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TEC-ELEC48-02 · Lesson 12 · 15 clock hours · practice

Circuit Troubleshooting Practical and Lab Report Completion

Objective. Students will independently diagnose and repair four faulted circuits of increasing complexity using systematic troubleshooting, and submit a complete, properly formatted lab report for the full course that earns a score of 70% or higher on the program rubric.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by an industry-certified professional or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Circuit Troubleshooting Practical and Lab Report Completion

Why Systematic Troubleshooting Matters

A technician who guesses randomly wastes time, risks damaging equipment, and can create new faults while hunting old ones. Systematic troubleshooting means following a repeatable logic process: observe symptoms, form a hypothesis, test that hypothesis with measurements, reach a conclusion, repair, and verify. This process is expected on the job and is embedded in the ETA CET and BICSI Installer competency frameworks.

Step 1 — Observe Symptoms Before Touching Anything

Before you pick up a probe or meter, look and listen. Is an LED dark that should be lit? Is a component visibly burned? Does the circuit smell of overheated insulation? Write down every observable symptom in your troubleshooting log. Symptoms tell you what the circuit is doing wrong; they narrow the list of possible causes before you make a single measurement.

Safety first. Before connecting or disconnecting any component, de-energize the circuit and follow lockout/tagout (LOTO) procedures. In this lab, DC bench power supplies are the energy source. Switch the supply off and verify output reads zero before probing internal nodes. Wear appropriate PPE — safety glasses at minimum. Capacitors in RC and RLC circuits can hold a charge after power is removed; discharge them through a current-limiting resistor before handling.

Step 2 — Form a Hypothesis

Use your symptom list to reason about which component or node is most likely faulty. In a series DC circuit, a complete open kills all current; a short across one resistor changes the voltage division. In a parallel DC circuit, an open branch affects total current but not the voltage across remaining branches. In an RC or RLC circuit, a faulty capacitor or inductor shifts the expected frequency response or time constant. Write your hypothesis as a specific, testable statement: "I believe R2 is open because there is full supply voltage across it and no voltage drop across R3."

Step 3 — Test with Measurements

Choose the minimum number of measurements that will confirm or eliminate your hypothesis. Common techniques:

Record every measurement — not just the ones that look interesting. A complete test record is required for your lab report and is what a senior technician or inspector will review.

Step 4 — Repair and Verify

Once you have identified the fault, make the repair (replace the component, reseat a connection, etc.). Then re-energize and re-measure the same nodes you measured before. The circuit should now match its design specification. Document the before and after readings side by side in your log. This verification step closes the loop and proves the repair was correct, not coincidental.

The Four Fault Stations

| Station | Circuit Type | Common Inserted Faults |

|---|---|---|

| 1 | Series DC | Open resistor, shorted resistor |

| 2 | Parallel DC | Open branch, wrong resistor value |

| 3 | RC | Open or shorted capacitor, wrong capacitor value |

| 4 | Series RLC | Open inductor, detuned component |

Spend no more than 30 minutes per station. If you are stuck, document what you have ruled out and move on. Partial credit is awarded for logical documented steps even if the final answer is not reached.

Lab Report Completion

Your bound lab log must contain, in order: a title page, a table of contents, individual lab sections (each with objective, schematic, data table, percent-error calculation, and conclusion), and a final summary conclusion for the full course. During the peer-review workshop, use the program rubric to score a classmate's report and receive a score on yours. Any section below 50% on the rubric must be revised before the skills demo assessment.

Percent error is calculated as:

`Percent Error = |Measured Value − Expected Value| / Expected Value × 100%`

Report percent error to one decimal place. Values consistently above 5 % should be explained in your conclusion — check for meter lead resistance, component tolerances, or power supply regulation.

Individual Debrief

Bring your troubleshooting log and the peer-reviewed lab report to your 10-minute debrief. Be ready to explain your reasoning at one fault station and to respond to one flagged item from the report review. Use the revision form to document any required changes. This conversation is not punitive; it mirrors the kind of technical dialogue you will have with supervisors and quality inspectors throughout your career.

Key terms8

Systematic troubleshootingA repeatable, logic-driven process for diagnosing circuit faults: observe symptoms, hypothesize, test, conclude, repair, and verify — in that order.
Half-split methodA fault-isolation technique in which the technician measures at the midpoint of a circuit to determine which half contains the fault, then repeats until the defective component is pinpointed.
Open circuit faultA break in the current path — resistance reads essentially infinite — causing loss of current through that branch and full supply voltage appearing across the open.
Short circuit faultAn unintended low-resistance path that bypasses a component, reducing voltage across it toward zero and increasing current in the affected branch.
Percent errorA measure of how closely a measured value agrees with the expected (theoretical) value, expressed as: |Measured − Expected| / Expected × 100%.
Lockout/tagout (LOTO)An OSHA-required safety procedure for de-energizing equipment and preventing accidental re-energization while a technician is working on it.
Voltage divisionIn a series resistor circuit, each resistor's voltage drop is proportional to its resistance relative to the total resistance; deviations from expected drops indicate a fault.
VerificationRe-measuring a repaired circuit to confirm it meets its original design specification, proving the repair corrected the actual fault rather than masking it.

Worked examples4

1. A series DC circuit has three resistors: R1 = 100 Ω, R2 = 220 Ω, R3 = 330 Ω, powered by a 12 V supply. Your DMM reads: V_R1 = 0.00 V, V_R2 = 0.00 V, V_R3 = 12.00 V. What fault does this indicate?
  1. Calculate expected current: I = 12 V / (100 + 220 + 330) Ω = 12 / 650 ≈ 0.0185 A.
  2. Calculate expected voltage drops: V_R1 = 0.0185 × 100 = 1.85 V; V_R2 = 0.0185 × 220 = 4.06 V; V_R3 = 0.0185 × 330 = 6.09 V.
  3. Compare measured to expected: R1 and R2 show 0 V instead of their expected drops; R3 shows the full 12 V instead of 6.09 V.
  4. Apply the rule: in a series circuit, full supply voltage appears across an open element because no current flows to create drops elsewhere.
  5. Conclusion: R3 is open. The voltmeter reads the full supply across it because no current path exists through R3, so no voltage is developed across R1 or R2.
Answer: R3 is open-circuited. The symptom of full supply voltage across one component and zero voltage across all others is the classic signature of an open in a series circuit.
2. In your RC lab, you measured an output voltage of 3.48 V across a capacitor at a specific frequency. The calculated expected value was 3.60 V. Calculate the percent error.
  1. Identify values: Measured = 3.48 V; Expected = 3.60 V.
  2. Find the absolute difference: |3.48 − 3.60| = 0.12 V.
  3. Divide by the expected value: 0.12 / 3.60 = 0.0333...
  4. Multiply by 100%: 0.0333 × 100% = 3.3%.
Answer: Percent error = 3.3%. This is within the typical ±5% acceptable range for bench measurements, so no fault is indicated — the small deviation is within component tolerance and meter accuracy limits.
3. A parallel DC circuit has two branches across a 9 V supply: Branch A = 180 Ω, Branch B = 360 Ω. You measure total current as 25 mA instead of the expected value. Is Branch B open or shorted?
  1. Calculate expected branch currents: I_A = 9 / 180 = 0.050 A = 50 mA; I_B = 9 / 360 = 0.025 A = 25 mA.
  2. Calculate expected total current: I_total = 50 + 25 = 75 mA.
  3. Compare measured total (25 mA) to expected (75 mA): measured is 50 mA less than expected.
  4. Note that 25 mA equals I_A alone (50 mA) — wait, re-check: 25 mA equals I_B alone. Re-examine: if Branch A were open, only Branch B conducts: I = 9/360 = 25 mA. That matches the measured value.
  5. Conclusion: Branch A (180 Ω) is open, leaving only Branch B conducting 25 mA.
Answer: Branch A is open. With only Branch B (360 Ω) conducting, total current = 9 V / 360 Ω = 25 mA, which matches the measurement exactly.
4. A student measures a resistor in-circuit and reads 560 Ω on the DMM. The color-coded value is 470 Ω ± 5%. Is the resistor within tolerance, and what is the percent error from the nominal?
  1. Calculate percent error: |560 − 470| / 470 × 100% = 90 / 470 × 100% = 19.1%.
  2. Determine the 5% tolerance band: 470 × 0.05 = 23.5 Ω; acceptable range = 470 − 23.5 to 470 + 23.5 = 446.5 Ω to 493.5 Ω.
  3. 560 Ω falls outside the 446.5–493.5 Ω range, so the resistor is out of tolerance.
  4. Note: in-circuit measurement can be misleading if parallel paths exist. De-energize, discharge any capacitors, then lift one resistor lead and re-measure to confirm.
Answer: Percent error is 19.1%, well outside the 5% tolerance. The resistor is out of specification (or a parallel path is affecting the in-circuit reading). Isolate the component and re-measure before replacing.

The math in this lesson1

Learners compute expected series current 12 V / 650 ohms and voltage drops, then compare to meter readings.
A-CED.4
Brush up in the math halls at newSchoolWorks.com:
In class: Rotating fault station practical: four pre-wired circuit boards (series DC, parallel DC, RC, and series RLC), each with one instructor-inserted fault; students rotate through stations, recording symptom, hypothesis, test steps, finding, and repair in a structured troubleshooting log—30 minutes per station. · Lab report workshop: students compile their data tables, schematics, percent-error calculations, and conclusions from all previous labs into a single bound lab log; peer review using the program rubric before final instructor review. · Individual debrief: each student meets with instructor for 10 minutes to discuss one troubleshooting decision from the fault stations and one item flagged on the lab report peer review; corrections documented on a revision form.
Materials: Four pre-faulted circuit boards, DMM, oscilloscope, function generator, DC power supply, troubleshooting log forms, compiled lab notebook from previous lessons, program lab report rubric
Check: All four faults correctly identified with logical documented steps (pass = 3 of 4 correct). Lab report rubric score ≥ 70%; any section below 50% requires revision before skills demo assessment.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

DC Circuit Theory and Troubleshooting
1. In a series DC circuit powered by 24 V, your DMM reads nearly the full 24 V across one resistor and approximately 0 V across all others. What does this most likely indicate?
Systematic Troubleshooting Methods
2. You are using the half-split method to locate a fault in a long series circuit. Your first measurement at the midpoint reads correctly. Where do you look next?
Electrical Safety and PPE
3. Before probing an RC circuit at the bench after switching off the power supply, what additional safety step is required?
Measurement and Lab Reporting
4. A student measures 4.85 V across a component that calculation predicts should be 5.00 V. What is the percent error?
DC Circuit Theory and Troubleshooting
5. In a parallel DC circuit, you measure the correct voltage across each branch but the total source current is lower than expected. Which fault is most consistent with this observation?
Technical Documentation and Reporting
6. During a lab report peer review, your partner's percent-error calculations are correct but the schematics section scores 45% on the program rubric. What must happen before the skills demo assessment?
Measurement and Troubleshooting Technique
7. When measuring a resistor suspected to be out of tolerance, you read an unexpectedly high value in-circuit. What is the correct next step before concluding the resistor is faulty?
AC Circuit Theory and Measurement
8. A series RLC circuit is driven at its resonant frequency. The technician suspects the inductor is open. Which oscilloscope observation best supports this hypothesis?
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