8 PWM Control: Signals and Duty-Cycle Behaviour

8.1 Start With the Decision

This section: Start With the Decision — PWM switches power on and off in short pulses.

PWM switches power on and off in short pulses. The motor winding current rises and falls between pulses; its average current and mechanical inertia shape torque and motion. Duty cycle is the share of each cycle spent on. It links the digital command to the motor’s physical motion.

8.2 Route Overview

This section: Route Overview — This is part 1 of 2.

This is part 1 of 2. Continue with PWM Control: Frequency and Driver Decisions.

8.3 Part Objectives

This section: Part Objectives

• Compute PWM duty cycle and average drive level.
• Explain how load response changes across a PWM cycle.

8.4 Start With the Story

This section: Start With the Story — Picture a small fan that must run gently at normal temperature and harder during a heat rise.

Prove the Physical Output at Three Settings

Picture a small fan that must run gently at normal temperature and harder during a heat rise. A changing command number is not enough; the reviewer needs the drive timing, supply condition, and measured fan result.

An actuator is a device that turns an electrical command into physical action. A microcontroller is a small computer built to read inputs and control outputs. Modulation means changing a signal over time to carry a setting. Duty cycle means the share of each switching period spent on.

Command the low, middle, and high settings. Stall the fan briefly within a safe test, lower the supply, repeat a setting, and restart the controller. Record command, timing, current, and physical response together, and keep any urgent heat action local.

This runway does not prove motor life or make one switching rate safe for every load. The deeper sections cover waveform timing, driver stages, resolution, filtering, load limits, and measurement.

Picture a fan that should start gently, run quietly, and still move enough air on a hot afternoon. PWM is the controller’s way to ask for less or more effort, but the physical result depends on frequency, duty cycle, driver losses, motor inertia, and the load.

Use PWM as an actuator command with evidence around it. The right setting is not just a number from 0 to 255; it is a duty, frequency, ramp, and feedback choice that keeps motion, brightness, heat, or position under control.

8.5 PWM Fundamentals

This section: PWM Fundamentals — Why use PWM?

PWM (Pulse Width Modulation) is a digital technique that simulates analog control by rapidly switching power ON and OFF:

• Duty cycle = Percentage of time the signal is HIGH (ON)
• Frequency = How many times per second the signal repeats
• Average power = Supply voltage x Duty cycle percentage

Why use PWM? Digital microcontrollers can’t produce true analog voltages (like 2.5V from a 5V supply). Instead, they rapidly switch between 0V and 5V. A motor or LED responds to the average voltage over time.

Math Bridge · guided foundationsWhy does full-voltage switching become smooth motion?Let Max separate the average drive from winding-current and torque ripple. Learn the maths

8.6 Try It: PWM Motor Speed Control

This section: Try It: PWM Motor Speed Control — Use the simulator as an evidence sequence.

Use the simulator as an evidence sequence. Begin with zero load and a moderate switching frequency, raise duty cycle while watching average voltage and speed, then add inertia and load to expose current demand. Reverse the H-bridge only after the shaft response is understood, and finally sweep frequency to distinguish audible whine from excessive switching loss. The useful result is a bounded operating region, not one attractive speed reading.

8.7 Interactive PWM Calculator

This section: Interactive PWM Calculator

8.8 PWM Applications by Frequency

This section: PWM Applications by Frequency

Different actuators require different PWM frequencies:

8.9 Servo PWM Timing

This section: Servo PWM Timing

Servo motors are a special case - they use pulse width for position:

Standard Servo:
- 1000 microseconds (1ms)   = 0 degrees (5% duty cycle)
- 1500 microseconds (1.5ms) = 90 degrees (7.5% duty cycle, center)
- 2000 microseconds (2ms)   = 180 degrees (10% duty cycle)
- Period: 20ms (50 Hz)

// Convert angle to pulse width
int angleToPulseWidth(int angle) {
  return map(angle, 0, 180, 1000, 2000);  // 1000-2000 microseconds
}

// Using ESP32Servo library (handles timing automatically)
#include <ESP32Servo.h>
Servo myServo;
myServo.attach(pin, 1000, 2000);  // min/max pulse width
myServo.write(90);                 // Move to 90 degrees

8.10 ESP32 PWM Configuration

This section: ESP32 PWM Configuration

// Arduino-ESP32 3.x assigns a channel when each pin is attached.
// Motor control - high frequency for silent operation
const int motorFreq = 20000;  // 20 kHz (above human hearing)
const int motorResolution = 8;  // 0-255

// LED control - lower frequency acceptable
const int ledFreq = 5000;  // 5 kHz
const int ledResolution = 10;  // 0-1023 (higher resolution for smooth fades)

void setup() {
  // ledcAttach(pin, frequency, resolution) selects a channel automatically.
  ledcAttach(MOTOR_PIN, motorFreq, motorResolution);
  ledcAttach(LED_PIN, ledFreq, ledResolution);
  // Use the ESP32Servo library for a hobby servo.
}

8.11 PWM Speed to Value Conversion

This section: PWM Speed to Value Conversion — Translate the requested percentage in two stages: first compute the maximum register value from resolution, then map the bounded 0-100% request into that integer range.

Translate the requested percentage in two stages: first compute the maximum register value from resolution, then map the bounded 0-100% request into that integer range. Compare the 8-, 10-, and 12-bit examples in order; all represent the same 75% command, but finer resolution supplies more selectable codes. The conversion changes command granularity, not supply voltage, motor torque margin, or proof of achieved speed.

// Convert percentage speed to PWM value
int speedToPWM(int speedPercent, int resolution) {
  int maxValue = (1 << resolution) - 1;  // 2^resolution - 1
  return map(speedPercent, 0, 100, 0, maxValue);
}

// Examples:
// 75% speed at 8-bit resolution:
int pwm8 = speedToPWM(75, 8);   // = 191 (out of 255)

// 75% speed at 10-bit resolution:
int pwm10 = speedToPWM(75, 10); // = 767 (out of 1023)

// 75% speed at 12-bit resolution:
int pwm12 = speedToPWM(75, 12); // = 3071 (out of 4095)

8.12 Why Duty Cycle Matters for IoT

This section: Why Duty Cycle Matters for IoT — Key insight: The actuator “sees” the average voltage, smoothed by its physical response time.

1. Energy Efficiency: LED at 50% duty cycle uses approximately 50% power, extending battery life
2. Precise Control: Achieve 256 speed levels (8-bit) or 1024 levels (10-bit) from a simple digital pin
3. No Analog Hardware: Digital MCUs can control analog-like behavior without DAC chips
4. Thermal Management: Lower duty cycles reduce heat in motors and drivers

Key insight: The actuator “sees” the average voltage, smoothed by its physical response time. A 1 kHz PWM signal switching between 0 V and 5 V at 50% duty has 2.5 V average, while the motor receives voltage pulses and responds according to its electrical and mechanical time constants.

8.13 Worked Example: Smart Greenhouse Ventilation Fan

This section: Worked Example: Smart Greenhouse Ventilation Fan — A greenhouse uses a 12V DC fan (rated 0.5A at full speed) controlled by an ESP32 via a MOSFET driver.

A greenhouse uses a 12V DC fan (rated 0.5A at full speed) controlled by an ESP32 via a MOSFET driver. The system adjusts fan speed based on temperature.

Step 1: Power at each operating point

Power = Voltage x Average Current = 12V x (Duty Cycle x 0.5A)

Step 2: Daily energy consumption

Assume a typical summer day: 6 hours off (night), 8 hours at 30%, 6 hours at 60%, 4 hours at 85%.

• Energy = (6 x 0) + (8 x 1.8) + (6 x 3.6) + (4 x 5.1) = 0 + 14.4 + 21.6 + 20.4 = 56.4 Wh/day

Compare to running the fan at full speed 24 hours: 6.0W x 24h = 144 Wh/day. PWM-based variable speed saves 61% of the daily energy.

Step 3: Why 20 kHz for this fan?

At lower PWM frequencies, the fan motor produces audible hum. A DC motor’s coils vibrate at the PWM frequency, and humans hear 20 Hz to 20 kHz. Choosing 20 kHz pushes the hum above hearing range, making the fan appear to run silently at any speed. The trade-off: higher PWM frequencies increase switching losses in the MOSFET driver (typically adding 0.1-0.3W at 20 kHz versus 1 kHz), but the acoustic benefit outweighs the small efficiency loss in an occupied greenhouse.

8.14 Continue to the Next Part

This section: Continue to the Next Part — Carry this evidence into PWM Control: Frequency and Driver Decisions, which begins with Choosing PWM Frequency: Decision Factors.

Carry this evidence into PWM Control: Frequency and Driver Decisions, which begins with Choosing PWM Frequency: Decision Factors.