DC Motors

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DC Motors   An interfacing method for turning on and off a DC motor via a microcontroller is shown below.   http://www.edsim51.com/8051Notes/images/5vMotorInterface.GIF  However, the above circuit will only work for a 5 V motor. If the supply voltage is changed (for example, if the supply is changed to 12 V to run a 12 V motor) then the motor will be on all the time because 5 V applied to the base of the p-n-p transistor is not enough to turn it off.       To interface to larger motors the following circuit may be used.   http://www.edsim51.com/8051Notes/images/12vMotorInterface.GIF  In the above example a 12 V DC motor is interfaced to a microcontroller. When the port pin is set (ie; is equal to 5 V) the p-n-p transistor is off which means the n-p-n transistor is also off. Therefore there is no path for current through the motor and the motor is off.   When the port pin is cleared the p-n-p transistor is on. This turns on the n-p-n transistor which allows current to flow through the motor to ground; the motor is on.   The value of R2 needs to be carefully chosen; too high and the current into the base of the n-p-n transistor will not be enough to turn on the transistor, too low and the circuit draws too much current.       Bi-directional DC Motor   A circuit diagram for interfacing a 12V DC motor to a microcontroller in a way that allows the controller to not only turn on and off the motor but also to set the direction in which the motor runs when it is on, is given below.   http://www.edsim51.com/8051Notes/images/bidirectionalMotor.GIF  The circuit is made up of a bridge. If both sides of the motor are at the same voltage the motor is off. So, if T1 and T3 are on, both sides of the motor are connected to 12 V and the motor is off. If T2 and T4 are on both sides of the motor are connected to ground and, again, the motor is off.   If T1 and T4 are on then the left side of the motor is at 12 V and the right side is at ground, therefore the motor runs in one direction. We will call this forward.   If T3 and T2 are on then the left side of the motor is at ground and the right side is at 12 V, therefore the motor runs in the opposite direction; ie, reverse.       The circuit is designed so that T1 and T2 cannot be on at the same time and T3 and T4 cannot be on at the same time. This is very important; if T1 and T2 were on at the same time there would be a short circuit between 12 V and ground and the transistors would burn out. The same is true for T3 and T4.       The truth table for the circuit with its two inputs, A and B, is given below.  


A

B

Motor

0

0

off

0

1 (5 V)

reverse

1 (5 V)

0

forward

1 (5 V)

1 (5 V)

off

  An explanation of the four entries in the table is given below:  

  • Entry 1:
    • With A at 0 (ie; ground) T5 is on which turns on T2; left side of motor is at ground. A at 0 also means T6 is off. Since there is no path for current through R3 and R4 there is no voltage drop across them which in turn means the base of T1 is at 12 V, hence it is off.
    • The right-hand side of the circuit is a mirror image of the left, therefore with B at 0, T4 is on and T3 is off - hence the right side of the motor is also at ground; the motor is off.
  • Entry 2:
    • A is still at 0 which means T1 is still off and T2 is still on; the left side of the motor is at ground.
    • With B at 5 V (ie; logic 1 on the port pin which is being used for B) T7 is off which means T4 is off. But T8 is on which generates a path for current through T8 to ground and also through R9 to the base of T3. There is a certain amount of voltage dropped across R9, but the base of T3 is close enough to ground for T3 to turn on; the right side of the motor is at 12 V.
    • The motor is therefore on and we stated above that ground on the left of the motor and 12 V on the right would be called reverse.
  • Entry 3:
    • This is the mirror image of entry 2, resulting in T1 on, T2 off, T3 off and T4 on; hence the left side of the motor is at 12 V and the right side is at ground - the motor runs forward.
  • Entry 4:
    • As in entry 3, with A at 5 V the left side of the motor is at 12 V.
    • As in entry 2, with B at 5 V the right side of the motor is at 12 V.
    • Therefore the motor is off.

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