What is the H-bridge topology and how does it reverse direction?
An H-bridge is the circuit that routes the two motor terminals between supply and chassis through four switches. When the two diagonal switches (top left + bottom right) are turned on, voltage is applied to the motor in one direction; the other diagonal pair feeds the motor with reversed polarity and the direction changes. The duty ratio of the switches also sets the speed.
It takes its name because the schematic view of the circuit looks like the letter H: the two vertical arms of the H are a pair of switches each (a leg) and the horizontal line in the middle is the motor.
Basic states:
- Forward: Top left and bottom right switches conducting; current flows through the motor from left to right.
- Reverse: Top right and bottom left switches conducting; current flows the other way and the motor turns in reverse.
- Coast: All switches off; the motor slows down through its own friction.
- Brake: The two lower switches conducting together; the motor terminals are shorted, the back-EMF produces opposing torque and the stop is faster.
Its advantage over reversing direction with a single relay is that direction and speed can be controlled in the same circuit, without contacts and without arcing. The critical risk is the upper and lower switch in the same leg conducting at the same time; this creates a short circuit between supply and chassis and is prevented with dead-time.
In the ST42, KS250 and PT500 modules direction control works on this principle, without mechanical contacts; the KS250 and PT500 additionally have speed adjustment via a pot, and the PT500 has an independent current limit for each direction through the A1/A2 pots.
The full topic: DC Motor Driver Selection Guide