Choosing Between DC Motors and Servos

Why the motor choice shapes your whole build

Every home robot eventually comes down to the same question: how does it move, and how precisely does it need to move? Motors are the muscles of your project, and the two you will meet first are DC motors and hobby servos. They look similar in a parts bag, they cost similar money, and they behave in completely different ways. Choosing the wrong one rarely breaks anything — it usually just means you spend a weekend rebuilding a chassis or rewriting code you had already tested.

The good news is that the decision is not complicated once you know what each type is designed to do. This guide walks through what each motor actually does, where it belongs in a robot, and the practical details — power, wiring, torque — that catch beginners out.

DC motors: simple, fast, and built for rolling

A plain DC motor spins continuously for as long as you feed it power, and stops when you cut it. Swap the two wires over and it spins the other way. That is the whole control interface, which makes it wonderfully easy to work with, but it also means the motor has no idea where it is or how far it has turned.

Speed is roughly proportional to voltage, so a 6V motor on a 6V battery pack runs at its rated speed, and dropping to 3V halves it. In practice you control speed with pulse-width modulation (PWM), flicking the power on and off thousands of times a second. Most beginners drive them through an H-bridge driver board, which handles direction and PWM in one neat package and stops you frying your microcontroller.

• Typical hobby sizes run from 3V to 12V, often quoted with a gearbox as “6V, 200 rpm”.
• Gearboxes trade speed for torque — a 200 rpm geared motor can push a small rover, a bare 12,000 rpm motor cannot.
• Stall current (the current drawn when the shaft is held still) can be several amps, so check your battery and driver rating.
• No position feedback unless you add an encoder or use a stepper motor instead.

DC motors are noisy, cheap and slightly imprecise, which is exactly what you want for wheels, tracks, rollers and anything that just needs to keep turning.

Servos: precise positions on demand

A servo is a small motor, a gearbox, a control circuit and a position sensor bundled into one box. You do not tell it to spin; you tell it to go to an angle and it holds that angle against a light load. Feed it a pulse roughly every 20 milliseconds, and the width of that pulse sets the position — about 1 ms for one end of travel, 1.5 ms for centre, 2 ms for the other.

Most standard servos move through about 180 degrees, with some 270-degree versions available. Torque is quoted in kilogram-centimetres and usually listed twice, at 4.8V and 6V, because more voltage means more torque and speed. If you overload one, it will buzz, jitter and eventually strip its plastic gears — which is why metal-geared servos are worth the extra few pounds for anything carrying real weight. Avoid stalling a servo against a hard stop; it draws a surprising amount of current and gets hot quickly.

One oddity worth knowing: continuous rotation servos look identical but have no positional feedback. You control speed and direction rather than angle, so they behave like a DC motor with a built-in driver. Handy for small wheels, useless for an arm joint.

Matching the motor to the job

The rule of thumb is simple: if a part must hold a position, use a servo. If it just needs to keep turning, use a DC motor.

• Drive wheels on a floor rover: DC motors with gearboxes, ideally with encoders if you want straight-line accuracy.
• Robot arm joints: servos, sized by torque, with the heaviest joint nearest the base.
• Grippers: a single servo, often with a simple linkage to convert rotation into a squeeze.
• Steering: a servo for car-style steering; continuous rotation servos only if you are doing tank-style skid steering.
• Sensor pan and tilt mounts: micro servos, which are light enough not to strain your chassis.
• Rollers, fans, pumps and conveyor belts: DC motors, every time.

Power, wiring and the mistakes beginners make

This is where most first builds wobble. Servos are power-hungry and twitchy, and if you run them from the same 5V regulator that feeds your microcontroller, you will get brownouts and random resets the moment they move. Give them their own battery or a separate regulator, and always connect all the grounds together — the signal wire has nothing to reference otherwise.

A few habits worth adopting early:

• Measure the current draw on a bench before you install anything, using a multimeter in series with the battery.
• Add a large capacitor across the servo supply to soak up sudden current spikes.
• Keep motor wires away from sensor wires where you can, and twist long runs to reduce electrical noise.
• Test each motor or servo on its own with a simple sketch before adding the next one.

A practical starting point for your first build

If you are ordering parts this week, a sensible opening kit is two 6V geared DC motors with matching wheels, an H-bridge driver board, and a 6V battery pack — that gets a rover rolling. Add one or two micro servos on a separate 5V supply for a moving sensor mount, and you have both technologies working side by side, which is exactly how most real home robots are built.

Once that is running reliably, upgrade the DC motors to encoder versions and you gain the ability to drive in straight lines and count distances. From there, bigger servos, better gearboxes and more sensors follow naturally. Your local hobby electronics retailer or a UK online components shop will stock all of it, and both types of motor are cheap enough to keep a spare or two in the drawer.