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Robotics Tutorials - Advanced - Hardware - Controlling DC Motors

The principles of DC motors are covered in the beginner and intermediate sections of this tutorial. This section will cover the electronics needed to interface them to a Basic X microcontoller or other digital chip.

The easiest way of controlling motors is using the Robocore. This board contains the driver electronics to control up to 4 DC motors or 2 stepper motors, and features direction LED's for easy debugging of circuits.

However, if you want to build your own motor drivers to use in custom projects there are various methods that you can use.

A circuit called an H bridge circuit will give you direction control of your DC motor. Below is the circuit diagram.

Here's how the circuit works.

Transistors are electronic switches, they allow you to turn on large voltages (the motor power supply) using a very small current (like the output pin of the Basic X). Each pair of transistors is connected to a pin on the micro controller and control the polarity of the current supplied to the motor. The actual components required would depend on the size of the motors that you want to use.

The diodes in the circuit are very important and are called fly back diodes. They are there to prevent voltage spikes from the motors from destroying the transistors. When a motor rotates and changes direction the coils of wire inside it act as a generator and produce a current. This current is called the back electro motive force, or back E.M.F. for short. This current travels back through the circuit, in the form of powerful voltage spikes, to the transistor. Now, because transistors only allow current to flow in one direction the current hits a bottle neck as the transistor tries to stop the reverse current. If this current is particularly large, say when your reversing the direction of the motor, it will simply blow the transistor, giving your new circuit a pretty short life.

The diodes job is to protect the transistor. It allows this back E.M.F. to bypass the transistor and travel safely back to the battery.

The circuit can be powered using whatever voltage is suitable for your motors. If you are using two power supplies, one to power your Basic X and another to power your motors, you must connect the ground of each supply together or your circuit may not work properly.

If you're looking for specific components to build the circuit the following will handle most motors up to about 12 volts.

Transistors: TIP41 NPN power transistor

Diodes: IN4002

Resistors: 2.2Kohm, 0.25 watt



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