Reverse power protection question with steppers

I’ve been mulling building a CNC milling machine and using an Pololu A4983 microstepping drivers with voltage regulators.

  • My first question is what exactly is the reverse power protection feature?

So, my problem is that the Sherline CNC mill requires a dual shaft NEMA 23 stepper, where you would attach a hand wheel on the backend shaft, if you wanted to turn the leadscrews manually. So, when manually feeding the mill, you are also directly rotating the stepper motors, generating some current back flow. There really isn’t an easy way to avoid rotating the steppers.

  • Does the reverse power protection feature allow you to manually turn the steppers, if you powered down the motor power supply? Would you also have to power down the driver? Or everything?

  • Can the standard A4983 driver WITHOUT the regulator handle the same scenario?



First off, we have a schematic for the board so that you can see for yourself exactly what the circuit is. The point of the protection is to prevent destruction of the board if you connect power to the board with the wrong polarity (+ and - switched). The reverse power protection does not have anything to do with manually turning the motor.

If you don’t drive the motor (there’s an enable line or something like that on the driver), you should be able to turn it by hand without having to shut anything down. The only potential issue is if you manually spin the motor faster than it can spin on its own at the voltage you are using. If you are concerned about it, you could look at your motor with an oscilloscope and spin the motor shaft to see how high of a voltage it generates.

- Jan

Ah, that clarifies things. Thanks for pointing out that the full schematic is available for view. I’m not an electronics expert by any means and had a hard time deciphering exactly what the circuit was doing.

So, if I understand this correctly, when the stepper driver is disabled via the ENABLE or SLEEP pins, the motor power supply is disconnected at the IC by a transistor, placing the stepper on an open circuit. This would allow you to rotate the stepper freely. When you state that there is a potential issue with generating motor voltage, is it regarding by reversing the voltage potential across the transistors? Or, do you mean something else?

Also, are there any issues with simultaneously enabling/disabling 3 separate stepper drivers with a single i/o off a microcontroller? Other than ensuring everything shares the same ground, I would think not.

The motor is still connected to the circuit, which has a maximum rating. For instance, if spinning the motor quickly enough gets you spikes past 35V, that could cause damage. If you have something like a 12V power supply and the motor output is 20V at the maximum speed you can spin it, that voltage can get shown to other things in your circuit. You can probably deal with that by putting a big cap on the power supply, but I think you might start feeling some kind of resistance once the voltage you’re generating is higher than your circuit supply voltage.

Having the same input on multiple drivers tied together should be fine. You always have to be aware of things like fan out and wire lengths, but if you have 3 units within a few inches of each other, you should be fine, especially if we are talking about slow signals like the enable line.

- Jan