An H-bridge is an electronic circuit which enables a voltage to be applied across a load in either direction. H-Bridges Circuit are often used to control the speed, position or torque of DC and stepper motors. An H-bridge is built with four switches (solid-state or mechanical). It takes a DC supply voltage and provides 4-quadrant control to a load connected between two pairs of power switching transistors. Below schematic diagram shows the Basic H-Bridge Circuit Diagram.
When the switches A1 and B2 switches and closed the A2 and B1 switches, a positive voltage will be applied across the motor. Because the switches allow current to flow bidirectionally, the voltage across the load and the direction of current through the load can be of either polarity.