~90 min. Build an RC circuit with a measurable time constant. Watch the capacitor charge through the resistor. Verify the predicted τ matches measurement.
Goal: build and measure an RC charging circuit; understand the time constant in plain English
Estimated time: 90 minutes
Prerequisites: labs 1-3 complete; multimeter and breadboard fluency
Steps
Step 1: Pick R and C (10 min)
For an easily-measurable time constant, aim for τ ≈ 10 seconds. With R = 100 kΩ and C = 100 μF: τ = 100,000 × 100×10⁻⁶ = 10 seconds. After 10 s the cap is at ~63% of final voltage; after 30 s it is at ~95%
If your kit has different values, pick combinations that give τ between 2 and 30 seconds (too fast to measure easily; too slow to be patient with)
Step 2: Identify the electrolytic capacitor and its polarity (10 min)
Electrolytic capacitors have a + and - lead. The + lead is longer; the - side often has a stripe on the body. Putting one in backward can cause it to pop; don't do that
Step 3: Wire the circuit (15 min)
From the + rail: through R (100 kΩ) to a tie-strip. From that tie-strip: the + side of the cap to that tie-strip; the - side of the cap to ground rail. The cap is "between" the resistor output and ground; this is the "charge through R, store in C" pattern
Initially: short the capacitor's + and - leads with a jumper wire for ~5 seconds; this drains any residual charge. Remove the jumper. The cap is now at 0 V
Step 4: Measure the charging (30 min)
Set the multimeter to DC voltage. Touch probes across the capacitor (+ to + lead, - to - lead). Start a timer at the same moment you connect the power (if not already connected). Record the voltage every 2 seconds for 60 seconds. Tabulate
If you have a smartphone with a stopwatch, use it. Lab notebook columns: time (s), voltage (V), predicted voltage (from V_final × (1 - e^(-t/τ)))
Step 5: Plot or check (15 min)
By hand or in a spreadsheet: plot measured voltage vs time. Compare to the predicted curve. Both should follow an exponential approach to the final value
At t = τ (10 s in this example), the cap voltage should be ~63% of final. At t = 3τ (30 s), ~95%. Confirm
Step 6: Discharge (10 min)
Disconnect the power supply (or leave it connected; the cap is now at V_final). Reconnect a wire from the cap's + lead to ground (or use a discharge resistor, ~1 kΩ, to keep the discharge gentle). The cap discharges through this path with τ = R_discharge × C. Watch it decay
Expected output
- Tabulated time vs voltage data showing exponential charging
- Comparison of measured τ to predicted τ
- Notebook reflection on the curve shape
Common pitfalls
- Cap in backward: don't. Read the polarity before plugging in
- Reading the cap value wrong: 100 μF, not 100 nF or 100 pF. The units matter; orders-of-magnitude difference
- Probe drift: hold the probes still while measuring. Moving them can make false readings
Stretch (optional)
- Try R = 10 kΩ and C = 1000 μF: τ = 10 s, but more total charge. The charging looks the same in time but stores more energy
- Discharge through a small LED + resistor; watch the LED dim as the cap drains
- Try ceramic capacitors (much smaller values, like 0.1 μF). The time constant will be too fast to measure without an oscilloscope; note that as a limit of multimeter-based timing
Lab 4.1 v0.1. The first time-based circuit. Time enters the curriculum.