DC Motors & the H-Bridge
A motor is an inductor that pushes back. Four switches around it give you forward, reverse, brake — and one catastrophic wrong move.
Builds on: 2.4 Inductors & Electromagnetism8.1 Build the PWM Dimmer3.2 Transistors
The motor, through your eyes
A brushed DC motor is Lesson 2.4 made muscular: current through coils in a magnetic field produces force; a commutator flips the current every half-turn so the push never stops. And because it is a coil, everything you know applies: it fights current changes, it kicks when switched off (flyback diodes are non-negotiable), and — the elegant part — when spun it generates voltage (2.4’s induction). That back-EMF grows with speed and opposes the supply, which is why an unloaded motor settles at the speed where back-EMF nearly equals the applied voltage: voltage sets speed. Load it down and it slows, back-EMF drops, more current flows: current is torque. Two sentences, whole discipline.
Direction needs four switches
One transistor (3.2) turns a motor on. Reversing it means flipping the current, and that takes the classic H-bridge: the motor is the crossbar, two switches above, two below.
Braking deserves a pause: shorting the spinning motor through the low-side switches makes its own back-EMF drive current against its motion — the motor becomes a generator fighting itself. Electric cars’ regenerative braking is this exact move, with the energy caught instead of burned. And the forbidden state: both switches of one side on is a dead short across the battery — shoot-through — which is why real driver chips (like the DRV8833 in your capstone) add hardware interlocks and dead-time.
You could build an H-bridge from 3.2’s parts — people do, once, for the scars. A $3 driver chip adds shoot-through protection, flyback diodes, current limiting and PWM inputs. Master the concept here; buy the armour for the robot.
⚡ Lab — Four Switches & the Forbidden State
An H-bridge with clickable switches and a motor that obeys — or a fuse that doesn’t forgive.
- Use the presets, then click switches manually and predict the motor before it moves.
- Spin it forward, then hit Brake — compare with Coast. Feel the difference regenerative braking exploits.
- Turn on Q1 and Q2 together. Once. 💥 (Then appreciate driver-chip interlocks.)