Switches & motion
DC motor
Turns current into rotation. Current through coils on the rotor makes it a magnet that is pushed round by fixed magnets; a switching contact (the commutator) flips the current every half turn to keep it spinning. Speed follows voltage; torque follows current.
Pins
- Two terminals
- : Swap them and it spins the other way.
Small brushed DC motors are metal cans with two solder tabs. Gear motors add a gearbox for more torque and less speed.
What to look for inside
- The voltage on the motor slowly rises and falls, and the speed follows it.
- Current in the rotor coils turns the rotor into an electromagnet, attracted and repelled by the fixed N and S magnets.
- Just as it lines up, the commutator reverses the coil current, so it is pushed onward instead of stopping.
The numbers that matter
| Spec | Typical |
|---|---|
Rated voltage Speed is roughly proportional to voltage. | 3 V · 6 V · 12 V |
No-load / stall current At start-up or when blocked it draws the stall current. Size the driver and supply for that. | 0.1 A / 1 to 3 A (small) |
Speed (no load) Usually geared down for real use. | 3000 to 15000 rpm |
Where you’ll use it
- Fans, pumps, toys and robots.
- Speed control with PWM through a MOSFET or an H-bridge (for both directions).
Common mistakes
- Driving a motor straight from a GPIO pin.
- No flyback diode. A motor is an inductor and kicks back when switched off.
- Forgetting the stall current. The supply droops and the microcontroller resets.
Lessons that use it
Lesson 3.2 shows the MOSFET that switches a motor; lesson 2.4 has a motor preset and shows the flyback spike.
- Lesson 3.2: MOSFET as a Switch: Gate Voltage & Heat
- Lesson 2.4: Inductors & the Flyback Diode Explained
Go deeper
The spinning motor makes its own back-voltage (k_e ω), which is why the current drops once it is up to speed.
Torque is proportional to current.