It is not clear to me what happens inside the stepper motor when it is manually and rapidly rotated in clockwise/counterclockwise, overcoming the torque of the motor.

As I understand, fly-back diodes are protecting the driver when the windings are not energized and the stepper is acting like a generator. So it seems to me that no additional protection is necessary if I manually rotate the shaft of the motor when it is fully powered. But I'm not 100% sure on this...


I ordered this little bipolar NEMA 11 stepper, driven from this driver. The shaft of the stepper is linked to the axis of a knob with a 5-to-1 ratio spur gear. No half-stepping, no micro-stepping.

I plan to use an 8-diodes-array to protect the driver board:

Texas Instruments UC3610

  • \$\begingroup\$ The diodes in the A4988 driver chip should be sufficient \$\endgroup\$
    – Kartman
    Commented Dec 12, 2020 at 10:48
  • 2
    \$\begingroup\$ At higher speeds, you do have to watch what happens when you're pushing voltage back into the power supply. Especially if that voltage exceeds the supply rating. Consider whether you need to dump generated power into a load resistor or charge a battery with it. \$\endgroup\$
    – user16324
    Commented Dec 12, 2020 at 12:20
  • \$\begingroup\$ @BrianDrummond it is difficult to evaluate the rpm-speed: if I turn manually rapidly the knob of 90° then the shaft of the motor will make 1,25 turns (because of the 1-to-5 spur gear). But... what is the speed of my hand??? \$\endgroup\$
    – gimpo
    Commented Dec 12, 2020 at 14:12
  • \$\begingroup\$ True, but you can easily measure voltage (DC easiest, after the diodes) \$\endgroup\$
    – user16324
    Commented Dec 12, 2020 at 14:13
  • \$\begingroup\$ @BrianDrummond: so it seems that I have to wait for the delivery of the stepper... Then I can do some measurements of current and voltage when I quickly turn the knob. Anyway, I was expecting that if the windings are already energized then the stepper cannot turn itself into a generator. After your comment I started thinking that I'm deeply wrong... \$\endgroup\$
    – gimpo
    Commented Dec 12, 2020 at 17:25

1 Answer 1


Well, through the comments, I think you answered your own question. If the stepper motor is powered up, the diodes in the driver chip (A4988) should direct any kick back into your battery. If you are using a DC power supply (from 120 VAC) then you need an electrolytic capacitor...this is shown as a 100 microfarad on the data sheet for the driver chip (between VBB1,VBB2 and ground). The DC power supply may have a capacitor on its output as well.

  • \$\begingroup\$ Thanks for summarizing up the "answers" in the comments. I plan to use the standard 12V-18Ah battery of my motorcycle. \$\endgroup\$
    – gimpo
    Commented Jan 11, 2021 at 9:34

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