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I was wanting to use this motor: https://www.circuitspecialists.com/nema_34_step_motor_85bygh450A-08.html rating shows: 4 amps and 1.05 Ohms
which would be 4.2V?

and possibly wanted to use this power supply: https://www.circuitspecialists.com/12-volt-8-amp-power-supply.html

it has an adjustable output Voltage control, which I believe I should adjust to get the V- and V+ to be 4.2V based on the motor.

I have previously controlled a stepper motor with the big easy driver from sparkfun: https://www.sparkfun.com/products/12859 I would set M1, M2, or M3 HIGH or LOW to microstep then pull step low and high to step. The power supply Im looking to use doesn't have those input pins that the driver had and my understanding get a little unclear at this point.

My question is how can I control the motor with a microController?

will I need to use transistors between the V+ and the motor to control it in one direction? Would I need transistors between V- and the motor also to control direction?

Or should I be looking for a driver still?

The part about looking for a driver that confuses me is that the driver I have used in the past had Voltage inputs to the board then the inputs from the MC would control the motor wires, but this power source has inputs for the motor wires but it doesnt seem to have inputs to control via a microController, so I am definitely misunderstanding something there

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My question is how can I control the motor with a microController?

Simple answer - you can't.

First, output voltage. Yes, you can (within limits) adjust the output voltage. What are those limits? From the data sheet:

Output Voltage Adjustment: +/- 10%

That is, by adjusting a small potentiometer on the supply, you can vary the output from 90% to 110% of the nominal 12 volts, or from 10.8 to 13.2 volts. This is nowhere near the 4.2 volts you're looking for.

Second, control inputs: there aren't any.

The way you use this is by applying power to turn it on, and removing power to turn it off. That's it.

Your observation that "it doesnt seem to have inputs to control via a microController" is spot on, but your hope that you can somehow use it as a controller anyways is what is causing you trouble.

TL;DR - You can't do it, and you need a controller as well as a power supply.

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Since you seem to be a newbie on stepper drivers and don’t indicate awareness of Imax avg with PWM at12V 30% is 4A and inductive motors integrate current and generate back EMF and Max current indicates ~ 60’C motor temp rise without fans , I suggest you buy a higher current driver and learn how it works with a CNC shield gCode Panel and reduced idle current. Your surge current will be 3x with 12V but could be made faster with high power and voltage supply where Imax can be 8-10x rated current then reduced to 10% duty cycle using higher V+ and consider that with heatsinks and microfan , but depends how fast and cool you need.

It would be best if you had a design performance spec for acceleration. Velocity, mass , friction then decided how to drive motor. Basic Physics

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