A motor driver lets a low-power controller, such as an Arduino or microcontroller, control a higher-current motor safely. The H-bridge is one of the most important motor driver circuits in robotics because it can make a DC motor spin forward or backward. It uses four electronic switches arranged around the motor so the voltage across the motor can be reversed.
This matters because mobile robots, grippers, wheels, and actuators all need controlled motion, not just simple on and off power.
In an H-bridge, turning on opposite pairs of switches sends current through the motor in one direction or the other. Speed is usually controlled with pulse-width modulation, or PWM, which rapidly switches the motor voltage on and off so the motor responds to the average power. Real motor drivers also need flyback diodes or internal protection circuits because motors are inductive loads that can produce dangerous voltage spikes.
Common driver ICs package the H-bridge switches, protection features, and control pins into a compact part that is easier and safer to use.
Key Facts
- An H-bridge uses four switches to reverse the polarity across a DC motor.
- Forward drive: close the high-side switch on one side and the low-side switch on the opposite side.
- Reverse drive: close the opposite diagonal pair of switches.
- Motor speed is controlled by PWM duty cycle: Vavg = D V supply, where D is from 0 to 1.
- Motor electrical power can be estimated by P = V I.
- Flyback diodes protect switches by giving inductive current a safe path when switching changes suddenly.
Vocabulary
- H-bridge
- A four-switch circuit that can reverse the direction of current through a DC motor.
- Motor driver
- An electronic circuit or chip that lets a control signal operate a motor using a separate higher-power supply.
- PWM
- Pulse-width modulation is a method of controlling average motor voltage by rapidly switching the supply on and off.
- Flyback diode
- A diode that provides a safe path for current from an inductive load when the switching circuit turns off or changes state.
- Shoot-through
- Shoot-through is a dangerous condition where the high-side and low-side switches on the same side turn on together and short the power supply.
Common Mistakes to Avoid
- Turning on both switches on the same side of the H-bridge, this is wrong because it creates shoot-through and can destroy the driver or power supply.
- Connecting a motor directly to a microcontroller pin, this is wrong because motors usually need far more current than logic pins can safely provide.
- Forgetting flyback protection, this is wrong because a spinning or switching motor can generate voltage spikes that damage transistors and ICs.
- Assuming PWM changes motor direction, this is wrong because PWM mainly changes average speed while the selected switch pair determines current direction.
Practice Questions
- 1 A 12 V motor is driven with PWM at a 25% duty cycle. Estimate the average voltage applied to the motor.
- 2 A motor draws 0.8 A from a 9 V supply while running. Estimate the electrical power delivered to the motor using P = V I.
- 3 In an H-bridge, explain which diagonal switch pair should be on to reverse the motor direction and why turning on two switches on the same side is unsafe.