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I have a stepper motor powered by a breakout board that uses the A4983 stepper motor driver (http://www.sparkfun.com/products/10735).

Sometimes when the power is disconnected and the drive wheels are back driven, all the LEDs on the the boards light up and it seems like the system is being powered by the stepper motors.

I have the board set up this way: Stepper motors connected to the stepper motor driver. Power coming from battery is directly hooked up to the driver's motor power supply. A 5v regulator comes off the same battery line and powers the micro controller and other 5v electronics.

Three questions:

1) what exactly is happening? I know when back driving the motor it will create a current, but this is a bipolar stepper, wouldn't the current be AC and thus destroy the circuit? (up until this point, everything still works just fine). How is the current flowing into my other electronics?

2) How harmful is this back driving? Are there serious voltage spikes that I should be worried about or is the stepper motor driver chip dealing with it?

3) how do i prevent or protect the circuits against harmful effects? I'd put a diodes between the motor driver and the motor but bipolar steppers are basically AC motors and that wouldn't work.

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The voltage coming out of your motor probably is AC, but that is being rectified by the kickback catch diodes in the driver circuit. Assuming this circuit was designed and wired up correctly, only the correct polarity voltage will be fed back onto the DC power input net. As long at this voltage doesn't exceed what the circuit is rated to operate at, there should be no problem with this.

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  • \$\begingroup\$ So would a pair of diodes at the motor driver's power input stop the reverse flow? Is it something that I should consider doing? \$\endgroup\$
    – Faken
    Commented Sep 29, 2011 at 18:37
  • \$\begingroup\$ Be careful with relying on those flyback/kickback diodes. If the motor is spun fast enough, it may exceed the power dissipation rating of them. That in turn may cause the diodes to become open circuit and possibly expose the board to AC. \$\endgroup\$
    – Thomas O
    Commented Sep 29, 2011 at 22:41
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    \$\begingroup\$ @Faken: If you put a blocking diode in front of the motor, it would prevent the inductive kickback from being shunted to the power supply. You'd have to put a snubber between the diode and the motor which would just waste the extra power instead of re-using it. It would also allow the motor to be driven in only one direction. \$\endgroup\$
    – Olin Lathrop
    Commented Sep 29, 2011 at 22:46
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    \$\begingroup\$ Well, this is a bipolar stepper motor so I can't put any sort of diodes in between the motor and the motor driver but what about between the batteries and the motor driver itself? \$\endgroup\$
    – Faken
    Commented Sep 30, 2011 at 17:55
  • \$\begingroup\$ @Faken: A diode directly in series with the battery would prevent the motor from trying to charge it and damaging it. However, it would not prevent the voltage from the motor driving the circuit. Note that none of this is a problem if the motor is never driven faster than the circuit would. \$\endgroup\$
    – Olin Lathrop
    Commented Sep 30, 2011 at 18:21
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Accidental powering of a board by motor-generators is often not physically harmful, provided that the motor isn't driven so fast as to subject the board to excessive voltage, but even when it is not harmful it can be a nuisance. It can be somewhat distracting, for example, if turning a motor that's attached to a powered-off devices causes it to freewheel until the device powers up and then start dynamic braking.

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  • \$\begingroup\$ The board's firmware may also turn on lights, fans, engage solenoids, etc. if there is sufficient power. \$\endgroup\$
    – Moby Disk
    Commented Oct 20, 2015 at 22:17
  • \$\begingroup\$ @MobyDisk: I can imagine that occurring, though the dynamic braking was a phenomenon I observed. I think the time I experienced this issue the dynamic braking clamped the voltage before it got high enough to operate the lights and beeper, but if it hadn't they might have been an additional distraction. \$\endgroup\$
    – supercat
    Commented Oct 21, 2015 at 3:58
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There are 3 parts to this question:

The first is adequately answered - the AC is rectified and powers the board.

As for the second and third: Yes, this can be dangerous. I've burnt boards this way.

Olin's answer says "As long at this voltage doesn't exceed what the circuit is rated to operate at..." - which is exactly the problem. Sparkfun lists that chip at 2A/30V. But you may be able to back-drive the motor to produce more than 2A.

There are 2 ways to prevent this:

  1. Add a physical brake solenoid that engages when power is off.
  2. Disconnect the driver chips from the motor power lines when power is off.

The first one prevents the external force (a human?) from back-driving the motor. The second solution prevents the back-driving force from hitting the driver chips.

I've worked on projects that employ both techniques. On one project, we had a pick-and-place device with a counterweight. If the counterweight was removed or lifted the elevator would fall with greater acceleration and top speed than the driver chips were intended to handle.

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