Pulse Width Modulation

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To control the speed of the motor, the switches of an H-bridge are opened and closed at different rates in order to apply different average voltages across the motor. This technique is called pulse width modulation. Let's look at the following diagrams:

Pwm1.jpg Pwm2.jpg

In the above diagrams, V is the voltage across the motor and t is time. By switching quickly, we can create an average voltage across across the motor. The speed of the motor can be adjusted by changing the pulse-width ratio:

Pulse-Width Ratio = \begin{matrix}\frac{t_{on}}{t_{period}}\end{matrix}

of the voltage applied across its terminals. If the motor only has to turn in 1 direction, we can just use a half bridge:

Half b.jpg

There are different ways to control the speed of a DC motor:

  1. switching from full positive to full negative</P>
  2. full positive to open</P>
  3. full negative to open</P>
  4. braking by shorting</P>
  5. free wheeling by leaving the circuit open.</P>

One of the possibilities is to use a full-bridge chip, such as the L6201 shown below:

L6201.jpg

When using a integrated chip, one of the concerns is heat sinking. It is because there are lots of power dissipation and we need to take the heat away. Usually, these chips come with some way to attach to piece of metal or copper (a heatsink).

For a practical circuit that controls a DC motor using PWM, see PWM Circuit Using PIC.

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