Key Engineering Takeaways
- •Pulse Width Modulation (PWM) rapidly toggles a digital signal between full ON and full OFF to simulate a variable analog voltage.
- •Duty Cycle is the percentage of time the signal is ON during one full period: Duty Cycle (%) = (T_on / T_total) × 100.
- •Average Voltage: V_avg = V_peak × Duty Cycle. At 50% duty cycle on 12V, V_avg = 6.0V.
- •PWM is highly efficient because driving transistors operate strictly in Saturation (fully ON) or Cutoff (fully OFF), minimizing heat loss.
- •For DC motors, use PWM frequencies above 20kHz to eliminate annoying audible coil whine.
Prerequisites
- • Basic Voltage and Transistor switching
Required Hardware / Tools
- • ESP32 / Arduino
- • N-MOSFET Driver Module
- • 12V DC Motor / LED Strip
- • Oscilloscope (optional)
What is Pulse Width Modulation (PWM)?
Microcontrollers are purely digital devices: their output pins can only produce **full HIGH (+3.3V / +5V)** or **full LOW (0V)**. They cannot directly output an analog +2.1V to run a motor at half speed.
**Pulse Width Modulation (PWM)** solves this limitation by switching the digital pin ON and OFF at high frequency (hundreds to thousands of times per second).
By varying the **width of the ON pulse** relative to the total period, we control the average electrical power delivered to the load.
Duty Cycle Formula & Effective Average Voltage
The key metric of any PWM signal is its **Duty Cycle (D)**:
**Formula:** Duty Cycle (D) = T_ON{T_ON + T_OFF} × 100%
### Effective Average Voltage:
**Formula:** V_average = V_supply × (Duty Cycle / 100)
### Examples with a 12V Motor Supply:
- **0% Duty Cycle**: T_ON = 0ms → V_avg = 0.0V (Motor Stopped)
- **25% Duty Cycle**: V_avg = 12V × 0.25 = 3.0V (Slow Speed)
- **50% Duty Cycle**: V_avg = 12V × 0.50 = 6.0V (Half Speed)
- **100% Duty Cycle**: V_avg = 12.0V (Full Speed)
Why PWM is 95%+ Efficient (Switching vs Linear Resistors)
Why not just use a variable resistor (potentiometer) to slow down a motor?
- **Linear Resistance**: If you drop 12V to 6V across a series resistor running a 2A motor, the resistor dissipates P = 6V × 2A = 12 Watts of boiling wasted heat! (50% efficiency).
- **PWM Switching**: A MOSFET switch is either **fully ON** (V_drop ≈ 0.05V → P ≈ 0.1W) or **fully OFF** (I = 0 to P = 0W). PWM achieves **>95% energy efficiency**, keeping batteries lasting hours without massive heat sinks.
Switching EfficiencyPWM switching delivers maximum torque at low speeds because the motor receives full-voltage kicks during every ON pulse.
Choosing PWM Frequencies for Motors, LEDs & Audio
### Frequency Selection Guide:
- **LED Dimming (100Hz - 1kHz)**: Human eyes perceive anything above 100Hz as solid, flicker-free light.
- **DC Brushed Motors (20kHz - 25kHz)**: Low frequencies (1kHz) cause the motor iron core to vibrate, emitting an irritating high-pitched acoustic squeal. Frequencies >20kHz are above the human hearing range (silent operation).
- **RC Servos (50Hz)**: Standard positional hobby servos expect a fixed 50Hz (20ms period) control pulse where pulse width (1.0ms - 2.0ms) maps directly to angle (-90° to +90°).
Frequently Asked Questions
What is PWM resolution (8-bit vs 10-bit vs 16-bit)?
PWM resolution defines the number of discrete steps between 0% and 100% duty cycle: 8-bit resolution provides 256 steps (0-255), 10-bit provides 1024 steps (0-1023), and 16-bit provides 65,536 ultra-fine control steps.
Tags:#PWM#Pulse Width Modulation#Motor Speed Control#Duty Cycle#LED Dimming#Servos