Practical case: Build a choke coil for DC

Materials

• 1x Ferrite or iron-powder toroidal core (≈ 20–30 mm OD)
• 1x Enameled copper wire, 0.6–0.8 mm (≈ 3–5 m)
• 1x Electrolytic capacitor, C1 = 470 µF, 25 V
• 1x Power resistor, R1 = 100 Ω, 5 W (load)
• 1x DC bench supply (9–12 V, ≥ 0.5 A)
• 1x Multimeter (DMM)
• 1x Oscilloscope with two probes (optional but recommended)
• 2–4x Alligator clip leads
• Heat-shrink or electrical tape; sandpaper or blade (to strip enamel)

Wiring guide

• Prepare L1 (the choke):
• Wind 40–60 tight turns on the toroid (spread evenly). Leave 5–7 cm leads.
• Scrape/tin the enamel ends thoroughly so they solder well.
• Connect the circuit:
• +V from the bench supply goes to the top node labeled +V.
• Insert L1 in series between +V and the output node (VOUT).
• Connect C1 from VOUT to GND (observe capacitor polarity: + to VOUT, − to GND).
• Connect R1 from VOUT to GND as the load.
• Bench supply negative goes to GND.
• Measurement abbreviations used on the schematic:
• VIN: node before L1 (input side of the choke). Probe tip touches the black dot labeled VIN; reference/ground clip to GND.
• VOUT: node after L1 (output to load). Probe tip touches the black dot labeled VOUT; reference/ground clip to GND.
• DMM usage:
• To measure DC voltage at VOUT, place red probe at VOUT (dot), black probe at GND.
• To measure DC current through R1, use I = VOUT / R1 (avoid breaking the circuit at Basic level).

Schematic

                 +V (+12 V)
                   │
               ● Vin
                   │
                ┌────┐               L1 3.3 mH / 0.5 A
                │    │
                │    │
                └──┬─┘
                   │
               ● IL
                   │
      ● Vout ──────┼───────────────┐
                   │               │
                 ┌─┴─┐             │           C1 1000 µF / 25 V
                 │   │             │
                 │   │             │
                 └─┬─┘             │
                   │               │
                 ┌─┴─┐             │           C2 100 nF
                 │   │             │
                 │   │             │
                 └─┬─┘             │
                   │             ┌─┴─┐         R1 120 Ω / 2 W (carga)
                   │             │   │
                   │             │   │
                   │             └─┬─┘
                   │               │
                  GND             GND

Measurements and tests

• Initial checks:
  • With the supply OFF, verify no wire strands short adjacent turns; ensure C1 polarity is correct.
  • Measure DC resistance (DCR) of L1 with the DMM (Ω mode); expect a small value (typically < 1 Ω).
• Power-up and DC operation:
  • Set the bench supply to 12 V and current limit ≥ 0.3 A. Power ON.
  • Measure VOUT: it should be approximately 12 V − (I × DCR of L1); with R1 = 100 Ω, I ≈ 12 V / 100 Ω = 120 mA.
• Ripple attenuation (scope):
  • VIN measurement: connect CH1 tip to the VIN dot, ground clip to GND. Measure AC ripple (Vripple_in, in mVpp).
  • VOUT measurement: connect CH2 tip to the VOUT dot, ground clip to GND. Measure AC ripple (Vripple_out, in mVpp).
  • Expected: Vripple_out < Vripple_in. Attenuation ≈ 20·log10(Vripple_out / Vripple_in) dB (negative is good).
• Thermal check:
  • After 2–3 minutes at 120 mA, L1 should be warm at most; R1 may run hot (it dissipates ≈ 1.4 W). Do not touch bare metal; briefly check by hand near the insulation only.

How many turns? A quick starting point

• If your core’s AL is known (nH/turn²), target N ≈ sqrt(L/AL). Example: AL = 1000 nH/turn², L ≈ 1 mH → N ≈ sqrt(1e6/1000) ≈ 32 turns.
• If AL is unknown, start with 50 turns; test ripple reduction. Add or remove turns to tune performance.
• Keep wire gauge thick enough for your current (0.6–0.8 mm is fine for a few hundred mA). Spread turns evenly to avoid hot spots.

Common mistakes

• Using too few turns: the choke acts like a short at low frequency and won’t reduce ripple.
• Very thin wire: excessive DCR causes unwanted voltage drop and heating.
• Long lead lengths to C1: move C1 physically close to VOUT and GND to improve filtering.
• Reversing electrolytic polarity: will overheat or vent—always double-check markings.

Safety notes

• Stay within the core’s current capability to avoid saturation (symptom: VOUT ripple increases sharply under load).
• R1 gets hot; keep it clear of plastics and fingers. Use a 5 W resistor or larger.
• Never short +V to GND. Set current limit on the bench supply.

Improvements

• Make a π-filter: add a second capacitor from +V to GND before L1 (CIN) for extra ripple suppression.
• Try different cores (ferrite vs iron powder) and turn counts to compare attenuation.
• Add a small ceramic (e.g., 100 nF) in parallel with C1 to better shunt high-frequency noise.

Validation: The schematic shows +V at top and GND at bottom, all nodes are connected, each component (L1, C1, R1) is identified, and measurement dots with abbreviations (VIN, VOUT) are placed directly on the circuit. Abbreviations are explained in the Wiring guide and used in Measurements and tests.

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Carlos Núñez Zorrilla

Electronics & Computer Engineer

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

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