Building an H-Bridge Control Circuit

A power reversing circuit is used to control which direction a DC motor turns. Power reversing circuits are incredibly versatile circuits and can be used for a wide variety of applications. For instance, these simple circuits can be used to control simple devices such as a crane winch; however, if you have two or more power reversing circuits you can control advanced robotic devices such as a battle-bots.

In the field of electoral engineering power reversing circuits are commonly referred to as an “H-Bridge”. This is because the most basic H-Bridge design contains four switching element, with the load at the centre, in an H-like configuration:

H-Bridge: Noun
A simple circuit that lets you control a DC motor to go backward or forward.

*A simple H-Bridge using 4 simple switches.* *The flow of electricity is marked with arrows.*

More advanced H-Bridge designs build on this one simple circuit design. A slightly more efficient design will replace the 4 one way switches with 2 SPDT (single pole double throw) switches, or with 1 DPDT (double through double pole) switch. However the most popular H-bridge design uses transistors and can be controlled using a simple control circuit or computerized microcontroller.

*H-Bridge designs using transistors*
[***NOTE:*** *although the circuit can work using only transistors, diodes are usually added to protect the transistors from power surges that are caused by the motor].*

There are numerous ways to build H-Bridge circuits, and each method has its own advantages and disadvantages. Some will have forward and reverse, while others will also have a neutral state. Some must be controlled by an operator while others can be controlled using an input signal from a microprocessor. The video below shows how an a diode based H-bridge design operates. This particular design could be controlled using a microprocessor or a simple 3-way switch.

A simulation of how electrons flow through an H-Bridge circuit that uses diode.

For instructional purposes we have designed a number of H-Bridge variations that you can use in your classroom. Each of these designs range in complexity and functionality which will allow you, the teacher, to differentiate assignments based on the students ability. Moreover, you can also have students complete various designs as a group so you can have the students compare and contrast the functionality of different approaches using the think, pair, share methodology.