Hi,
I am using the “Makerverse Motor Driver, 2 Channel” for driving a DC motor for an exercise bike. The motor is moving a magnet over a metallic flywheel to alter the resistance when spinning. The motor drive voltage is 5V and the motor driver operates in PWM / Dir mode. In practice the average motor drive voltage is around 2V. I have measured the motor current to around 0.1 - 0.2 A.
The other day the magnet stopped moving and after some debugging I found that the motor driver output stage did not work anymore. I replaced the motor driver with a new but same board and now it works again.
I am wondering if I need to protect the motor driver output with external diodes to protect from back EMF? What is your view?
What would happen if the motor was driven to far and the movement was forced to a stop. Would the current limiting kick in and protect the driver in that case? In my setup that should not happen as I have limited the allowable movement of the motor in software (it is using a potentiometer to read the motor position). I am just trying to exclude other reasons for the failure.
Kind regards,
Tony
tcnygard@gmail.com
Hi Tony
Measuring the motor voltage with a DMM does not mean much. The voltage you see you have correctly described as the AVERAGE of the PWM drive voltage. The actual situation is a bit more complex that this. he same applies to the current measurement.
The driver IC takes care of this with a combination of switching control and Mosfet body diodes. If the motor is driven in both directions, as I think it is, fitting protection diodes is not straight forward and I would advise against fitting any extra.
The current required would be the stall current of the motor, which incidentally is the same as the start up inrush current as you are applying power while the motor is stopped. This is only for a very short time until the motor moves.
The key word here is “SHOULD”.
If this is your version of a “Limit Switch” forger it. This would never pass muster. If a failure occurred involving the right 2 Mosfets (which commonly fail short circuit) the motor will go hard on and no software will stop it. Probably go to the end of travel at some speed and bend things when it can go no further. Limit switches MUST remove power to the motor and / or the supply to the controller. To be effective a limit switch must also apply a brake to the motor to prevent over run which could cause the switch to restore power when the motor goes past it.
I am assuming here that this motor only operates when you want to change the load on the spinning disk. Which I also assume is driven by the pedals and the person using the apparatus.
Cheers Bob
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