How H-Bridges Work: DC Motor Direction & PWM Control

What is an H-Bridge?

A DC motor rotates clockwise or counter-clockwise depending on the polarity of the voltage applied to its two terminals. If you swap the positive and negative connections, the motor reverses direction. In a physical circuit, you cannot manually swap wires at kilohertz speeds — so we use an H-bridge.

An H-bridge is an electronic circuit consisting of four switches arranged in an "H" topology, with the motor positioned in the center crossbar. By closing specific pairs of switches, we can steer current through the motor in either direction from a single, static DC power source.

The Four H-Bridge Switch States

To control the motor safely and correctly, the switches in the H-bridge (which are physically transistors like BJTs or MOSFETs) are activated in specific combinations:

L298N vs Modern MOSFET H-Bridges

In consumer robotics modules, you will often find two major families of H-bridges: BJT-based (like the vintage L298N) and MOSFET-based (like the modern TB6612FNG or DRV8833).

The L298N is a classic BJT dual H-bridge. However, it is highly inefficient by modern standards because of its bipolar junction transistor structure. BJT saturation voltage drop (V_CE(sat)) causes a **1.5V to 3V drop** across the IC. If you connect a 6V battery to an L298N, your motor may only receive 3.5V, and the lost energy is released as heat, requiring a bulky metal heat sink.

Modern MOSFET H-bridges have extremely low Drain-Source On-Resistance (R_DS(on), often under 100mΩ). Because of this, the voltage drop is negligible, the chip remains cool, and battery efficiency is drastically improved, making them far superior for mobile robotic chassis.

Speed Control Using PWM

While the H-bridge switches direction by closing diagonal pairs, it controls **speed** using Pulse Width Modulation (PWM).

By rapidly turning the active high-side switch (S1 or S2) ON and OFF thousands of times per second (usually at 1kHz to 20kHz), the average voltage applied to the motor winding is varied proportional to the duty cycle. For instance, at a 75% duty cycle on a 12V motor, the motor acts as an integrator, averaging the pulses to spin at approximately 75% of its top speed.

Inductive Spikes & Flyback Diodes

A DC motor is an inductive load made of wire coils. Inductors resist sudden changes in electric current. When you open a closed switch in an H-bridge to turn the motor off, the magnetic field in the motor coil collapses, creating a massive high-voltage reverse spike (called Back-EMF or kickback).

To prevent this voltage spike (which can easily exceed 100V) from breaching and frying the transistors in the H-bridge, **flyback diodes** must be placed in reverse bias across each of the four switches. These diodes provide a safe recirculation pathway for the decaying current until it dissipates. Integrated chips (like the L293D and TB6612) contain these diodes internally, whereas raw L298N boards include them as external components.

Practical Arduino Wiring with L298N

To control a motor with an Arduino Uno, connect three pins: two digital direction pins (e.g., IN1, IN2) and one PWM-capable speed pin connected to Enable A (ENA).

**Basic Arduino Control Code Example:**

// Pin Definitions
const int ENA = 9;  // PWM pin for speed control
const int IN1 = 8;  // Direction Pin 1
const int IN2 = 7;  // Direction Pin 2

void setup() {
  pinMode(ENA, OUTPUT);
  pinMode(IN1, OUTPUT);
  pinMode(IN2, OUTPUT);
}

void loop() {
  // Move Forward at 75% speed (191 out of 255)
  digitalWrite(IN1, HIGH);
  digitalWrite(IN2, LOW);
  analogWrite(ENA, 191); 
  delay(3000);

  // Hard Brake
  digitalWrite(IN1, HIGH);
  digitalWrite(IN2, HIGH);
  delay(1000);

  // Move Reverse at 50% speed (127 out of 255)
  digitalWrite(IN1, LOW);
  digitalWrite(IN2, HIGH);
  analogWrite(ENA, 127);
  delay(3000);

  // Coast Stop
  digitalWrite(IN1, LOW);
  digitalWrite(IN2, LOW);
  analogWrite(ENA, 0);
  delay(2000);
}

Robotics Hardware & H-Bridge Component Availability in India

Whether you are a student, hobbyist, or engineering professional, sourcing high-quality motor driver boards like the L298N, TB6612FNG, or DRV8833 is highly convenient across major technology hubs in India. You can easily find these components at local electronics markets and leading online stores:

Frequently Asked Questions

Conclusion

The H-bridge is a fundamental building block of physical computing, bridge topologies, and high-power electronic drive systems. Understanding how to manage the four operating states, selecting modern MOSFET ICs over obsolete BJT L298N drivers, and using PWM for speed control will empower you to design powerful, efficient robotic drivetrains and motor controllers.

Ready to control actuators? Pair your H-bridge with the Arduino Uno, dive deeper into the physics of Pulse Width Modulation, or protect your circuits using discrete MOSFET switches.