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I have a stepper motor and it is rated at 2 amps per phase. I also have a stepper driver which is rated at 750 miliamps per phase. To make matters worse I have a wall wart power supply rated as 9V @ 650miliamps. That power is converted by a voltage regulator PCB down to a usable 3.3 V at an unknown amperage.

My question is, what effect will underpowering the motor have?
Will it affect the speed of the motor or just holding torque?

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up vote 10 down vote accepted

Running a stepper motor at under it's rated current will affect both it's maximum speed and torque (both holding and dynamic), though depending on the driver type, it mostly effects the torque.

Firstly, The stepper motor driver you are linking requires power rails of 7-30V. It's logic interface is 5 or 3.3v.

Furthermore, your stepper motor is 3.6 ohms per winding, so even with 3.3V rails, your motors are going to pull ~900 ma, which will cause power rail brownouts.

Fortunately, in this context, the stepper motor controller IC provides chopped current limiting, which is a technique which limits the maximum amount of current it runs through the stepper motor. Therefore, you can run the stepper motor you have off the wall-wart you have, a the cost of reduced torque.

In a system which uses current chopping, the voltage largely determines the motors maximum speed, and the current the torque. There is interactions, but they are fairly small in effect size (assuming you are not at extremely low voltage or current), proportionate to the effect of changing the relevant characteristic directly.

I urge you to carefully study the A3967 datasheet (the A3967 is the stepper motor IC the linked driver uses).

Furthermore, the designer of that stepper motor driver provides a fairly decent introduction into the concepts of how the device works here, which, from your question, I don't think you have really read. You should carefully read it before you hook anything up, lest you damage one of your parts.

Additionally, the stepper motor driver you link already has a voltage regulator, so you do not even need the voltage regulator you included in your post (SFE sku: PRT-00114).
See the schematic for the Stepper Driver Here. The voltage regulator is IC2.

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I understand more now from asking 1 question than from reading all that documentation. – user3045 Feb 18 '11 at 10:12
I don't mind the question about how stepper motors behave, I'm just pointing out that it seems you have order some unneeded parts, since you haven't really read the documentation. – Connor Wolf Feb 19 '11 at 6:54

According to the product page, it uses an LM317, which has a output rated at 1.5A. Since your total power input is 9V * 0.65A, hopefully when it is regulated down to 3.3V you will get the maximum current.

Driving this stepper with 3.3v will definitely have an effect on your holding torque. I'd expect it to adversely affect the speed of the motor under load as well.

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Would that be the output of the the board or just the IC? – user3045 Feb 18 '11 at 10:13
Hopefully both. I've never seen the output of a voltage regulator be different than the output of the pins that the powered-device is connected to on the board. – Dave Feb 18 '11 at 13:56

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