Practical case: Charging and discharging a capacitor

Materials

• 1× Breadboard
• 1× DC power supply, +5 V (or USB 5 V with breakout)
• 1× R1 = 1 kΩ, 1/4 W resistor (charge path)
• 1× R2 = 1 kΩ, 1/4 W resistor (discharge path)
• 1× C1 = 100 µF, ≥16 V electrolytic capacitor
• 1× S1 = Pushbutton (normally open, NO)
• 6× Jumper wires
• 1× Multimeter (DC volts) and/or 1× Oscilloscope with probe

Wiring guide

• Place R1 so one end will go to +5 V and the other to a central node (the capacitor node).
• Connect the free end of R1 to a free row that will be the node V_C (capacitor node).
• Insert C1 so its positive lead goes to node V_C and its negative lead goes to the ground rail. Observe polarity: “+” to V_C, “−” to GND.
• Build the discharge branch: connect one end of R2 to node V_C; connect the other end of R2 to one terminal of S1; connect the other terminal of S1 to the ground rail.
• Connect the power supply +5 V to the breadboard’s +V rail and the supply ground to the GND rail.
• Abbreviations used in the schematic:
• V_C = capacitor voltage at the node (measure here with the red probe).
• REF = reference ground point (connect black probe here).
• For a multimeter: set to DC volts, red probe to the black dot labeled V_C, black probe to the black dot labeled REF.
• For an oscilloscope: CH1 probe tip to V_C, CH1 ground clip to REF; timebase around 50 ms/div to 200 ms/div to see the RC curve.

Schematic

                +5 V
                 │
               ┌┴┐
               │ │   S1 = Pulsador NA (carga)
               │ │
               └┬┘
                 │
               ┌┴┐
               │ │   R1 = 10 kΩ (carga)
               │ │
               └┬┘
                 │
                 ● V_C
                 ├───────────────┬
                 │               │
                 │              ┌┴┐
                 │              │ │   S2 = Pulsador NA (descarga)
                 │              │ │
                 │              └┬┘
                 │               │
                ┌┴┐            ┌┴┐
                │ │            │ │   R2 = 10 kΩ (descarga)
                │ │   C1 =     │ │
                │ │   100 μF   └┬┘
                └┬┘              │
                 │               │
                GND             GND

Measurements and tests

• Before powering on:

  • Verify C1 polarity: positive to V_C, negative to GND.
  • Confirm there is no short between +V and GND with the multimeter continuity mode (power supply off).

• Charging (R1–C1):

  • Power on +5 V with S1 released (open).
  • Observe V_C:
    • With a multimeter: V_C will rise smoothly toward +5 V.
    • With an oscilloscope: an exponential rise V_C(t) = 5 V · (1 − e^(−t/τ)).
  • The time constant τ_charge = R1·C1 = 1 kΩ · 100 µF ≈ 0.10 s (100 ms).
  • Check points:
    • At t = τ (≈0.1 s), V_C ≈ 0.63 · 5 V ≈ 3.15 V.
    • By t ≈ 5τ (≈0.5 s), V_C ≈ 99% of 5 V.

• Discharging (R2–S1–C1):

  • Press and hold S1 to connect R2 to GND and discharge the capacitor.
  • Observe V_C:
    • With a multimeter: V_C will fall toward 0 V.
    • With an oscilloscope: an exponential decay V_C(t) = V0 · e^(−t/τ).
  • The time constant τ_discharge = R2·C1 = 1 kΩ · 100 µF ≈ 0.10 s.
  • Check points:
    • At t = τ (≈0.1 s) after pressing S1, V_C ≈ 37% of its initial value.
    • After ≈5τ (≈0.5 s), V_C is near 0 V.

• Repeatability and symmetry:

  • Release S1 to let the capacitor charge again through R1.
  • Compare charge and discharge curves; with R1 = R2, the time constants should match.

• Optional checks:

  • Change R1 or R2 (e.g., to 4.7 kΩ) and verify τ scales linearly with R.
  • Try a smaller capacitor (10 µF) and note the faster transitions.

Common mistakes

• Reversing the electrolytic capacitor polarity (can damage the capacitor).
• Forgetting the ground reference for the meter/scope (always connect to REF).
• Misplacing the pushbutton so its internal contacts short +V to GND; ensure S1 is only between R2 and GND.
• Using a multimeter in current mode across the capacitor node V_C (will short the circuit). Use voltage mode.

Safety notes

• Keep the supply at 5 V. Higher voltages increase inrush/discharge currents and stress components.
• Always discharge C1 (hold S1 for a second) before re-wiring.
• Do not short V_C directly to GND without a resistor; R2 limits discharge current.

Improvements

• Replace S1+R2 with an SPDT toggle to select CHARGE (to +V via R1) or DISCHARGE (to GND via R2) hands-free.
• Add an LED with a series resistor from V_C to GND to visualize the charge level; note it will slightly alter τ.
• Use a data acquisition or oscilloscope cursors to measure τ accurately and compare with R·C.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

Follow me:

Who we are