Confirming validity of schematic for dual stepper motor PCB

Thread Starter

ProbablyHuman

Joined Jan 13, 2021
1
I'm trying to make my first schematic for a PCB that would sit atop an RPI and allow for control of two stepper motors. I have an LM2576 to take 12v from the motor supply and step it down to 5v for the RPI, and I took the schematic from Pololu's DRV8225 as the driver for the motors.

I did however modify a couple of things, such as add a single capacitor to act as voltage spike protection for both the LM2576 and the two DRV8225 ics. I also tethered reset and sleep to 5v on the board, and enable as I would want to keep the driver running so long as the board is on.

I am just very worried that if I order the PCB there's no turning back, so I want to make sure I have everything right before I start. My biggest ask is that on Pololu's schematic for the DRV8225, they have this circuit separate from the ic off to the side, I assumed that if it changed the power supply, I would only need one per board despite having two of the driver ics.

Here is my schematic so far, which I have designed in easy eda
My Schematic.png
 

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Sensacell

Joined Jun 19, 2012
2,718
Truth is- every new PCB is a test PCB

You cannot anticipate every problem, I take the attitude that the first version is always going to be wrong in some way.
Make one quickly, test it, fix the problems, and make the final version. It's faster than trying to agonize over every detail.
 

Phil-S

Joined Dec 4, 2015
180
Have you actually tested the proposed circuit as a prototype?
You would be a lot more confident if you saw it working first
 

jeffl_2

Joined Sep 17, 2013
32
The DRV8825 (NOT "8225"!) is a microstepper control. Your design is treating it as if it were a legacy "R/L" design where the way you limit the motor current is by restricting the drive voltage to the "nameplate" voltage. But a microstepper has a dynamic current regulator so you're not restricted like that, you can increase the chip supply (and therefore coil drive) voltage so instead of "nameplate" voltage you drive it with "maximum compliance" to help you build up the current quickly despite the limitation of the coil inductance. In my testing this allows microstepper control to get as much as 25 times as much power to the shaft (11.8 times the speed, 2.2 times the torque) as a non-microstepped "R/L" design, but you would want to use the highest voltage supply (40-45 volts) in order to leverage that, on the other hand you can only go as high as the chip's coil current vs. voltage curve allows you to, on this part you can only go to 24V at 2.5A. And a high current means you want to make sure that you do a decent job putting a ground plane under the center of the chip to help the thermal dissipation (PCB heat sink). This ultimately also bears on the selection of the current/voltage tradeoff of the stepper motor coils themselves. I know I've raised a lot of issues here but I wanted to make sure you understand you have a lot of options that impact directly on performance that you can make now that you may not have after you finalize the PCB. Also there are a lot of inexpensive imported assembled microsteppers that use various chips to offer you drive options that go from 40 volts and 3.5 amps (TB6600 class) to as high as 80 volts and 8 amperes (DSP-based).
 
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