Practical case: Inductor to limit current in DC motor

Materials

• 1 × DC power supply V1 (e.g., 12 V, ≥3 A) or a 12 V battery
• 1 × DC motor M1 (rated 6–12 V, stall current ≤ inductor rating)
• 1 × Power inductor L1 (≈1 mH, saturation/continuous current ≥ motor stall current)
• 1 × Shunt resistor RS (0.1 Ω, 5 W, low-inductance)
• 1 × Flyback diode D1 (across motor, e.g., 1N5408 or SB560; Schottky preferred at low voltage)
• 1 × Flyback diode D2 (across L1, same rating as D1)
• 1 × Switch S1 (SPST, current rating ≥ motor stall current)
• 1 × Multimeter (with mV range; optional Min/Max or oscilloscope for transients)
• Hook-up wire, screw terminals or soldered joints (avoid solderless breadboards for high current)

Wiring guide

• Connect V1 positive to S1 input. Connect S1 output to L1 top.
• Connect L1 bottom to RS top. Connect RS bottom to M1 top. Connect M1 bottom to V1 ground.
• D1 (across motor): cathode to M1 top (node between RS and M1), anode to ground.
• D2 (across inductor): cathode to L1 top (node between S1 and L1), anode to L1 bottom (node between L1 and RS).
• Abbreviations used for measurement points:
• VM+: node at the motor’s positive terminal (top of M1). VM−: motor negative (ground near M1).
• VRS+: node at the top of the shunt (between L1 and RS). VRS−: node at the bottom of the shunt (between RS and M1).
• To measure motor current: IM = VRS / RS = (VRS+ − VRS−) / 0.1 Ω.
• Ensure correct polarity of diodes (stripe = cathode). Keep all high-current paths short and with thick wire.

Schematic

                 +12 V
                  │
                ┌─┴─┐
                │   │  S1 (SPST)
                │   │
                └─┬─┘
                  │
                  ├───────────────────────────┐
                  │                           │
                ┌─┴─┐                         │
                │   │  L1 2.2 mH / 2 A        │
                │   │                         │
                └─┬─┘                         │
                  │                           │
                  ● Vm                        │
                  │                           │
                ┌─┴─┐                         │
                │   │  M1 Motor 6–12 V        │
                │   │                         │
                └─┬─┘                         │
                  │                           │
                  ● Vsh                       │
                  │                           │
                ┌─┴─┐                         │
                │   │  R1 0.10 Ω / 2 W        │
                │   │                         │
                └─┬─┘                         │
                  │                           │
                 GND                        ┌─┴─┐
                                            │   │  D1 Schottky 3 A
                                            │   │
                                            └─┬─┘
                                              │
                                             GND

Measurements and tests

• Safety pre-check:
  • Verify diode orientations (D1 cathode at M1 top; D2 cathode at L1 top).
  • Confirm no shorts; measure RS value (~0.1 Ω) with power OFF.
• Baseline (optional, without L1):
  • Temporarily bypass L1 with a jumper (S1 output to RS top). Power ON and observe:
    • VRS step at startup; compute IM,peak ≈ VRS / 0.1 Ω.
    • Remove power and jumper before continuing.
• With inductor (current limiting):
  • Power ON with L1 in series. Observe:
    • VM: place probes at VM+ and VM−. Expect VM to rise faster than current; motor spins up smoothly.
    • VRS: measure between VRS+ and VRS−. Expect a current ramp (slower rise), IM(t) = (VRS/0.1 Ω).
• Quantitative checks:
  • IM,steady ≈ (V1 − VM,steady)/Rpath is the load current after spin-up; confirm that IM,peak (with L1) < IM,peak (no L1).
  • If using an oscilloscope: CH1 across RS to see current ramp; CH2 across M1 for VM. Note reduced di/dt with L1.
• Understanding abbreviations:
  • VM is motor voltage measured between VM+ and VM−.
  • VRS is shunt voltage measured between VRS+ and VRS−.
  • IM = VRS / RS (use RS = 0.1 Ω).

Common mistakes

• Using an inductor with insufficient current rating (saturates, losing limiting effect).
• Omitting D1/D2 or reversing them (spikes can damage the switch or supply).
• Measuring current by placing a multimeter in series at high current without proper rating (use the shunt).
• Building on a solderless breadboard (high resistance/heat; use screw terminals or soldered connections).

Improvements and variations

• Try different L values (0.47–2.2 mH) to see the trade-off between startup smoothness and torque response.
• Add a small ceramic (e.g., 100 nF) across motor brushes to reduce EMI.
• Replace S1 with a MOSFET and PWM; L1 then acts with the motor as an output filter to reduce current ripple.

Validation: The schematic uses Unicode box-drawing, shows +V at top and GND at bottom, labels each component to match the materials, provides closed connections, and includes clearly placed black-dot measurement points (VM+, VM−, VRS+, VRS−) explained in the guide.

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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