# Powering two stepper motors with A4988 drivers

New to this site and wanted advice and sanity checking on my project design. I know there are similar questions to this but I couldn’t find one that was closely similar to mine.

I’m creating a device the uses two NEMA 17 stepper motors (https://www.omc-stepperonline.com/nema-17-stepper-motor-bipolar-l48mm-w-gear-raio-1001-planetary-gearbox-17hs19-1684s-pg100.html) with chunky 100:1 gearboxes in addition to using 1/16 microstepping. The end result of this is to achieve a stepper motor that turns very slowly (minutes per revolution).

Each motor is rated at 2.8V and 1.68A per phase (3.36A total since two phase) and I’m using an A4988 driver for each of them (https://www.amazon.co.uk/gp/product/B07FKXXJ41/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1) (note that these are Chinese versions where RCS = 0.1Ω). Since the A4988 drivers are limited to 1A without additional cooling, I’ve used the onboard potentiometer to reduce Vref to just below 0.9V to prevent overheating (using the equation Vref = 8 x Imax x RCS from Pololu).

This should mean that consequentially the torque output from each motor is reduced by a factor of 1/(3.36) = 0.3 (as torque α current) since each motor is only being supplied with 1A (which is not an issue due to the large gearing ratio giving a suitable output torque). I think 1/16th microstepping might also reduce the torque slightly, but I couldn’t find any conclusive sources that didn’t contradict each other on this.

This gives a power requirement of 2.8 x 1 = 2.8W per motor, hence 5.6W in total. So, I plan to use a power source that provides at least 3A (surplus on the 2A required by the drivers) and rated at a voltage of 12V-24V to run the stepper motors. I’m also aware that I need to make sure I power the drivers in parallel from this source, and not in series to reduce strain / overheating.

I’ve included two power sources I was thinking of using:

2: 24V, 15A, 360W PSU Transformer Adaptor (https://www.amazon.co.uk/NEWSTYLE-Universal-Regulated-Switching-Transformer/dp/B0758BNMLH/ref=sr_1_3?keywords=PSU+24V&qid=1565084433&s=electronics&sr=1-3)

Overall question is: would either of these power supplies be suitable, I think the second might be a bit overkill but i'm not sure.

Here’s the circuit diagram for reference:

Apologies for the long post, just needed to get all the information out of my head. I’m curious to see if I’ve made any errors in my calculations / assumptions, so any help is greatly appreciated.