for troubleshooting large

circuits

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Identifying Possible Malfunctions Using Circuit Extracts Format for circuit extracts • Rectangular • Power source on left (only)• Load device on right (only)• Conductor on top and bottom to include switches

and tie points Size • Only one per normal size sheet of paper, on both

sides • Large enough to read at a glance

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Labeling • Must include tie points • All components must be labeled • All switches will be drawn in the position they are

shown in on the main schematic • Include all wire colors and wire numbers

Advantage in troubleshooting • Easier to parallel troubleshoot (sides of line easily

open) • Concentrates attention on circuit to be troubleshot

rather than entire schematic

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Use of voltmeter (locating the problem) Measures the voltage difference between the two

leads Black lead - reference lead - common Red lead - testing lead Set meter to read what you would normally expect to

find Voltmeter compares the voltage on the red lead to

the voltage on the black lead

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A voltmeter is like a load device When the meter indicates a voltage level all

conductors between the power source and meter leads must be good NOTE: Start meter at highest setting for safety and about ghost voltage

If the meter does not read a voltage and power is applied, there is an open between the power source and the meter leads NOTE: A multimeter set to ohms works differently - the meter is the power source and it checks the resistance to current flow between the meter leads

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Determine reference point Know a good point in functioning area of circuit If entire unit does not work place leads across

incoming power - if meter reads voltage, leave one lead in place for reference

When moving into a different power supply you must move your reference lead to within the power supply to be tested

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Parallel Troubleshooting NOTE: Before ANY type of troubleshooting takes

place, all switches and or controlling devices MUST be in the proper position as to energize the load device

The most common method involves keeping meter parallel to power source and load device - one lead on each side of incoming power line

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Procedure • Connect reference lead • Check other side of line with testing lead

Start at first known point closest to power source Check point by point out to load device If voltage read up to load device - that side of the line is

good as a reference and then check the other side of the line up to the load device

If the meter reading is 0 volts the open is between the last place you read voltage and the first place 0 volts was read

If voltage is read to both sides of a load device and it does not energize - LOAD DEVICE IS OPEN

• When the meter indicates a voltage level it is energized

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TROUBLESHOOTINGFigure 3. Basic DC Circuit 2

Series Troubleshooting Keeps leads on the same incoming line Zero voltage means that the line between the two

test leads are good Applied voltage read on a meter means there is

an open somewhere between the two testing leads

Procedure • Determine which line you will check first and connect

the reference lead as close to the power source as possible

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Insure that the reference is good by checking the opposite line for applied voltage

Check point by point out to the load device until applied voltage is found

If applied voltage is found; then the open is between where the voltage was found and the last reading of zero volts

If zero volts is found up to the load device, then the side of the line you are testing is good and the procedure should be repeated on the opposite side of the line

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Loop Troubleshooting Method Loop troubleshooting is a combination of

both series and parallel methods As with either of the above methods, the

object is to locate an open by observing a difference of potential where there should be none

Procedure • Connect the reference lead as close to the incoming power

supply as possible (usually the neutral or dc negative side) • Test for incoming voltage at the power supply

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Move along the hot line and parallel troubleshoot this line

Once at the load device (if an open hasn't been found already) cross over the load device and troubleshoot in the series method until an open is found Note: If You Read Applied Voltage On The Top Of The

Load DEVICE, AND ZERO volts on the bottom of the load device, and the load device is not energizing the load device itself is open.

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Figure 4. Basic DC Circuit 3

Use of the Oscilloscope in Fault Isolation Displays voltage in the form of a signal

Black lead - reference lead - common Probes - 10x or 1x Oscilloscope compares the voltage on the probe tip

to the voltage on the reference lead and displays it as a signal

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An oscilloscope is like a load device When the oscilloscope displays a proper reading, it is

energized When the oscilloscope displays a proper reading, all

conductors between the power source and leads must be good

If the oscilloscope does not displays the proper reading, and power is applied, there is an open between the power source and the leads

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Determine reference point Known good in functioning area of circuit If entire unit does not work place leads across

incoming power - if meter reads voltage, leave one lead in place for reference

Meter Readings NOTE: It is good practice to determine what

voltage readings should be present before meter is placed in the circuit

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Opens An open is a break in the path of current flow

Infinite resistance No current flow Drops the applied voltage

If an open is present in a circuit, the circuit will not function

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Figure 5. Basic DC Circuit 4

Shorts • A short is an abnormal path for current flow

No resistance Maximum current flow No voltage drop

• Types of shorts • Damaging: will change the resistance of a circuit and

may cause fuse(S) to blow Non-Damaging: will not change the resistance of a circuit

Across a load device (or power supply) - changes the resistance of a circuit and may cause fuse(s) to blow

From one circuit to another - ie, high voltage circuit to a low voltage circuit - may not change circuit operation but could cause a safety hazard

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Figure 7. Non Damaging Short

Figure 6. Damaging Short

Pictures of faults (diagrams) Show faults between two nearest point Label all points and wires Use proper schematic symbols If fault is a load device - device must be label State whether the fault is an open or a short

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Half-Split Analysis The use of half-split analysis is fundamental to

logical troubleshooting it is relatively simple always applicable when troubleshooting an

instrument or system which consists of a series of functions

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Figure 8. Application of Half-split Analysis

When troubleshooting a functional-series circuit, make your first check at the point where the circuit divides (split the circuit in half) If the desired response is present, the problem is

located between the checkpoint and the output and the limits can be moved to encompass this area

If the desired response is not present, the problem is probably located between the input and the checkpoint and again, the limits can be moved to encompass this area

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Figure 9. Taking the Next Step with Half-split Analysis

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Figure 10. Localizing the Problem with Half-split Analysis

After we have reduced the limits of the problem to the smallest circuit area, we look at the section of the circuit schematic and make individual resistance, voltage and current checks to determine the particular faulty components

Feedback Analysis When analyzing a feedback-controlled system or

network, break (disconnect) the feedback loop • If the output of the system or network does not

change, the problem is in the feedback loop • If the output changes radically, the problem is in the

system or the circuit

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The first check of the feedback-controlled system is restricted to its output Consists of measuring it as you disconnect the

feedback loop This measurement might be a voltmeter, or

oscilloscope Always remember feedback is used to control

When you remove affective control, you have rapid and large change

When you remove ineffective control, you have slow and minute change

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Conclusions The expert troubleshooter utilizes all of his

or her senses to isolate and eliminate a problem as rapidly as possible

They use the techniques of: • Circuit analysis • Series troubleshooting • Parallel troubleshooting • Loop troubleshooting • Half-split analysis • Feedback analysis

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To set initial limits within which his problem is located and the relentlessly reduces those limits until he has reduced the problem to a specific defect

With practice of the techniques detailed here, you can improve your effectiveness and, therefore, your value within the hospital.

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NOTE: Four Steps for Success Know what you expect to see at any given point prior

to taking a measurement Insure your test equipment is set to read / display

the proper voltage or signal Insure that your reference is good by checking the

incoming voltage or signal Insure that all controlling and/or switching devices

are in the proper position to energize the load device that you are troubleshooting

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