Measure inductive drive current and flyback

Follow the actuator guide's driver-current and flyback questions through a transistor-switched inductive-load model.

Follow both the current through the inductive load and the voltage across its switch at turn-off., your practice guide

Follow both the current through the inductive load and the voltage across its switch at turn-off.
Predict the reading, then compare it with the measurement.

Falstad CircuitJS

Third party Tool

Follow the actuator guide's driver-current and flyback questions through a transistor-switched inductive-load model.

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Version tested: Falstad CircuitJS live in Chromium 151 on 2026-10-08; CircuitJS1 element API and scope readouts; deterministic synthetic coil model, seed 0. Date: 2026-10-08.

Open the prepared circuit.

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Steps

Screens captured against Falstad CircuitJS Falstad CircuitJS live in Chromium 151 on 2026-10-08; CircuitJS1 element API and scope readouts; deterministic synthetic coil model, seed 0 on 2026-10-08; the tool may have moved on — the text steps are the contract.

  1. Step 1

    Do
    Open the protected circuit canvas and inspect the 5 V supply and coil.
    You will see
    Supply source: 5 V DC. Inductive load model: 50 mH coil and 20 Ω series resistor. Drive source: 100 Hz square, 50% duty. All component values are synthetic teaching inputs, seed 0.
    Why it matters
    The model exposes the load path and switch before comparing protection.

  2. Step 2

    Do
    Point at the inductor on the canvas and read its drive current.
    You will see
    With diode: sampled coil-current peak=115.680 mA (scope Max about 115.689 mA). Near the end of ON: coil=115.676 mA; MOSFET current=115.674 mA; Vds=2.674 V. Effective MOSFET on-resistance then is 2.674 V / 0.115674 A = 23.11 Ω. With the 20 Ω resistor, Rtotal≈43.11 Ω and 50 mH / Rtotal≈1.16 ms near the peak. The 5 ms ON interval begins near 0.06 mA left after the previous OFF interval. Ideal 5 V / 20 Ω = 250 mA assumes a switch with zero voltage drop. Falstad model steady current≈115.83 mA; its nonlinear 5 ms rise predicts≈115.69 mA. These are circuit-model values, not a motor stall rating.
    Why it matters
    The MOSFET drops about 2.67 V at the peak, leaving about 2.33 V across the 20 Ω resistor. Its resistance changes with current; the 115.7 mA peak agrees with the model rise, while the driver carries that current during ON.

  3. Step 3

    Do
    Point at the diode on the circuit canvas and inspect the voltage scope.
    You will see
    With diode: sampled switch-node peak=5.697 V. Live scope read Max=5.699 V in the protected run. The diode conducts after switch-off as coil current decays. The switch node stays close to the 5 V rail plus diode drop.
    Why it matters
    A flyback path limits voltage while stored magnetic energy dissipates.

  4. Step 4

    Do
    Open the unprotected circuit canvas and inspect the switch voltage scope.
    You will see
    Without diode: sampled switch-node peak=95.860 V. The live scope showed a 258.669 V maximum in one run. The 10 nF parasitic capacitor and 1 MΩ leak path keep this idealized model finite. The turn-off ring includes negative current samples down to -105.788 mA.
    Why it matters
    Removing the diode lets coil energy charge the switch-node capacitance and ring.

  5. Step 5

    Do
    Return to the protected circuit canvas and compare the switch readout.
    You will see
    With diode: scope maximum was 5.699 V. Without diode: scope maximum was 258.669 V in the comparison run. The protected coil-current peak was 115.680 mA. The 5 V, 50 mH, 20 Ω values are model choices, not hardware limits.
    Why it matters
    The voltage comparison supports a flyback path without claiming a hardware safety rating.

Chapter checks

These questions refer to the chapter’s examples. Use the return links to review their answers.

  1. A standard hobby servo receives a 1.5ms pulse at 50 Hz. What position will the servo move to?

    Return to the chapter’s knowledge check

  2. A high-torque servo has a stall current of 2.5A. If you are building a 3-servo robotic arm, what minimum power supply current rating do you need?

    Return to the chapter’s knowledge check

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