Circuit Card Academy

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Guided Practice Labs

Start with prediction and measurement on a small passive circuit. These labs use a two-AA battery holder (about 3 V), an inline 100 Ω current-limiting resistor rated at least ¼ W, three 1 kΩ resistors (two for the divider and one for Lab 3), a breadboard and an appropriate DMM. No mains, aircraft hardware, lithium packs, power conversion, current injection into an unknown board, soldering or intentional shorts. Check the battery holder polarity and resistor markings. Disconnect power before moving wires. Stop if anything warms, leaks or smells unusual.

Current-limited passive dividerBattery positive through a 100 ohm series limiter, then R1 1 kilohm to midpoint M, then R2 1 kilohm to battery negative. Measure voltage relative to battery negative. 2 × AA100 ΩR1: 1 kΩR2: 1 kΩMReference: battery negative
The 100 Ω limiter is part of the calculation. This is a learning circuit, not an avionics test fixture.

Lab 1 — Predict, measure, explain

  1. With batteries disconnected, measure the resistors individually. Record nominal, measured and tolerance. Return the meter lead to the voltage/resistance jack if needed.
  2. Draw the series path before assembling it. Check the unpowered connections against the diagram. The midpoint must not be connected directly to either battery terminal.
  3. Predict total resistance (2.1 kΩ), current (about 1.43 mA at exactly 3.0 V) and M-to-negative voltage (about 1.43 V). These are predictions, not acceptance limits.
  4. Connect the battery, select DC volts and measure the battery voltage and midpoint relative to battery negative. Never place a meter in current mode across the source. Infer current from the resistor voltage rather than opening the circuit for an ammeter in this first lab.
  5. Recalculate using the measured source and resistor values. Record expected vs measured and explain differences using tolerance, contact quality and meter loading. Disconnect the battery.
Worked check for a 2.90 V battery and nominal resistors

I = 2.90 / 2100 = 1.38 mA. Midpoint voltage = I × 1000 = 1.38 V. The 100 Ω limiter drops about 0.138 V. An answer of exactly half the battery voltage has omitted the limiter.

Lab 2 — A deliberate open, with power removed

  1. Disconnect the battery. Open the R1 connection at the midpoint. Verify no residual voltage before using resistance mode.
  2. Write what continuity should show across the disconnected joint and across each resistor. Check the meter on a known open and known connection first.
  3. Locate the open using measurements between named nodes. Reconnect only after disconnecting the meter and keeping the battery isolated.
  4. Repeat the Lab 1 powered voltage check. Explain why a repaired connection still needs a functional check.
What the open establishes

An open measurement across the intentionally disconnected joint localizes the break. It does not prove that a resistor is internally open. A floating midpoint can produce a misleading powered reading, so avoid diagnosing from one voltage alone.

Lab 3 — Loading changes the answer

With the battery disconnected, connect a second 1 kΩ resistor in parallel with R2. Predict the lower equivalent resistance (500 Ω), total resistance (1.6 kΩ), and midpoint at exactly 3 V (0.9375 V). Check connections, power briefly, measure, then disconnect. Explain why this lower voltage is expected loading, not a broken source.

Keep an evidence record

Use the Repair Case Log for each lab: circuit identity, expected result and its assumptions, measured result, instrument setup, conclusion and verification. Label the entry “practice lab.” A completed lab is self-reported practice; workplace competence needs observation against an applicable procedure.

Next: Fault Drills. Your objective is a justified diagnosis, not the smallest number of clicks.