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What Is an H-Bridge?

An H-bridge allows a robotic arm to move in multiple directions.
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  • Written By: Jason C. Chavis
  • Edited By: Bronwyn Harris
  • Last Modified Date: 20 September 2014
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An H-bridge is a type of electronic circuit that is shaped like an “H” and is used to allow voltage to travel in both directions across a board. These are used most readily in situations requiring the motion of a device such as a direct current (DC) motor to move in two directions, usually backward and forwards. Built as an integrated circuit, an H-bridge generally includes four solid-state or mechanical switches, making it work with reverse polarity devices. When two adjacent switches open, a positive voltage is applied. As these two close and the other two open, a negative voltage is applied.

Constructing an H-bridge requires the use of reverse polarity devices such as metal-oxide-semiconductor field-effect transistors (MOSFET). These are typically negatively-charged since they have less resistance than a positively-charged MOSFET. The H-bridge itself is divided into two sections, an upper bridge and a lower bridge. Each side needs one switch to open to change the polarity. The MOSFET itself is responsible for this switching.

The design for an H-bridge can be modified to produce a “half bridge.” This is used to amplify the signal that is being loaded through an electronic device, rather than specifically changing polarity. This half H-bridge utilizes energy from the power supply of the device itself and uses it to modulate the output through the shape of the signal.

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The most prevalent use for an H-bridge is in the field of robotics or other similar mechanical operations. These types of designs require movement in different directions, unlike other devices, which generally function in a single manner. Since the integrated circuit can change polarity, it can make the motors of the mechanism operate in both directions. This allows a designer to make more use of motion than otherwise possible. For example, a robot that plays soccer needs to move its “foot” backward to get ready to impact the ball and forward to release the kick.

Using an H-bridge as the controller for a motor has been made easier in the early 21st century with the development of the MIT Miniboard and other similar designs. Stemming from the design of the MIT 6.270 robot design project, the Miniboard works as the base controller module for almost any type of robotic device. Since the board is composed of a simple configuration with attachments and an integrated circuit, it can be used as the principal piece for home and educational design projects.

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