A4988 Stepper Driver

Current Quick Formula.

The A4988 limits per-phase current using the reference voltage (Vref) on the trim pot:

I_max = Vref / (8 × R_SENSE)

• R_SENSE is the value of the little current-sense resistors on the module (look for markings like R050 = 0.05 Ω, R068 = 0.068 Ω, R100 = 0.10 Ω).
• This current is the peak per phase limit (what the driver enforces); choose a target around 70–85% of your motor’s rated phase current for reliability/cooling.

What You Need.

• Multimeter (DC volts, 0–2 V range).
• Small (ideally ceramic) screwdriver.
• Your control board powered with VDD (logic) and VMOT (motor).

Steps (do this once per driver).

• Power the board (VMOT + VDD). It’s fine if motors are connected but never plug/unplug a motor with power on.
• Place black probe on GND, red probe on the metal of the trimpot (or the exposed Vref pad if present).
• Read Vref.
• Adjust: turn the trimpot clockwise to increase, counter-clockwise to decrease (some clones reverse this—watch the meter).
• Set to your target Vref using the tables below.
• Test: run the axis. If you get missed steps, bump Vref a little; if the motor/driver runs hot or noisy, lower it. Add a heatsink/fan if you’re above ~1 A.

Common Targets (Handy Cheat Sheet).

If R_SENSE = 0.05 Ω (R050):

Vref = I × 0.40

0.50 A → 0.20 V

0.80 A → 0.32 V

1.00 A → 0.40 V

1.20 A → 0.48 V

1.50 A → 0.60 V (aggressive; cooling required)

If R_SENSE = 0.068 Ω (R068):

Vref = I × 0.544

0.50 A → 0.27 V

0.80 A → 0.44 V

1.00 A → 0.54 V

1.20 A → 0.65 V

If R_SENSE = 0.10 Ω (R100):

Vref = I × 0.80

0.50 A → 0.40 V

0.80 A → 0.64 V

1.00 A → 0.80 V

1.20 A → 0.96 V

Practical Tips.

• Start modest (e.g., 0.8–1.0 A) and tune up only if you see skipped steps.
• Microstepping doesn’t change the limit you set—it just changes how the current waveform is apportioned—so you still tune to the motor’s phase rating.
• Keep the driver below ~90 °C case temp; if it’s too hot to touch or you get thermal shutdowns, lower Vref or improve cooling.
• For quiet, reliable printing: proper belt tension + reasonable acceleration/jerk often helps more than cranking current.