Why do we need four switches? In reality, we only need one switch to turn the DC Motor On and OFF. Applications where this is all that is needed, will happily use a single switch and move on. However, the great deal of applications out there not only require the control of On and Off, but also of direction. For this purpose, a single switch is no longer enough.
At first it may be apparent that two switches should suffice. However, a carefull look will show this is not the case. Since we must apply both a GND connection as well as a positive voltage (VBB) connection to either of the two DC Motor terminals, four switches are the bare minimum. This configuration guarantees that each terminal on the DC Motor has access to both GND and VBB.
In order for the DC Motor to move in one direction, voltage of one polarity is applied. If the opposing polarity is applied, then the motor spins in the opposing direction. Hence, the importance of being able to apply two voltage polarities to the same motor.
The picture above shows how this is attained. If we turn switches Q1 and Q4, current flows from the left terminal on the motor, to the right terminal on the motor (left side of the picture). This results in the motor turning clockwise. If on the other hand we enable switches Q2 and Q3, then current flows from the right motor terminal to the left motor terminal, resulting on a counterclockwise motor shaft rotation.
Care must be taken so that VBB is never connected to GND as this is a hard short and it will most certainly destroy the power source along with the switches. An event in which switches are connecting VBB to GND is called Shoot-Through. Either enabling switches Q1 and Q3 or switches Q2 and Q4 at the same time, would result in Shoot Through. Shoot Through HAS to be avoided at all cost. Proper H Bridge design will take this important aspect into consideration and will later play a role with how fast the switches can be closed and opened.
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