To drive a servomotor with an Arduino, you need a 5V to 8.4V power supply and a 50Hz PWM signal with a pulse width between 1000µs and 2000µs. For standard micro servos (like the SG90), you can power one or two directly from the Arduino's 5V pin. However, for high-torque digital servos (like the 20kg DS3218) or arrays of three or more, you must use a dedicated PWM driver like the PCA9685 and an external 5V/6V power supply capable of delivering at least 1A per servo. Attempting to run high-torque servos directly from the Arduino's onboard linear regulator will cause immediate brownouts and potential board damage.
Matching the Servomotor to Your Arduino Load Profile
A common mistake in embedded robotics is treating stepper motors and servos as interchangeable. They are fundamentally different architectures. A stepper motor holds position via magnetic detents in an open-loop system; if it misses a step, the controller never knows. A servomotor, by contrast, uses an internal potentiometer or magnetic encoder and an H-bridge to actively measure and correct position errors in a closed loop. If an external force pushes a servo off its target, the internal circuitry instantly applies current to fight back and restore the position.
Motor Type Comparison for Embedded Projects
| Motor Type | Torque Curve Characteristics | Control Needs | Typical Cost (2026) |
|---|---|---|---|
| Hobby Servo (Analog/Digital) | Peak torque at stall; drops to zero at max speed. | Closed-loop 50Hz PWM pulse width (1000-2000µs). | $3 - $35 |
| Stepper Motor (NEMA 17) | High holding torque at zero speed; drops sharply at high RPM. | Open-loop step/direction pulses via dedicated chopper driver. | $15 - $45 |
| DC Gear Motor | Linear torque drop-off as speed increases. | Requires external encoder + PID loop for position control. | $8 - $25 |
| Industrial AC Servo | Flat, constant torque curve up to rated base speed. | Dedicated 3-phase drive + absolute encoder feedback. | $300+ |
Sizing Rule of Thumb and Worked Load Example
Never size a servo based purely on the static weight of your payload. Dynamic movement, acceleration, and gravity acting on the lever arm multiply the required torque. The industry rule of thumb is to calculate the static stall torque at the joint, then apply a 2.5x safety factor for dynamic loads.
Scenario: Lifting a 500g (0.5kg) payload at the end of a 15cm (0.15m) forearm.
1. Calculate Force: Mass × Gravity = 0.5kg × 9.81 m/s² = 4.905 N.
2. Calculate Static Torque: Force × Distance = 4.905 N × 0.15m = 0.735 N·m.
3. Convert to kg·cm: (0.735 N·m × 100) / 9.81 = 7.5 kg·cm.
4. Apply 2.5x Safety Factor: 7.5 kg·cm × 2.5 = 18.75 kg·cm required.
Selection: A standard SG90 (1.8 kg·cm) will strip its plastic gears instantly. An MG996R (10-13 kg·cm) will stall and overheat. You need a DS3218 (20 kg·cm) or a Feetech SCS15 (14.5 kg·cm, but higher efficiency) to handle this load reliably.
Wiring, Terminals, and Driver Selection
Getting the physical connections right is where most hobbyist builds fail. The standard JR/Futaba 3-pin connector used on 95% of hobby servos follows a strict color code, though variations exist depending on the manufacturer.
Terminal Identification
| Function | Standard JR Color | Alternative Color | Arduino / Driver Connection |
|---|---|---|---|
| Ground (GND) | Brown | Black | Common ground with Arduino and Power Supply |
| Power (VCC) | Red | Red | External 5V-6V BEC or Buck Converter (NOT Arduino 5V) |
| Signal (PWM) | Orange | White or Yellow | PCA9685 PWM output or Arduino Digital Pin (D9/D10) |
Driver Demands: Direct GPIO vs. PCA9685
The Arduino Servo library relies on the ATmega328P's Timer1 hardware to generate the precise 50Hz pulses required. While it can simulate PWM on any digital pin, driving more than two high-torque servos directly from the Arduino's USB or onboard linear regulator will cause severe voltage sag.
For any build requiring 3 or more servos, or any servo drawing over 1A, use the PCA9685 16-channel I2C PWM driver. The Adafruit PCA9685 guide details how this chip offloads the timing burden from the Arduino's CPU, communicating via I2C (default address 0x40) and handling the high-current PWM switching locally. You only need to connect the Arduino's SDA/SCL pins and a common ground to the driver board, while the servo power is fed directly into the driver's screw terminals.
Failure Signatures: Diagnosing Servo Faults
- Hum or Jitter at Idle: This is almost always caused by power supply ripple or an inadequate ground return path. The 50Hz/60Hz mains noise is coupling into the signal line. Fix: Solder a 1000µF electrolytic capacitor directly across the VCC and GND rails at the servo power input, and ensure your ground wire is at least 18 AWG.
- Overheat (Hot to touch without moving): The internal control board is detecting a position error—usually due to mechanical binding, gear backlash, or a worn internal potentiometer. The H-bridge is continuously driving the motor to reach the target, drawing stall current (often 2A+) without moving. Fix: Remove the mechanical load and check for physical obstruction. If it still overheats, the internal pot is failing.
- Stall (Clicking or dead stop): The external load has exceeded the motor's magnetic detent and gear train limit. The servo will emit a loud clicking sound as gear teeth skip, or stop dead. Fix: Your load calculation was wrong, or the mechanical advantage is insufficient. Upgrading the servo or adding a gear reduction is required.
Frequently Asked Questions
Can I power a servomotor for Arduino directly from the 5V pin?
Only if you are using a single micro servo like the SG90 (which draws ~200mA peak) and powering the Arduino via a robust USB-C port or external barrel jack. If you are using a standard metal-gear servo like the MG996R (which can spike to 2.5A under load), pulling that current through the Arduino's onboard 5V linear regulator will cause it to overheat, trigger thermal shutdown, and brownout the microcontroller, leading to erratic behavior and memory corruption.
Why is my Arduino resetting when the servo moves?
This is a classic brownout. When a servo starts moving, it draws a massive inrush current. If your power supply cannot deliver it instantly, the voltage on the 5V rail dips below the ATmega328P's brownout detection threshold (usually around 4.3V), causing the chip to reset. To fix this, use an external BEC (Battery Eliminator Circuit) or a high-quality buck converter rated for at least 3A, and place a large bulk capacitor (1000µF to 2200µF) on the servo power rail to handle transient spikes.
What is the difference between 180-degree and 360-degree continuous rotation servos in code?
In a standard 180-degree servo, the pulse width maps to an absolute physical angle (e.g., 1000µs = 0°, 1500µs = 90°, 2000µs = 180°). In a 360-degree continuous rotation servo, the internal potentiometer has been removed or bypassed. The pulse width now maps to speed and direction. A 1500µs pulse means "stop", values below 1500µs spin the motor backward (with lower numbers being faster), and values above 1500µs spin it forward. You lose all absolute position feedback with a 360-degree variant.
Do I need a serial bus servo instead of standard PWM?
If your project involves more than 6 joints (like a hexapod or humanoid robot), standard PWM wiring becomes a nightmare of cable management, and the PCA9685 requires 16 separate heavy-gauge power traces. Serial bus servos (like the Feetech SCS series or Dynamixel) use a single UART data line daisy-chained through all servos. They allow you to read back real-time telemetry—including internal temperature, current draw, and exact position—allowing your Arduino to detect stalls and prevent overheating before physical damage occurs. They are more expensive ($15-$40 each) but essential for advanced kinematics.
