DC Motors on Embedded Systems Development

PWM Speed Control

Mon, 01 Jan 0001 00:00:00 +0000

PWM Speed Control#

A brushed DC motor’s speed is approximately proportional to the average voltage across its terminals. Rather than varying a supply voltage directly — which wastes power as heat in a linear regulator — the standard embedded approach is pulse-width modulation: a transistor switches the full supply voltage on and off at a fixed frequency, and the motor’s inductance smooths the pulsed current into a nearly steady flow. The duty cycle sets the effective voltage, and therefore the speed.

H-Bridge Circuits

Mon, 01 Jan 0001 00:00:00 +0000

H-Bridge Circuits#

An H-bridge is four switches arranged in an “H” topology around a motor, allowing current to flow through the motor in either direction. By activating diagonal pairs of switches, the bridge reverses the motor’s polarity without changing the supply wiring. This is the standard circuit for bidirectional DC motor control, and it appears in everything from toy cars to industrial drives.

Basic H-Bridge Topology#

 V+
 │
 ┌────┴────┐
 │ │
 Q1(HS) Q3(HS)
 │ │
 ├── Motor ─┤
 │ │
 Q2(LS) Q4(LS)
 │ │
 └────┬────┘
 │
 GND

State	Q1	Q2	Q3	Q4	Motor
Forward	ON	OFF	OFF	ON	CW rotation
Reverse	OFF	ON	ON	OFF	CCW rotation
Brake (low-side)	OFF	ON	OFF	ON	Shorted to GND — dynamic braking
Coast	OFF	OFF	OFF	OFF	Freewheeling — motor coasts

Shoot-Through Protection#

If both the high-side and low-side switches in the same leg turn on simultaneously, a dead short appears across the supply — often called shoot-through. This can destroy MOSFETs in microseconds. Protection requires dead time: a brief interval (typically 0.5–2 µs) where both switches in a leg are off before the complementary switch turns on.

Current Sensing & Limiting

Mon, 01 Jan 0001 00:00:00 +0000

Current Sensing & Limiting#

Knowing how much current a motor draws — and being able to limit it — is essential for protecting both the motor and the drive electronics. Stall current in a brushed DC motor can be 5–10× the running current; a 12 V motor with 1 Ω winding resistance pulls 12 A at stall, even if it runs at 1–2 A under normal load. Without current limiting, a stall event can overheat windings, saturate H-bridge MOSFETs, blow fuses, or trip power supplies.

Back-EMF & Braking

Mon, 01 Jan 0001 00:00:00 +0000

Back-EMF & Braking#

A spinning DC motor is also a generator. The rotating armature produces a voltage proportional to speed — the back-EMF (electromotive force). This voltage opposes the applied voltage and is the fundamental mechanism that regulates motor current: as speed increases, back-EMF rises, net voltage across the winding drops, and current decreases. Understanding back-EMF is essential for speed measurement without encoders, controlled deceleration, and preventing voltage spikes from damaging drive electronics.