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Decrease voltage but keeping current


#1

Hello everyone,
I am new to pololu forum and to electronic world also, so please excuse me if this question is too easy, but I looked for a long time and I don’t know what to do.

My problem is the next one :

I bought a Nema 17 stepper motor (this one : https://www.pololu.com/product/2267 ) and it’s driver (this one : https://www.pololu.com/product/2967), and I want to make them work with a nrf51822 BLE device, actually I can make it work with the nrf51dk from nordic semi conductors when I plug it in USB on my laptop but I would like to make it work with AAA batteries, that delivers 1.8A of current and 6V (I have 4 of them).
The problem is that the nrf51822 BLE device accepts 3.6V as maximum voltage, and the Nema17 2.6V, and I don’t want to break them both with a too high voltage, so I tried to reduce it with a voltage divider but it reduce the current to 0A, so nothing work after.
So I need to reduce the voltage to make everything safe but I also need to keep the current at it’s maximum level.
I was thinking to use a voltage regulator like this one : https://www.newark.com/rohm/ba33dd0t/fixed-ldo-voltage-regulator-3v/dp/08X1226

But I Wonder if the output current of 2A is not too high for my stepper motor.
I really hope that someone can help me,
Thank you very much!


#2

Alkaline AAA (or AA) batteries cannot supply that amount of current. Don’t forget that each winding of the motor draws current, up to 1.7 Amperes each.

Use a different, higher voltage power supply, as limited by the stepper driver, and carefully follow the directions on the motor driver product page to set the current limit to less than 1.5 Amperes.


#3

What kind of power supply would you recommand me, because it is for mbed device, so it needs to be a battery, but as I told before I am a complete beginner and have no idea what is the best.
Also you are saying that each of the winding of the stepper motor draw current up to 1.7A, but the AAA batteries that I use draw a current of 1.8Amp on my ampermeter, so I Don’t know why it won’t be enough.
And on the documentation it is written 1.68A for rated current, is it really Dangerous for the motor to use a few more?
Thank for your attention and your reply, I really appreciate =)


#4

That would be for a “short circuit” using the current scale of your multimeter.

To supply each winding with 1.5 Amperes (the upper limit of the motor driver you have), the power supply should be able to provide 3 Amperes. 5xAA NiMH batteries should work, or use a 7.4V LiPo battery intended for radio control vehicles.


#5

I Don’t understand why I would need 7.4V for a LiPo battery, why such a high voltage whereas I only need 2.8V for the motor?
Also I will certainly need to use a LDO to regulate the voltage and the current output of the corresponding LDO that I found is 1.5A, so we would I need 3A in input?
Thanks!


#6

The voltage rating of the stepping motor is largely meaningless, as such motors are current driven.

The motor driver takes care of all the requirements, but you MUST adjust it correctly by following the Pololu engineer’s clear instructions.

You should study this tutorial to learn more about how steppers work: http://homepage.divms.uiowa.edu/~jones/step/


#7

Thank you very much, this is helpfull!