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Problem with output voltage of simple PWM DC motor speed controller

Guest ravi_bhat

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Guest ravi_bhat


I found the simple PWM DC motor speed control circuit very useful. I am planning to use this plan to control my small 12V DC geared motor to control its speed precisely. The application of this motor will be in an equatorial drive for my telescope that I am building. There is no need to vary the speed of the motor every now and then, but the speed of the motor has to remain constant once set.

I made the circuit described here - http://www.electronics-lab.com/projects/motor_light/005/index.html, on a breadboard. The required motor speed for my project is achieved at a duty cycle of 25% approx. I am able to achieve this degree of control using this circuit. However, the problem I am facing is, when I reduce the duty cycle to anything below 100%, the output voltage (across the motor) is not stable. i.e., when I have a 100% duty cycle, voltage across the motor remains very stable at 12V, but when I set 25% duty cycle the voltage fluctuates between 2.8V to 3.2V, which makes the motor speed also vary somewhat. This will be a disaster for the telescope drive. I am using an auto-ranging DMM for measuring the voltage and duty cycle at the motor (I don't have access to an oscilloscope). The motor draws about 20mA current at no load and it will run at near no-load condition in the telescope drive. What can I do to get a constant voltage for my motor?

Thanks in advance,

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