# Bi-directional Control Of Motors And The H-Bridge

Posted on Feb 6, 2014

Reverse the DC power connections on such a motor and its shaft will rotate in the opposite direction. One way to control direction is to use switches arranged in the following manner: This configuration is called an H-Bridge due to its shape. Let`s say that the motor runs forward when its + terminal is connected to Motor V+ and its - terminal is connected to ground. It will run in reverse when the opposite is true.

Turn on switch A and switch D and the motor will run forward. Turn on switch B and switch C and it will run in reverse. The following table shows all of the possibilities. A 1 means a switch is on, and a 0 means it`s off: Only a few of the possibilities are needed. In fact, seven of the combinations must be avoided because they short out the power supply. We can use the forward and reverse, one of the offs and, optionally, one of the brakes. The braking action might need a little explaining. A motor`s armature will pass through the fields of the magnets when it is turning, as will the wire that`s wound around the armature. That will induce an electrical current in the wire. Spin the shaft of a motor and you can measure a voltage at its terminals; it has become a generator. Short the motor`s terminals and you will feel it resist your attempts to spin the shaft. That`s because the magnet that`s formed by the current induced in the armature winding is opposite to that of the motor`s field magnet. The opposite poles attract, resulting in a braking action. Both of the brake combinations short the terminals of the motor together. There are a variety of devices that can be used for switches, including common mechanical switches, transistors and relays. The TIP120 will work for low currents, but a heat sink is needed for high current. The reason is that most transistors have a voltage drop between their collector and emitter of...

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