H-Bridge Motor Controllers

The H-bridge is a simple but effective method for controlling the direction of a motor. This system uses four switches (usually relays per motor). When two of the switches on either side of the motor are closed the motor turns in one direction. When the opposite two relays are closed the motor turns in the opposite direction. The H-bridge reverses the flow of electricity that is traveling through a motor. This reversal results in a change in direction of the motor because the permanent magnets in the motor will either repel or attract the motor coils depending on the polarity of the incoming power source.

In the two schematics bellow the closed relays are indicated by red arrows, the direction of the motor by the green arrow and the flow of electricity by the yellow line.

Usually when a circuit is hooked up like this the two opposite relays that control the motor direction are wired together so that they close at the same time.

If you want to control the speed of the motor as well you can add a potentiometer (variable resister) between the connection traveling from the motor's power source and the positive (+) side of the H-bridge.