MakerKiln

Stepper Driver Vref

The voltage to set on an A4988, DRV8825, TMC2209 or TMC2208, worked from the sense resistor that is actually on your board. Every formula here is quoted from the maker's own datasheet.

Driver

What you want

Set Vref tolive
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Formula used:

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Local onlyThe arithmetic runs in this tab, your driver and sense resistor choice stay in this browser's local storage, and the page works offline once loaded. Nothing is uploaded.

Setting the current on a stepper driver is two numbers multiplied together, and almost every wrong answer on the internet comes from the same place: someone hard-coded the sense resistor. That resistor is the pair of small black rectangles beside the driver chip, its value differs between manufacturers and between production runs of the same board, and the current you get scales directly with it. Get it wrong by a third and your motor runs a third hot or a third weak.

So this tool will not guess it. The sense resistance is a required input, the presets say which manufacturer published which value, and the ones we could not source are marked as uncited rather than dressed up as facts.

Three things worth knowing before you turn the pot

The A4988 changed in 2017, and most guides did not

Pololu's own product page states that all units manufactured since 2017 have 0.068 ohm current sense resistors, and that units made before that had green 0.050 ohm resistors. Plenty of pages still print 0.05 for a genuine board and 0.1 for a clone. On a real post-2017 board those two guesses land in opposite directions: 0.050 gives you about 74 percent of the current you asked for, and 0.100 gives you about 147 percent of it. The second one is the dangerous direction, because nothing on the machine tells you it is happening until something is too hot to touch.

"Vref = Irms x 1.41" is a TMC2209 rule for one specific board

The datasheet formula is Irms = 325 mV / (Rsense + 20 mohm) x 1/√2 x Vref / 2.5 V. That 20 mohm is resistance inside the chip, between the sense pin and the comparator, and it is added to whatever resistor you fitted. At Rsense = 0.110 ohm, which is what BIGTREETECH, FYSETC and Watterott all use, the whole constant collapses to exactly √2 and you get the familiar 1.41. Change the resistor and the shortcut breaks, because the fixed 20 mohm does not scale with it: on a 0.150 ohm board the 1.41 rule delivers 76 percent of the current you asked for, and on a 0.100 ohm board it delivers 108 percent.

Your DRV8825 is probably running below the voltage TI recommends

TI's recommended operating conditions for the DRV8825 give V(VREF) a minimum of 1 V and a maximum of 3.5 V, with a footnote that the part is "operational at VREF between 0 to 1 V, but accuracy is degraded". On a Pololu carrier with 0.100 ohm resistors, 1 V is a 2 A current limit. Almost every 3D printer axis is set well below that, which means almost every DRV8825 in a printer is running in the region TI describes as degraded. It works, and people have run them that way for a decade. It is worth knowing anyway, because it tells you not to spend an evening chasing a five percent current discrepancy that the datasheet never promised to hold.

Where every number on this page came from

Read on 19 September 2026. If one of these pages changes, this one is wrong until we re-read it.

  • A4988: Vref = 8 × current limit × Rsense; 0.068 ohm since 2017, 0.050 ohm (green) before; coil current in full step is 70 percent of the limit Pololu A4988 carrier product page · pololu.com/product/1182
  • DRV8825: current limit = Vref / (5 × Rsense); sense amplifier gain 5 V/V; V(VREF) recommended 1 V to 3.5 V, "operational at VREF between 0 to 1 V, but accuracy is degraded"; worked examples at 0.25 ohm and 0.2 ohm Texas Instruments DRV8825 datasheet SLVSA73F (April 2010, revised July 2014), sections 7.3, 8.3.2 and 9.2.2.2 · ti.com DRV8825 datasheet
  • DRV8825 carrier: sense resistors are 0.100 ohm, so Vref = current limit / 2; carrier held to 2.2 A by those resistors Pololu DRV8825 carrier product page · pololu.com/product/2133
  • TMC2209: Irms = 325 mV / (Rsense + 20 mohm) × 1/√2 × Vref / 2.5 V; full-scale sense voltage 325 mV at vsense = 0; internal 20 mohm between the sense pin and the comparator; a reference below 1 V gives "reduced analog performance" TMC2209 datasheet Rev. 1.09 (16 February 2023), section 9 and the DC characteristics table on page 79 · the PDF we read, Adafruit's mirror of the Analog Devices document
  • TMC2209: leaving Vref open does not give full current. Revision 1.07 of the datasheet corrected the open-pin behavior "from 100% of current (2.5V) to 73% (1.8V)" TMC2209 datasheet Rev. 1.09, document revision table · same PDF
  • TMC2208 and the rest of the TMC220x family: the same standalone formula, printed with the same 20 mohm term TMC220X / TMC222X datasheet Rev. 1.01 (16 November 2016), section 9 · the PDF we read, MikroElektronika's mirror. Noted, not resolved: that document's own DC characteristics table on page 72 gives the internal resistance as 30 mohm, which disagrees with the 20 mohm in its formula. We use the 20 mohm the formula prints. On a 0.110 ohm board the choice moves the answer by about 7 percent, and if you have a later revision of that datasheet it is worth checking which figure it carries.
  • 0.110 ohm sense resistors on the common TMC2209 boards BIGTREETECH wiki ("Our 2209 drivers utilise 110 mΩ RSENSE resistors") · FYSETC Silent2209 wiki, which also publishes the matching "Vref = Irms x 1.41" and the instruction not to have the motor connected while adjusting · Watterott SilentStepStick documentation
  • Sense resistor values for clone boards Not sourced, and marked that way in the tool. A clone's resistor is whatever its maker fitted, and there is no datasheet to appeal to. Read the marking on the part: R050 is 0.050 ohm, R068 is 0.068 ohm, R100 is 0.100 ohm, R110 is 0.110 ohm.

Questions this tool gets asked

What Vref should I set for a 1 A stepper motor?

There is no single answer, because Vref depends on the sense resistor fitted to your board. On a Pololu A4988 made in 2017 or later the sense resistors are 0.068 ohm, so a 1 A current limit is 8 x 1 x 0.068 = 0.544 V. On a Pololu DRV8825 the resistors are 0.100 ohm and the same 1 A limit is 0.500 V. On a BIGTREETECH or FYSETC TMC2209 with 0.110 ohm resistors, 1 A RMS is 1.414 V. Same motor, three different voltages. Read the resistor value off your own board and enter it above.

How do I find my driver's sense resistor value?

Look at the two small black rectangles either side of the driver chip, usually just below it. The value is printed on them in a shorthand where the letter R stands for the decimal point: R050 is 0.050 ohm, R068 is 0.068 ohm, R100 is 0.100 ohm and R110 is 0.110 ohm. If you bought a genuine Pololu board, its product page states the value. If you bought a clone, the marking on the part is the only thing that is actually true about your board, and it is worth a magnifying glass.

Is Vref = Irms x 1.41 correct for a TMC2209?

Only on a board with 0.110 ohm sense resistors, which is what BIGTREETECH, FYSETC and Watterott fit. The datasheet formula is Irms = 325 mV / (Rsense + 20 mohm) x 1 / root 2 x Vref / 2.5 V, and at Rsense = 0.110 the constant collapses to exactly root 2, which is where the 1.41 comes from. On a 0.150 ohm board the same rule gives you 76 percent of the current you asked for; on a 0.100 ohm board it gives you 108 percent. The 20 mohm term is internal resistance inside the chip and it does not scale with your resistor, which is why the shortcut cannot be reused.

Why does the A4988 formula use 0.068 and not 0.05?

Because Pololu changed the part. Its product page states that all units manufactured since 2017 have 0.068 ohm current sense resistors, and that units made before that had green 0.050 ohm resistors. A lot of guides still print the older figure. If you use 0.050 in the formula on a board that is really 0.068, you set about 74 percent of the current you wanted. If you use the 0.100 that some pages quote as a common value, you set about 147 percent of it, which is the direction that overheats a motor.

Should the motor be connected when I measure Vref?

No. Measure with logic power applied and the motor unplugged, black probe on ground and red probe on the Vref point. FYSETC's instruction for its Silent2209 is explicit about not having the motor connected while you adjust the pot. Never unplug a stepper motor from a driver that is powered up. Pololu's carriers bring the reference voltage out to a via circled on the underside silkscreen, which is easier and safer to touch than the pot itself.

What is the difference between the current limit and the coil current?

On an A4988 or DRV8825 the number the formula gives you is the current limit, which is the magnitude of the current vector across both coils. Pololu states that in full step both coils are on and held to roughly 70 percent of that limit, so a 1.4 A limit measures about 1 A in one coil. It also means that when you switch to microstepping, the current in a single coil can rise back up to the full limit. This tool shows both numbers so you can see which one you are looking at.

Why does my DRV8825 Vref come out below 1 volt?

Because the Pololu carrier's 0.100 ohm resistors put a 1 V reference at a 2 A current limit, and most 3D printer axes run well under that. It is worth knowing that TI's recommended operating conditions give V(VREF) a minimum of 1 V, with the footnote that the part is operational between 0 and 1 V but accuracy is degraded. The driver still works. It just means the last few percent of current accuracy is not something the datasheet promises at that setting, so chasing a 5 percent error there is chasing noise.

Does anything I enter leave my browser?

No. The arithmetic runs in this tab, your driver and sense resistor choice are remembered in this browser's local storage only, and the page works offline once loaded. Nothing is uploaded.

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