I have two questions regarding my first design - a simple speed/direction controller for a small (24V, 0.7A) stepper motor.

How it works:

12V from DC plug is converted to 5V to power Atmega and a4988 (driver) and to 24V to power motor. Atmega controlls stepper speed and direction by reading two user inputs: 3-pos rotary switch (CW, Off, CCW) and 10k potentiometer (speed control).

enter image description here Questions:

  1. are decoupling capacitors ok? anything is missing?
  2. I'm aiming at EMC compliance in the near future, what other obvious things I should consider at this stage? (my next question will be about PCB design but first things first)

Thank you!


step-up converter

voltage regulator


a4988 stepper driver module

a4988 complete driver


  • \$\begingroup\$ LOTS of problems with boost converter. Examine all your impedances and current limits. Start with motor DCR, Cap ESR, Inductor DCR, U4 Ron. Then examine step responses. E-caps are too small and must specify low ESR directly under /beside IC. \$\endgroup\$ Aug 13, 2020 at 20:33
  • \$\begingroup\$ Hi @Tony I often see your answers for topics of my interest and I really appreciate what you're doing, thx! Regarding your comment - bipolar motor DCR 4.2Ohm/phase, C13 900mOhm (well.. not low), Inductor DCR 36mOhm, U4 Ron 80-150mOhm. Questions: boost conv caps (C3,C13) size is from U4 spec, they should be bigger or just low ESR? Motor is 2.9v 0.7A but on a prototype board with similar components (atmega, a4988, mt3608) it shows 0.1-0.2A max for 24V \$\endgroup\$
    – Jannki
    Aug 14, 2020 at 7:57
  • \$\begingroup\$ The resulting RLC determine the resonant frequency and Q of the switched filter SMPS which may be analyzed on Falstad’s Bode for filter response estimates then copied to time domain for comparison of overshoot and risetime. I recall my stepper drivers with DRV8866 using 150uF across the IC with low ESR T=ESR*C <2us. But for C13 there is also your lead-lag compensation, so the entire converter dynamics for over shoot and resistive surge drops. It can resonate with subharmonics with no load if not careful but not sure if you reduce holding current after steps \$\endgroup\$ Aug 14, 2020 at 12:22


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