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I'm having some problem powering my stepper motor through arduino motor shield with external power supply. I already cut the Vin trace on the motor shield, so the main board and the motor shield is not now powered separately. The stepper motor is rated at 12v, 1.7A.

Using a variable power supply, I've noticed some very odd things. When I restrict the power supply to 5V, 1A, the overall draw seems to be around 0.7-0.9A. When I upped the limit to 2A, the draw was around 1.6A. Understandably, when the stepper motor is braking, the full current is drawn.

However, when I supply 12V, the current draw seems to be whatever I set the limit to. If it is 1A, it always is 1A. When it's 2A or more, it always draws that too. at 12V 2A, the shield is reaching it's limit and gets very hot.

For now, i'm limiting my source to either 12V 1A, or using 5V (since the load isn't that much). But I want to figure out why is it behaving this way. I'm probably missing something very basic here, so any help is appreciated.

Motor Shield: http://arduino.cc/en/Main/ArduinoMotorShieldR3 Stepper Motor: http://www.phidgets.com/products.php?product_id=3319_1

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  • \$\begingroup\$ It's probably worth adding more details on the type of stepper motor and how it's connected. Any chance when braking you're driving multiple coils? That might be a likely cause. \$\endgroup\$ – PeterJ Jul 28 '13 at 6:09
  • \$\begingroup\$ I just noticed on the stepper motor page: Connecting the motor directly to a power supply will destroy the motor and void the warranty. If you want to check your motor make sure it is connected to a constant current / chopper drive controller. \$\endgroup\$ – PeterJ Jul 28 '13 at 6:44
  • \$\begingroup\$ The motor itself is a 4 wire, bipolar stepper motor. Yes, when braking, both the coils are active, and hence draw full current. But during normal operation, the draw should be much lower (like in the case of 5V). \$\endgroup\$ – Tauseef Latif Jul 28 '13 at 6:47
  • \$\begingroup\$ I thought initially it might have been unipolar and you were driving all the coils. Wait to see what answers you get, I've never done much with bipolar motors but it sounds like this one needs a constant current source. It might be possible to do a chopper drive under software. \$\endgroup\$ – PeterJ Jul 28 '13 at 6:53
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I actually figured it out. The stepper motor itself cannot regulate current and so has no way of stepping down to a safer level. It is the job of the motor controller to provide safe current level to the motor. Since I was powering the motor externally, the motor controller may not have been providing proper current regulation.

And since the coil resistance is so low for these motors, anything more than ~1.5A will start to cause problems.

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