Driving motors with Arduino: servo vs DC vs stepper, drivers and separate power

TUREFF• •0

Motors are where many Arduino projects go wrong, and usually for the same reason: someone wires a motor straight to a pin or the board's 5 V supply and the board resets, misbehaves or is damaged. The rule to remember is simple. The Arduino gives the orders; something else supplies the muscle.

Why a pin cannot drive a motor

An Arduino pin is designed to supply a small signal current, tens of milliamps at most and less on some newer boards. Even a small hobby motor can draw far more than that, and much more again at the moment it starts or stalls. Motors also cause trouble electrically:

They create electrical noise that can reset the microcontroller.
When a motor coil switches off, it can push a voltage spike back into the circuit. Drivers include protection (flyback diodes) against this.

So every motor goes through a driver of some kind, and usually from its own power supply.

Servo motors

A hobby servo turns to a set angle, usually across about half a turn, and holds it. Inside are a small motor, gears and its own control circuit, so the Arduino only sends a control pulse on one wire. The built-in Servo library handles that. Good for: steering, flaps, pointing a sensor, small robotic arms. Wiring: three wires, usually power, ground and signal. The signal wire goes to an Arduino pin. A single small servo doing light work is often run from the board in tutorials, but anything larger or more than one servo should have a separate supply of the right voltage. Continuous-rotation servos exist too; they spin rather than hold an angle.

DC motors

A plain DC motor spins when powered and reverses if you swap the wires. Speed is controlled with PWM, switching the power on and off very fast, via analogWrite. Good for: wheels, fans, pumps. To run one you need a driver:

a transistor or MOSFET circuit with a flyback diode, to switch one direction only
an H-bridge motor driver module, to run forwards and backwards with speed control

Pick a driver rated comfortably above the motor's stall current, not just its running current.

Stepper motors

A stepper moves in fixed steps, so you can turn it a precise amount without any sensor. They hold position firmly when powered, and they draw current even when standing still, so they get warm. Good for: 3D printers, camera sliders, plotters, anything needing repeatable positions. A stepper always needs a driver board. Small ones come with a matching driver in many kits; larger ones use dedicated stepper drivers where you set a current limit. Set that limit before running the motor. The Stepper library, or more capable third-party libraries, send the steps.

Separate power, shared ground

The safe pattern for any motor project:

Power the motor and driver from a supply matched to the motor's rated voltage and able to deliver its current.
Power the Arduino from USB or its own input as usual.
Connect the grounds of the two supplies together, so the control signal has a common reference.

Without a shared ground, the driver may not understand the Arduino's signals at all. Never connect the motor supply's positive line to the Arduino's 5 V pin. Batteries need respect: lithium cells can deliver large currents into a short and get dangerously hot. Use a holder with protection or a fuse, and never leave a charging lithium pack unattended.

Quick comparison

Servo: set an angle; easy; limited rotation.
DC motor: continuous spin and speed; needs an H-bridge for reversing.
Stepper: precise steps and holding; needs a stepper driver; warms up.

Common symptoms

Board resets when the motor starts: power the motor separately and add the driver's recommended capacitors.
Motor hums but does not turn: supply too weak, or stepper wiring order wrong.
Driver very hot: check its rating and current limit.

What motor are you trying to drive, and from what supply? Share your setup or your questions in the comments.