3D Printing Calculators

FDM calibration & firmware

Stepper Motor Vref Calculator

Set the tiny potentiometer from a number, not from “a bit more than the last printer”. Each driver family turns Vref into current differently: A4988 and DRV8825 set a peak current limit, TMC2208/2209 scale an RMS current through a different reference. Measure Vref on the probe pad with the driver powered.

Inputs

Units
Ω

Read the board. 0.11 Ω is common on TMC sticks; A4988 carriers range from 0.05 to 0.2 Ω and DRV8825 sticks are usually 0.1 Ω.

A

RMS current per coil, often 50–80% of the motor’s rated current. At about 70%, the peak (and the A4988/DRV8825 current limit) equals the rated current. 0.8 A is a mild NEMA 17 starting point.

Results

Vref
1.131V

DC voltage from GND to the Vref pad/pot wiper. On TMC2208/2209, a result above 2.5 V is out of reach for that sense resistor.

Peak current
1.131A

I_rms × √2, the top of the coil-current sine. On A4988 and DRV8825 this is the current limit Vref sets; in full-step mode the coils only reach about 71% of it.

The Result is indicative and educational. It does not constitute a design, a technical opinion or medical advice and does not replace calculations or verification by a person with the appropriate qualifications and authorisations. Before purchase, installation or use in practice – in particular in electrical installations – the Result should be verified by a qualified specialist. Details: Terms of Service, point 4.

Example products

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Calculation method

Microstepping drivers push each coil current along a sine, so its peak is √2 × the RMS current you ask for. A4988 and DRV8825 set that peak, their current limit, from Vref: I_limit = Vref / (8 × R_sense) on the A4988 and Vref / (5 × R_sense) on the DRV8825, so Vref = 8 × R_sense × I_peak or 5 × R_sense × I_peak. TMC2208 and TMC2209 in standalone mode scale their full-scale current by Vref / 2.5 V. Their datasheet gives I_rms = (Vref / 2.5 V) × 0.325 V / (R_sense + 0.02 Ω) / √2, where 0.02 Ω is internal resistance in series with the sense resistor, so Vref = 2.5 × √2 × I_rms × (R_sense + 0.02) / 0.325. On the common 0.11 Ω sticks that is about 1.41 × I_rms. Sense resistance is in ohms, current in amperes and Vref in volts; none of these fields change with the imperial toggle.

Exact formula

I_peak = √2 × I_rms. A4988: Vref = 8 × R_sense × I_peak. DRV8825: Vref = 5 × R_sense × I_peak. TMC2208/TMC2209: Vref = 2.5 V × I_peak × (R_sense + 0.02 Ω) / 0.325 V. Limits: Confirm R_sense on the silkscreen. TMC VREF stops scaling at 2.5 V. Not for TMC current set over UART.

StealthChop vs torque

TMC in stealth mode can stall at high speed even when Vref is “correct”. Raise current or use spreadCycle for fast travel.

Frequently asked questions

Klipper already has run_current.

If you drive TMC over UART, ignore this pot math and set run_current in amps. This page is for standalone / silent-stepstick pots.

My motor is 1.5 A rated.

Start around 0.8–1.1 A RMS unless the motor stays cool and you need the torque. Heat is the limit, not the label.

A4988 vs DRV8825 — same formula?

No. The A4988 limits current to Vref / (8 × R_sense), the DRV8825 to Vref / (5 × R_sense). With the same 0.1 Ω sense resistors, a DRV8825 needs 5/8 of the A4988’s Vref for the same current. Both limits are peak currents, so the tool multiplies your RMS target by √2 first.

Why do TMC guides say Vref = I_rms × 1.41?

That is this formula with 0.11 Ω sense resistors: 2.5 × √2 × 0.13 / 0.325 ≈ 1.41. With 0.15 Ω resistors the multiplier is about 1.85, so reusing the 1.41 rule on those boards leaves the motor under-powered.

Where do I probe Vref?

Ground clip on a GND pin, probe on the pot wiper or labelled VREF pad. Drivers must be powered. Do not short the neighbouring pins.

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