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Total newbie, issue with Pololu step-motor, trying to build a simple motorized syringe pump

Hi everyone,

thanks for your help.
I am really, REALLY ignorant about all this, as I am a neuroscientist and I have just a minimal knowledge about electronics. I tried googling around and studying a bit, but I am still confused. I write asking your help because I can’t seem to be able to figure out a couple of things in a small project. I am trying to build a motorized syringe pump to deliver sugar solution to reward animals (simplified version of what is described here: https://github.com/chen42/openbehavior/blob/master/operantLicking/Assembly.md ).
In the “POWER” paragraph, they indicate that I would need to cut a 12V AC-DC converter and connect it to a DC-DC step-down set to 5V. Basically the output (5V) should go towards a mini-USB connector that (from what I understand) should power the raspberryPi. On the side of the input to the DC-DC step-down, from the figure, I should connect wires that will power the Pololu step-motor. If I understand correctly, this would bring the 12V to the step motor, right? From the specifications in the Pololu webpage, it looks like the step-motor needs only a minimum of 3.8V, but from the Q&A I understand that a higher voltage can and should be applied, as the motor (through the step motor controller) will draw power (670mA) as needed.
Now, my question is: instead of using the 12V AC-DC converter and then scaling down the DC-DC with the step-down, can I use the AC-DC converter that outputs 5V that I got with the raspberryPi? In principle, could I just open the wire, branch it (doubling the wires, basically) and power both the raspberry Pi and the step-motor? Would this work, or am I completely not understanding how all this works?

Thanks a lot.

Massimo

Hello, Massimo.

Based on what you told me about your setup it sounds fine to power both your Raspberry Pi and your stepper motor through a sufficient 5V AC-DC converter instead of a 12V AC-DC converter and a DC-DC step-down regulator. However, there a few trade-offs you are making by doing that you should be aware of.

First, by reducing the stepper motor driver’s supply voltage you will probably reduce the maximum step rate you can get from your motor, and the current draw from your converter will be greater since it has to supply the same power from a lower voltage. You also will need to keep track of how far your Raspberry Pi is from your converter to make sure the voltage drop across the wires is not significant enough to cause problems.

- Patrick