How to use a 3.8V stepper with Marlin / RAMPS?

Question

I recently purchased a small stepper motor with a hollow shaft, without realizing it ran at 3.8V. I'm looking for the easiest way to integrate it with the ecosystem around the 12V steppers used in 3d printing, including control by Marlin firmware, and also compatibility with mainstream printer boards.

The rated specs are as follows:

    2 phase 4 wires
    1.8 step angle
    Rated Voltage: 3.8V
    Rated Current: 0.8A
    Body Length: 30mm
    Shaft Diameter: 5mm

I have only A4988 drivers, and I was under the impression that those drivers could only work at 12V. I have two spare DC-DC buck converters that could drop a 12V output from something to 3.8V. But I'm not sure what wires to do that for.

I know that motors have "A" and "B" coils for movement. Do I just use two DC-DC bucks off an A4988 driver to power this thing? So, after the output from the A4988 driver, I would connect 1A and 1B to IN+ and IN- on the DC-DC buck, and 2A 2B on IN+ IN- on the second DC-DC buck.

Is there a better way to integrate this motor into an existing 3D printer board based on RAMPS? I'm using MKS Gen and MKS Sgen boards

Using the pins before the driver might not be such a good idea if 800 mA is needed to run the printer. I don't know whether those pins connect to 12V or to the microcontroller output.

Answer 1

The 3.8 V rating does not mean what you think it does. "Rated voltage" has a very specific technical meaning. For a 3D stepper motor to work properly, the rated voltage of the stepper motors actually needs to be significantly lower than the supply voltage of the stepper drivers.

These steppers are perfectly compatible with a standard 12V RAMPS setup with A4988 system. You do not need to and should not mess around with buck converters. All you need to do is adjust the potentiometer on your stepper drivers to limit the current to at most 0.8 A.

The rated voltage in some sense refers to the average voltage the stepper motor should see, but to actually run them at reasonable speeds you need to supply short bursts of higher voltage. A stepper driver and motor actually form a sort of buck converter themselves, and in some sense do the conversion from 12V to the rated voltage for you.