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I got this little car I built using two of these motors and A4988 drivers.

It was running with a 12V power supply, but I'm trying to get it on a battery and running for about 8 hours. thought I could use one of those phone power banks that give 5V, but I tried using a 5V power supply and it doesn't seem to work.

Thought maybe 5V is not enough for the driver, but they say here that it should do: Can I drive a low voltage stepper motor with an A4988 driver?

Any idea what the problem could be? (I'm also open for other solutions)

By the way, don't know if it matters but the power also goes to the Arduino controlling the thing.

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  • \$\begingroup\$ Where does it say 5V should do? I don't see it. The A4988 has a minimum load supply rating of 8V. \$\endgroup\$ – Tut Aug 18 '14 at 10:25
  • \$\begingroup\$ Could you clarify what you mean by "it doesn't seem to work"? Does NOTHING happen? Do the steppers try to turn but lose steps? Does the driver smoke? How much current does your 5V supply provide - enough to power the Arduino, the driver, and the stepper? Do you have effective filtering/bypass capacitors in place to prevent stepper loading from dropping out your Arduino? \$\endgroup\$ – TDHofstetter Aug 18 '14 at 14:09
  • \$\begingroup\$ Maybe I didn't interpretare correctly the thread I directed to when it said "you only have to ensure the power-supply voltage for the driver is > then the rated voltage on the stepper. Nothing happens at all. The power supply i used is one for mobile phones giving 1.2A. I think it should do... \$\endgroup\$ – Muaddib Aug 18 '14 at 18:22
  • \$\begingroup\$ ..."you only have to ensure the power-supply voltage for the driver is > then the rated voltage on the stepper"... This statement is almost true (doesn't take into account voltage drops in the driver circuit), but is intended to show the minimum requirements, for the stepper motor, in order to drive it to it's rated current (using a chopper drive). You still need to honor the minimum voltage requirements of the driver that you choose. In the case of the A4988, this is 8V. \$\endgroup\$ – Tut Aug 18 '14 at 19:03
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    \$\begingroup\$ Fundamentally, stepper motors are a poor choice for the drive wheels of a mobile, battery powered robot. \$\endgroup\$ – Chris Stratton Aug 19 '14 at 18:48
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The answer to the question you linked to explains that a 2.55V rated motor could work with the A4988 driver.

Answers continue on to explain the real constraint for a stepper is thermal, it could be 'cooked to death' by too much power.

The answers also explain the A4988 can be set to limit the current, so that a 2.55V stepper could be driven from a 8-35V power supply without damage.

The problem is trying to drive the A4988 with 5V, as Tut wrote, the A4988 minimum drive is 8V.

Looking at Allegro's web site for Bipolar Stepper Motor Drivers the A4980 is designed to operate at 3.3V or above.

I do not read Chinese, and I can not find any mention of the rated voltage of the stepper you linked to.

It looks like the motor could be made to turn at 5V. However getting that stepper to work reliably, with sufficient torque, at a high enough speed for your purpose, might be a problem at 5V. I would try a higher voltage than 5V, maybe 8.4V of NiMh, as an experiment, using the A48988 drivers. (Edit: 7 x 1.2V NiMh is 8.4V not 8.6V corrected)

The current consumption of a motor is highly variable. In the absence of a datasheet, a reasonable guess is half-max speed is maximum efficiency, and happens around 1/2 maximum current.

One 0.45A motor might consume 0.225A if it is running at maximum efficiency, and so 8 hours would be 1.8Ah.

The maximum current is 0.45A, so one motor running at maximum current (and hence maximum power) would consume 8*0.45A = 3.6Ah.

Choose a battery, for each motor, in between those two.

The Remote Control model approach is to use LiPo batteries, which run at a nominal 7.4V (for 2 cells in series) or 11.1V for 3 cells in series. They are light, and can charge quickly. They need a LiPo charger, and they must never, ever be run too low.

(Have you access to any test equipment, for example a bench PSU which would let you 'dial in' the voltage and current?)

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  • \$\begingroup\$ I must have misinterpreted the thread I directed to... I can't read chinese either and derived the voltage from the current and resistance. Is it wrong to do in this case? I could try a 8.6V but that would probably not get me even close to 8 hours... I could also change the motors and/or drivers. No bench PSU unfortunately. (and thanks for the great answer btw) \$\endgroup\$ – Muaddib Aug 18 '14 at 18:33
  • \$\begingroup\$ @user51505 "... derived the voltage from the current and resistance. Is it wrong to do in this case?" Probably. As I, and the thread you pointed to explained, the problem for steppers is overheating. However DEPENDING ON WHAT YOU WANT TO DO the power supply might have to provide a much higher voltage than I*R would give. So you can not do simple IR calculations and derive the voltage. The back-emf of the motor will reduce the current, so the motor may need a higher voltage to drive enough current. Stepper motor drivers solve overheating by limiting current, but it needs voltage headroom. \$\endgroup\$ – gbulmer Aug 19 '14 at 0:28
  • \$\begingroup\$ @user51505 - " I could try a 8.6V but that would probably not get me even close to 8 hours" How do you derive that? Let's use your simple calculation. Each motor might use 0.45A per hour, two motors = 0.9A/hour * 8 hours = 7.2Ahours. Voltage didn't appear anywhere in that estimate, and I only used the values from the motors datasheet. \$\endgroup\$ – gbulmer Aug 19 '14 at 0:31
  • \$\begingroup\$ ok, sorry, each time I get a comment here it send me on a long google search and I learn something basic but new to me about batteries or motors :) so if I get 8.6V batteries and the ones I see are about 3000mAh, I'll need to stack about 7 of them in parallel to get to the run time I need. Am I getting this right? \$\endgroup\$ – Muaddib Aug 19 '14 at 1:08
  • \$\begingroup\$ @user51505 - you are getting closer!-) I suggested using something rechargeable, with a voltage big enough to operate the A4988 so that you could do an experiment to get some facts. The only facts are 1) it works at 12V, and 2) it doesn't work at 5V. That failure might be due to the A4988. Only you know what you want to do with the car, and it is likely hard to explain to us. Typically steppers need some voltage headroom to perform well, so a battery voltage > 8V seemed useful. Batteries, or if you have a 9V >1A 'wall-wart' that you can afford to fail, might give some facts to make progress. \$\endgroup\$ – gbulmer Aug 19 '14 at 1:20

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