Practical case: Eliminate flicker in LEDs

Materials

• 1× LED (5 mm, any color; red assumed ≈2.0 V forward drop)
• 1× Resistor 330 Ω, 1/4 W (LED current limiter)
• 1× Electrolytic capacitor 470 µF, ≥10 V (main filter, C1)
• 1× Ceramic capacitor 100 nF, ≥10 V (optional high‑frequency bypass, parallel to C1)
• 1× Breadboard and jumper wires
• 1× Signal generator (sine, 1 Vpp, 100 Hz, +5 V DC offset) or a 5 V supply with noticeable ripple
• 1× Oscilloscope (2 channels recommended)
• 1× Digital multimeter (DMM)

Wiring guide

• Build the LED branch:
• From +V rail to R1 (330 Ω), then from R1 to LED1 anode, then LED1 cathode to GND.
• Add the capacitor:
• C1 (470 µF) from +V rail to GND, observing polarity: “+” to +V, “−” to GND.
• Optional: place 100 nF ceramic in parallel with C1 (also from +V to GND).
• Connect the ripple source:
• Signal generator output (Vin) to the +V rail; generator ground to GND.
• Configure generator: sine, 100 Hz, 1 Vpp, +5 V DC offset.
• Define measurement abbreviations you will use:
• V_SUP: voltage at the +V rail relative to GND (ripple you are smoothing).
• V_A: voltage at the node between R1 and LED1 (LED anode) relative to GND.
• V_R: voltage drop across R1; compute as V_R = V_SUP − V_A.
• I_LED: LED current; compute as I_LED = V_R / 330 Ω.
• Oscilloscope probing:
• Ch1 probe tip to V_SUP dot; Ch1 ground clip to GND.
• Ch2 probe tip to V_A dot; Ch2 ground clip to GND.
• DMM:
• Measure DC voltages as needed; to get I_LED, measure V_R and divide by 330 Ω.

Schematic

            +5 V
             │
             ● VCC
             │
             │                 ┌┴┐
             │─────────────────│ │  R1 = 330 Ω (limitador)
             │                 │ │
             │                 └┬┘
             │                  │
            ┌┴┐                 ● I_LED
            │ │  C1 = 220 µF    │
            │ │  (electrolítico)┌┴┐
            └┬┘                 │ │  LED1 rojo
             │                  │ │
            ┌┴┐                 └┬┘
            │ │  C2 = 100 nF     │───● V_LED
            │ │  (cerámico)      │
            └┬┘                  │
             │                   │
             └───────────────────┴────────────────
                                                GND

Measurements and tests

• Baseline (without C1):

  • Remove C1 (and 100 nF if installed).
  • Observe V_SUP ripple with Ch1; note its peak‑to‑peak value (ΔV_SUP,pp ≈ 1 Vpp).
  • Observe V_A with Ch2; you should see a similar 100 Hz ripple, and the LED will visibly flicker.
  • Measure V_R with DMM by reading V_SUP and V_A and computing V_R = V_SUP − V_A.
  • Compute I_LED ≈ V_R / 330 Ω. Expect average around (5 V − 2 V)/330 Ω ≈ 9 mA, modulated by ripple.

• With capacitor filtering (install C1):

  • Install C1 from +V to GND observing polarity. Optionally add the 100 nF ceramic in parallel.
  • Observe V_SUP ripple with Ch1 again; it should drop significantly.
  • Observe V_A with Ch2; ripple should be much smaller and LED flicker should be gone or barely noticeable.
  • Measure V_R and recompute I_LED; current ripple should be reduced. Example expectation:
    • Ripple estimate: ΔV_SUP,pp ≈ I_LOAD / (C1 × f_ripple) ≈ 0.009 A / (470 µF × 100 Hz) ≈ 0.19 Vpp.
    • If flicker remains, try 1000 µF or larger.

• Quantify improvement:

  • Reduction factor ≈ ΔV_SUP,pp (no C1) / ΔV_SUP,pp (with C1).
  • Note visual result: steady LED with reduced current ripple.

• Optional sweep:

  • Vary sine frequency (50–200 Hz) and record the smallest C1 that keeps flicker invisible.
  • Verify the inverse relationship: larger C1 → less ripple; higher f → less ripple.

Why the capacitor fixes flicker

• The electrolytic capacitor across the supply stores energy and fills in the valleys of the low‑frequency ripple.
• The LED current is set by R1 and the instantaneous supply voltage. Reducing ripple at +V directly reduces current modulation, eliminating visible flicker.
• A small 100 nF ceramic in parallel improves high‑frequency decoupling but does not affect 100/120 Hz ripple; the large µF value does the heavy lifting.

Common mistakes

• Reversed electrolytic polarity (C1 “+” must go to +V). This can damage the capacitor.
• Putting C1 in series with the LED or resistor. It must be from +V to GND (in parallel with the load/supply).
• Omitting the series resistor: never drive an LED directly from a voltage source without current limiting.
• Using too small a capacitor value and expecting no flicker at low frequencies.
• Scope ground not connected to the circuit GND; always clip to GND for accurate waveforms.

Safety and good practices

• Ensure C1 voltage rating comfortably exceeds the rail (≥10 V for a 5 V rail).
• Discharge C1 before rewiring; short it briefly with a 1 kΩ resistor to avoid sparks.
• Keep capacitor leads short to minimize series resistance and improve filtering.

Improvements

• Add a small series resistor (e.g., 10–22 Ω) between the source and +V to form an RC filter with C1 for even better ripple suppression.
• Use a constant‑current LED driver for maximum immunity to supply ripple.
• If multiple LEDs flicker, decouple each module locally with its own C1 near the load.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

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

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