Motor stall current vs motor driver's peak output current


I am very new to robotics and electronics in general, and I am doing a robotics project, which is basically a two-wheeled robot. Due to circumstances beyond my control I am using a certain motor driver: A3950. Its datasheet says, in particular, that:

  • A3950 is capable of peak output currents to ±2.8A.
  • Driver features overcurrent protection.

Now I would really like to use the Pololo 19:1 37Dx52L motor with this driver.
However, the motor spec states that its stall current is 5A. I am wondering, much of a problem may this stall current turn out to be for the selected driver.

In particular, I would be grateful if someone could help me with the following questions.

  1. Do I understand correctly, that “stall current = 5A” means that, unless current is limited, there will certainly be a 5A current spike when the motor is started.
  2. Is this spike short enough (assuming the motor will propel a 2kg robot) to be unnoticeable?
  3. Do I understand correctly that if I change the polarity in order to use the motor for braking, this will also lead to stall torque conditions and, consequently, 5A current?
  4. Is it true that I can always limit the current by simply carefully programming the appropriate PWM signals at both startup and braking?
  5. In general, are there some other useful things I might have to know in this setting? Just a link describing similar situation would be helpful, as my googling skills failed me on that.

Thank you!


I don’t think that driver will handle this motor very well. I did not see any rating besides the 2.8A peak; many drivers have continuous current ratings of less than half the peak. Judging by the on-resistance, I don’t think you should count on getting more than about 1.5A out of this driver. Also, the over-current detection can trip as low as 3A, which could be quite annoying to deal with.

Here are answers to your questions:

  1. Pretty much.
  2. It depends on how good your system is at “noticing” these things. I suspect that it could be a problem with the driver you have.
  3. If you flip power while the motor is running, you’ll get double the stall current, or 10A.
  4. Probably to some extent, but it’s not always practical.
  5. I don’t have any links to recommend. Something like the A3950 is a fairly complicated device, and you have to take a lot on trust (e.g. in the datasheet). There can be all kinds of subtle problems even when you think you’re using everything within specifications; you’re asking for a lot more trouble when you know you’re well past the maximum ratings. I recommend going with a more capable driver, such as the MC33926 (the link is to a dual carrier, but we have single ones, too), or to a smaller motor with a stall current under 3A.

- Jan


I just want to add that the motors you are looking at can run at 6 V, in which case the stall current is approximately 2.5 A. If you absolutely must use those motors with that motor driver, please consider using them at something closer to 6 V (I don’t know how well your driver will work at 6 V, though, so 8 or 9 V might actually end up working better).

- Ben