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Motor drivers: from small to big

Driving a motor means giving it the right current at the right moment. At its simplest one transistor sets only the speed; each step adds an idea: direction, steps, electronic commutation, making a new 3-phase supply, and finally closing the position loop with an encoder. Go through the steps in order and play each simulation.

⚙️ Motor drivers, from small to big

Six steps from a single transistor to an industrial servo. Every step adds one idea to the previous one: PWM → direction → steps → electronic commutation → making a new AC supply → closing the loop with an encoder. Pick a step and play with its simulation.

① DC motor + MOSFET1–100 W · speed only② H-bridge1–500 W · direction + speed③ Stepper driver0.3–5 A/phase · position (open loop)④ Brushless (BLDC) ESC10 W – 5 kW · 3 half-bridges⑤ AC motor drive (VFD)0.2 kW – MW · 3-phase⑥ Servo drive50 W – 15 kW · closed-loop position

⛶ Enlarge

PWM duty%60

With diodeRemove diode

Average voltage

7.2 V

Speed (approx.)

1800 rpm

Q1 status

cool

🧠 The logic

A single N-channel MOSFET sits between the motor and ground (a low-side switch). The processor drives the gate with PWM; the MOSFET turns on and off thousands of times a second. The on-time ratio (duty) sets the average voltage the motor sees, and so its speed: 50 % duty at 12 V ≈ 6 V average. The direction never changes; for that you need an H-bridge. The motor is also an inductor: when the MOSFET turns off, the current can't stop instantly and a high-voltage spike appears. A flyback diode across the motor gives that current a path; it keeps circulating through the motor via the diode and the MOSFET is protected.

🔩 Typical parts / ICs

IRLZ44N / IRLR7843 (logic-level)AO3400 (SOT-23)1N5819 / SS34 (Schottky)TIP120 (Darlington, eski)ULN2003100 Ω gate + 10 kΩ pull-down

📍 Where you meet it

Fans and 4-wire PC fans, toy cars, small pumps, vibration motors, aquarium air pumps, Arduino projects.

🔧 Typical faults

• Motor ignores PWM, always full speed: MOSFET drain–source shorted (0 both ways on diode test).
• The MOSFET overheats and dies in minutes: flyback diode missing, reversed, or a slow diode (1N4007) used.
• MOSFET runs hot, motor weak: a non-logic-level MOSFET (IRF540) driven from 3.3/5 V stays half on.
• The motor twitches at power-up: no pull-down resistor on the gate.
• The processor resets when the motor runs: brush-spark noise; add 100 nF across the motor and an e-cap on the supply.

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📷 Real-world view

Fotoğraf: Quel.soler, CC BY-SA 4.0, Wikimedia CommonsFotoğraf: Kushagra Keshari, CC BY-SA 4.0, Wikimedia CommonsFotoğraf: 1sfoerster, CC BY-SA 3.0, Wikimedia CommonsFotoğraf: C J Cowie, CC BY-SA 3.0, Wikimedia Commons

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⚠ Safety: A motor is an inductor: cutting its current makes a high-voltage spike, so never skip the flyback/protection diode. A short on a LiPo or other battery can start a fire. In VFDs and servo drives the DC bus holds 540–800 V for minutes after switch-off: wait and measure.