Speed control of dc motor using pulse width modulation (pwm)Dc motor finds a wide application in various field. The major use of dc motor is found in cpu fans, fumes extinguisher and many other dc operated appliances.The speed of motor needs to be adjusted as per the requirement of the user and the respective condition. Many fans find such speed adjustment as an automated function but there are also the cases where the speed is to be adjusted manually.There are various method for adjusting the speed of dc motor. One of the simpliest method is to add a series resistance along the motor. Though simple this method does not find its applicability when we want to use any devices like microcontrollers or any other digital equipment for automating our DC fan speed. So there comes the use of pulse width modulation technique of dc motor control for efficient speed control.

Circuit diagram for pulse width modulation method of dc motor speed control

How does the circuit work?IC 555 is an integrated circuit that is utilized in this circuit to operate a dc motor. Here, IC 555 is operated in astable mode where the circuit can be used as a pulse width modulator with few small adjustment to the circuit. The frequency of operation of the circuit is provided by the passive parameters of resistances and capacitances attached to it. The resistance between pin-7 and pin-8, the resistance between pin-6 and pin-7 and the capacitance between pin-2 and the ground govern the frequency of operation and duty cycle of the IC 555 in astable mode. The duty cycle is governed by the resistor which is in between pin-6 and pin-7 of the IC 555 timer. So, by taking advantage of the circuits working, we can change the 555 astable multivibrator into a pulse width modulator by using a variable resistor instead of a constant resistor in between pin-6 and pin-7.This modulator is very cheap and advantageous in the sense that it can be utilized as an astable multivibrator with little hardware and little cost. If the desire is for more advance pulse width modulator than it is better to choose the microcontroller based pulse width modulation than this. The duty cycle of this circuit can be changed by varying the resistance between pin-7 and pin-6. Increase in duty cycle increases the speed of motor and decrease in duty cycle decreases the speed of motor.However the circuit is not so much sensitive and accurate. for accurate and sophisticated pulse width modulator it is advised to use microcontroller.