Bipolar Stepper Motor w/ Darlington Array

A big lesson in the ordering of parts was learned, but the result is that I have ended up with three bipolar stepper motors and a lot of ULN2003A and no power supply. My understanding is that I will need to switch the Darlington array with an H-Bridge. I'd like to double check first before going through the reordering process. I don't have much experience with darlington arrays so I'd greatly appreciate any advice.

[Mod edit
Please note that @randomuser1219 has 3 questions open on related but separate subjects. These are:


Let the ULN2003A stay deep down in a drawer. They are almost useless for steppers. They also loose a lot of voltage. Buy stepper drivers.

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use something like

or if you want an almost silent running stepper-motor use TMC2209

For all stepper-motordrivers it is very important to connect/disconnect the motorwires only if the power-supply is shut-off.
If you connect or disconnect the motorwires while the power-supply is switched on you get the electric-fence-effect of high voltage-spikes that will immidiately damage/destroy the driver

You can drive them by two inputs direction and step
each pulse results in one step or microstep. Those drivers can be configured to make 1/16 or even smaller microsteps which means one rotation is 200*16 = 3200 steps or a single step is 360/3200=0,1125 degree.
best regards Stefan

Thank you! So two of the motors are coded to run in conjunction and the third has a separate task. They all run off of an Arduino nano, so is there a solution where I could use one driver for all three motors?

no. Just one motor per driver.
Three stepper-motors require three stepperdrivers

You cannot really control a bipolar stepper with a ULN2x03 anyway, can you? :face_with_raised_eyebrow:

The original order was for two unipolar steppers but the person in charge of purchasing found a better "deal" than the link I gave them (Three bipolar steppers for a cheaper price). They didn't understand that unipolar and bipolar stepper motors are not equivalent. So I'm working with what I have.

I quoogled for bipolar stepper motor ULN2003
and found this website with very good introductional material about unipolar and bipolar stepper-motors and how to drive them.

the driver A4988 has the same basic principle of step/direction-input as the DRV8825 and TMC2209. The A4988 has less current than the DRV8825

Well explaining to them now we have to buy new drivers to make the bipolar-stepper-motors work is an important experience "A quick decision for a "cheaper" solution - might turn out to be more expensive"

next thing is to compare the torque of your unipolar-stepper-motor and the torque of their "cheaper" bipolar ones.

best regards Stefan

Stepper motors can be very different regarding their electrical characteristics. Its not sufficent to distinguish between 'bipolar' and 'unipolar'.

Please provide a link to the motors.

I am very puzzled how this motor can be a replacement for a unipolar stepper controlled by a ULN2003. I don't know any unipolar stepper that is comparable in size and power that could be driven by an ULN2003 (maybe with paralleld outputs? ). Even the drivers suggested above are already at or beyond their power limit with the linked stepper.
What is the application and what was the original intended motor?

The ULN2003 was something that was definitely a mistake, I do not think it would work the way I would want it to. Here is the link to the original motor specs ( I'm pretty new to stepper motors, so I really don't know much outside of what youtube has taught me.

I am building an arm that can rotate 180 at the base and 90 degrees vertically. So one stepper will be at the base, and on top of the rotating pad will be two steppers that will move in synchronization to move the arm, which will likely be quite heavy and weighted at the end (thus the two steppers as opposed to one).

Well 1,26 Nm is quite some power. If you are planning to put such a stepper-motor to the free end of an arm you have to deal with a lot of torque simply created through the length of the arm multiplied with the weight of the arm.
Can you post a picture of how this arm will look like and post the dimensions?

I think if you want motors on the foreend of a longer arm you should use a strong RC-servo-motor
RC-servo-Motors that can pull 30 kG will be in the range of 60-100 grams of weight.

example here an example at 6V 30 kg weight only 63 gram 10% of the stepper-motor

Please post a picture of the mechanic how your device will look like.
It highly depends on the design if this will work with stepper-motors.

usually such robot-arms are tooth-belt-driven which enables to keep all the weight of the motors in the base
See example here

or something like this
best regards Stefan

While my project is by no means a secret or important, I am not permitted to post dimensions at this time or the CAD models. I did the math to figure out the holding torque the motors will need to have based on loading. The choice to use stepper motors was not wholly mine, I'm just the research student. I code, CAD, and get things to work to the specification of the project. This is a proof of concept, so I just need to get it to work, then I can go in and optimize. So as of now its a breadboard that will probably melt, and the materials I have been provided with and a limited budget for ordering.

If what I do works, the money will come and I can begin getting into motor selection.

If you ordered steppers with Darlingtons. They may the best possible choice or you got the wrong motor's.
I think steppers and drivers get some misinformed responses.

Stepper drivers can be expensive pin saving devices. Or the best solution.
But any suggestion WITHOUT understanding you APPLICATION is useless and may waste time and money.

If the steppers can run your project successfully with the the darlintons then the are the perfect solution.

If you find they are too weak then you can get more power by increasing voltage.

Increasing voltage means you undersized your motors or you have correctly engineered your selection.
Higher voltage requires a chopper stepper driver.

Darlintons only allow nameplate volts and amps.PCB bus ba

Stepper drivers offer microstepping which makes slow speed operation smoother but reduces high speed top end.

I am curious about the thrust bearings you intend to use to support the weight and movement of your arm. Do you have a drawing of the assembly?

Clearly some actual engineering is needed to know torques and loading.
the 'package' of stepper with Darlinton are a great package and work well when there is not a high torque requirement. it seems you did not get that package.

Post #10, shows 2.8 amps and screams for a high power stepper driver.
2.5 volts is also excellent voltage for a motor, you can drive it with 24 volts and get some power out of it (10 times nameplate is a good voltage starting point)

a robotic arm is not going to need high speed unless you are tossing baseballs or something.

as a note, I am miffed that no one asked for a link to the motors you have. It is impossible to offer guidance when the parts are unknown.

In Post12 you linked to a bipolar motor. that is a good looking motor. If that is the motor you have, then you will need a driver able to run it.
pololu offers a wide array of drivers with much information on how-to. should answer your questions.

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Good thing I'm an engineer. I am using the motors linked in post 12 along with TB6612FNG as the driver and have scrapped the Darlington array idea. I've included the preliminary CAD drawing, the arm does not bend and is not to scale. The base rotates 180 and the arm rotates 90. Scale is completely disproportionate, but the idea is the same. There will be no need for high speed movement.

Oh, I apologize, the link to the motors I have and will be using is this: Nema 23 Bipolar 1.8deg 1.26Nm ( 2.8A 2.5V 57x57x56mm ...
thank you for your assistance