Practical case: Use of inductor to smooth LED current

Materials

• 1 × 5 V DC supply (USB power bank or bench supply)
• 1 × Red LED, D1 (typ. Vf ≈ 2.0 V @ 10–20 mA)
• 1 × Series inductor, L1 = 100 µH (≥100 mA, low DCR)
• 1 × Current‑limit resistor, R1 = 150 Ω (¼ W)
• 1 × NPN transistor, Q1 = 2N2222 (or PN2222A)
• 1 × Base resistor, RB = 1 kΩ
• 1 × Sense resistor, RS = 10 Ω (¼ W)
• 1 × Schottky diode, D2 = 1N5819 (freewheel)
• 1 × PWM source (Arduino pin, or function generator 0–5 V), 200 Hz–5 kHz
• 1 × Oscilloscope (2 channels) and 1 × Multimeter
• Breadboard and jumper wires

Wiring guide

• Build the main current path in series from +5 V downwards: +5 V → L1 → R1 → D1 (anode up, cathode down) → Q1 collector → RS → GND.
• Connect Q1 emitter to the top of RS; RS bottom to GND.
• Add the freewheel diode D2 from the node between D1 and Q1 (Q1 collector) up to +5 V: D2 anode at the collector node, D2 cathode at +5 V.
• Drive Q1 base from the PWM source through RB: PWM output → RB → Q1 base. Connect PWM ground to GND.
• Typical PWM setup: 0–5 V square wave, 500 Hz, duty 20–80%. Start at 50%.
• Abbreviations used in the schematic and how to probe them:
• PWM: control signal at Q1 base side of RB; scope CH1 here (reference to GND).
• V_SW: switching node at Q1 collector (between D1 and Q1); scope CH2 here (reference to GND).
• V_RS: voltage at the top of RS (node between Q1 emitter and RS). Measure with DMM to compute LED current I_LED = V_RS / RS.

Schematic

            +5V ────────────────┬─────────────────────────────
                                │
                                │
                              ┌─┴─┐
                              │   │  L1 10 mH
                              │   │
                              └─┬─┘
                                │
                           ● VLED+
                                │
                              ┌─┴─┐
                              │   │  D1 LED
                              │   │
                              └─┬─┘
                           ● VLED-
                                │
                              ┌─┴─┐
                              │   │  R1 10 Ω
                              │   │
                              └─┬─┘
                           ● VR1+
                                │
                                ├───────────────┐
                                │               │
                              ┌─┴─┐             │
                              │   │  Q1 N-MOSFET│
                              │   │  2N7000     │
                              └─┬─┘             │
                                │               │
                           ● VR1-               │
                                │               │
                               GND            ┌─┴─┐
                                              │   │  D2 1N5819
                                              │   │
                                              └─┬─┘
                                                │
                                                │
                                                │
            PWM 0–5V                           A│K
             ┌─┐                                 │
             │ │  V1                             │
             │ │                                 │
             └─┘                                 │
               │                                 │
               └───────────── Gate Q1 ───────────┘
               │
              GND

Measurements and tests

• Initial checks:

  • Verify D1 orientation (anode to R1, cathode to Q1). Verify D2 orientation (anode at V_SW node, cathode at +5 V).
  • Confirm common ground between PWM source and the circuit.

• Oscilloscope setup:

  • CH1 on PWM (● PWM) to confirm duty/frequency.
  • CH2 on switching node (● V_SW) to see the collector waveform.

• Measure current ripple with the inductor:

  • Set PWM to 500 Hz, 50% duty. Measure V_RS (● V_RS) with the DMM; compute I_LED(avg) = V_RS / 10 Ω.
  • Observe CH2 (V_SW): you should see a square wave between near 0 V (Q1 on) and near +5 V minus D2 drop (Q1 off, diode conducting).
  • The LED current will be smoother because L1 resists rapid change; flicker should be reduced.

• Compare (optional control):

  • Briefly bypass L1 with a jumper (keep D2 in place). Re-measure V_RS and observe its waveform (it will follow PWM more abruptly).
  • Remove the jumper to restore L1 and confirm current smoothing (lower ripple, steadier light).

• Parameter exploration:

  • Increase PWM frequency to 1–2 kHz: ripple decreases further for the same L1.
  • Change duty cycle: I_LED(avg) should scale with duty. Verify by V_RS / 10 Ω.

• Notes on abbreviations:

  • V_RS: node voltage at the top of RS; I_LED = V_RS / RS because RS bottom is at GND.
  • PWM: base drive signal to Q1 via RB.
  • V_SW: Q1 collector node where D2 connects; shows diode conduction during “off” time.

Common mistakes

• Omitting or reversing D2: the inductor will create high voltage spikes at V_SW and can damage Q1.
• Using an iron-core or saturating inductor: saturation raises ripple; prefer a small ferrite inductor.
• No common ground between PWM source and the power stage: Q1 won’t switch properly.
• Oversized RS: too large wastes power and reduces LED current; too small makes current hard to measure.

Safety and improvements

• Keep LED current under 20 mA unless your LED is rated higher; adjust R1 accordingly.
• If the transistor overheats, reduce duty or use a logic‑level N‑MOSFET for lower loss.
• For even smoother current, increase L1 moderately (e.g., 220 µH) or raise PWM frequency.

Validation: All components are labeled per the Materials list; +5 V is at the top, GND at the bottom. Measurement dots (PWM, V_SW, V_RS) are placed directly on wires, and all connections are continuous with no floating nodes.

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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