The Circuit Schematic Diagram of PWM Motor Control

April 19th, 2010

Open loop or closed loop can be implemented for speed control application for DC motors. The closed loop or often called servo controller or feedback control gives the best performance. It is because the loop will maintain the actual speed to follow the reference. For example in Personal Computer’s fan speed control, an open loop controller is sufficient if the load is constant. The following PWM motor control circuit uses the Pulse Width Modulation (PWM), that will give a better efficiency than using linear driver. Following figure shows the circuit diagram :

PWM motor control circuit diagram

The infamous 555 IC is used in this circuit. However, it is configured in unusual way. It can be seen that the output from PWM is taken from the discharge pin, an open collector point that should be pulled high using a resistor connected to Vcc, while pin 3 – the original output pin – is functioned to discharge the timing capacitor. The recommended supply voltage for the 555 IC is between 9 to 12 volts. And if that voltage value is being used by the motor, you can connect the +V motor to the 555 supply voltage. The frequency of this controller is around 144Hz. The frequency can be changed by changing the C1 capacitor using smaller caps for higher frequency. If you allow the MOSFET to rise its case temperature to 100 degree Celcius, the motor current can be as high as 37 amperes. Therefore, enough heat sink should be provided to maintain the temperature below this point. It is also a good idea to provide protector diode in parallel with the motor in order to short the back emf generated by the DC motor that might cause a voltage spike which can burn the MOSFET. The simple pwm control circuit gives you a variable speed drives for motion control application. [Source : dprg.org]