Speed Control


Thread Starter


anyone have any suggestions on controlling the speed of a motor run using an H-bridge and powered by a 9-volt supply?

Johan Bengtsson

Well, I assume you are talking about a DC motor.
A DC motor have a speed roughly proportional to voltage (it will be lower whith higher load, but that is the next part of the story).

If you have some concern about losses I would suggest you pulse width modulate (PWM) your transistors to get the desired average voltage, the PWM frequency have to be high enough for the motor not to be able to change speed during a pulse (some kHz for a larger motor and more for a smaller one)
Don't forget the free wheel diodes for each transistor (yes you need 4 of them).
Also, put one large and one smaller capacitor across the power supply but none across the output to the motor.

If it is a really low power motor and you can accept the losses by continous control of the transistors (and don't want the PWM part) then you have to make a similar approach but I really think it would be easier in this case to make
the PWM.

If you want to compensate the speed for increased load you will have to either measure the current (can be done at some common point) or measure speed. If this is necesary and you need help, repost a new question about that...

/Johan Bengtsson

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Thomas Leibner

I did that job using a stepper drive and a SGS L297/L298 driver. The H bridge enables control over the polarity and on/off of each motor windings. Furthermore I tried software pulse width modulation at 10kHz to adjust the motor current and torque. Software done with RTLinux and RTAI. Frontend with LabVIEW/Linux. In Produktion use for over 12 months. Please see http://lvlposixio.sourceforge.net for details.

I'm also working on a similar project .

Can anyone suggest what transistors and diodes to use for a pwm h-bridge.