I have built a uC controlled H-bridge circuit to control a DC motor. I have coded in a delay before starting the motor in order to eliminate the possibility of an instantaneous reversal of polarity. My question is what is the minimum delay time that I should code?

Thanks

 

that depends on the rotating mass, less mass, shorter time, more mass, longer time. thats why machine tools use a plugging switch or relay to detect zero apeed. a fixed mass could be done with a timer, a varying mass needs a zero speed detector.

 

that depends on the rotating mass, less mass, shorter time, more mass, longer time. thats why machine tools use a plugging switch or relay to detect zero apeed. a fixed mass could be done with a timer, a varying mass needs a zero speed detector.

It's a small gear head motor with the output rotating at 40 rpm; the load is very light. Would 100 msec be safe?

 

If reversing, I would expect you would have to wait for most of a second. Why not measure the back emf from the motor using the ADC on the MCU, and reverse only when the back emf drops to zero. Another possibility is dynamic braking by switching the H-bridge into the appropriate state.

 

If the process is always the same, all you need to know/time the stopping time of the motor, a DC motor generates a voltage as the same polarity of the applied power, this voltage is directly proportional to the rpm of the motor, on a small motor it should not even require a dead stop condition.
Max.