Motor drivers and inductive loads

• Specify a low-voltage DC motor driver from stall current and control requirements, then diagnose supply, control, or load-path faults from measurements.

• diodes-transistors
• power-regulation
• digital-interfaces

A DC motor converts electrical input into torque and motion. Its winding has resistance and inductance, while rotation generates back electromotive force that opposes the supply. At startup or stall, speed and back-EMF are near zero, so current is limited mainly by winding resistance and the driver. That stall current can be several times the free-running current and is the first electrical number to check.

A single low-side transistor controls one direction. An H-bridge uses four switching paths to apply either polarity across the motor, allowing forward and reverse. Some switch combinations coast the motor by opening its current path; others brake by providing a low-resistance path that dissipates motion. Shoot-through—turning on the high and low switch of the same bridge leg together—shorts the supply, so driver logic inserts safe switching behavior. Integrated drivers add flyback paths, current limiting, and thermal protection, but their current ratings and voltage drops still matter.

Pulse-width modulation (PWM) rapidly switches drive on and off. Duty cycle changes average applied voltage in a simplified model, but motor current, torque ripple, switching loss, and audible noise depend on inductance and frequency. For a toy car, choosing a driver above the measured or specified stall current with realistic thermal margin is more important than matching only free-run current. A vacuum-blower analogue may need one direction but a much larger startup budget.

A common misconception is that the motor draws only what the software commands. The mechanical load and electrical path determine current; a jam can force stall current at any duty cycle. Another failure is powering motor and logic from separate sources without the required common reference, or allowing motor noise to reset the controller. Separate the power path from control signals, place local decoupling as recommended by the driver manufacturer, keep high-current loops short, and test with current limiting. The DC-motor simulation is a bounded linear model, not a winding-temperature or driver-safety guarantee.

Problem

A 6 V motor has 3 Ω winding resistance. A driver adds 0.6 Ω total path resistance. Estimate stall current and driver conduction loss if its resistance accounts for the 0.6 Ω.

Steps

1. At stall, model back-EMF as zero and total resistance as 3.0 + 0.6 = 3.6 Ω.
2. Stall current is 6 V / 3.6 Ω = 1.67 A.
3. Driver conduction loss is I²R = (1.67 A)^2 × 0.6 Ω ≈ 1.67 W.
4. The motor receives about 5.0 V because driver drop is 1.67 A × 0.6 Ω ≈ 1.0 V.

Result

The simple stall model predicts 1.67 A and about 1.67 W dissipated in the driver path—large enough to make thermal limits central.

Sanity check

Motor plus driver voltage drops are about 5.0 V + 1.0 V = 6.0 V, and current is lower than the 2 A predicted if driver resistance were ignored.

A motor runs freely at 200 mA but its datasheet gives 1.8 A stall current. Which value should dominate minimum driver peak-current selection?

A. The 1.8 A stall current, with suitable duration and thermal margin B. The 200 mA free-run current only C. The arithmetic average, 1.0 A D. Neither; motor current is set entirely by PWM code

Your answer: ______________________________

Reasoning:

Goal

Characterize direction, startup voltage sag, and a deliberate control fault using a protected low-voltage motor-driver setup.

Equipment

• 3–6 V small DC motor with documented current limits
• compatible integrated motor-driver board rated above stall current
• current-limited low-voltage bench supply or protected battery holder
• multimeter
• switches or logic jumpers
• light paper flag
• driver documentation

Procedure

1. With power off, verify driver pinout, motor voltage range, stall-current compatibility, and shared ground; attach only a light paper flag for direction visibility.
2. Set a conservative current limit if using a bench supply, command coast, then power logic and motor supply in the documented sequence.
3. Command forward briefly and record actual supply voltage before and during startup plus running current; repeat reverse after the motor stops.
4. Power down. Only if the driver documents a pull-up or pull-down establishing a known input level, disconnect that direction-control signal and predict the result from the truth table. Otherwise keep the input connected and impose a documented safe logic level with the switch or resistor arrangement; never leave an undocumented input floating. Repower briefly and compare the result with the prediction.
5. Restore the input and verify both directions without mechanically stalling the shaft.

What to observe

• Expected: startup current exceeds unloaded running current and supply voltage may dip; actual peak may be missed by a slow meter and must be labeled accordingly.
• Expected fault clue: the deliberately imposed, known input level gives the documented coast, brake, or direction state. A floating input has no guaranteed state; the truth table alone does not define it.

Transfer challenge

Specify a driver for a two-motor toy car from voltage, per-motor stall current, simultaneous direction changes, braking need, logic voltage, and source peak capability.

Cautions

• Use only a documented low-voltage motor and driver with guarded or lightweight attachments; keep hair, fingers, and wires away from rotation.
• Do not stall the motor by hand, reverse it while spinning, or probe mains-powered appliances such as a vacuum cleaner or printer.

Learner: ____________________ Date: ____________________

Evidence artifact or file: ____________________________________________

Notes: