A stepper driver sends the motor a current, and too much cooks the motor while too little loses steps. Vref is the little voltage that sets it. This gives the target Vref for your driver and motor, so you turn the pot to a number instead of guessing.
How to use it
- Choose your driver (A4988, DRV8825) and its sense resistor value if it differs from the default.
- Enter the motor's rated current, and the running fraction you want (70 to 90 percent is usual).
- Read the target Vref.
- Power the board, measure Vref between the pot and ground, and turn the pot to match.
What it does
The right formula per driver
A4988 and DRV8825 use different multipliers and often different sense resistors. Pick the driver and the maths is handled.
Running current, not rated
It sets to a safe fraction of the motor rating by default, which is what keeps motors cool and prints reliable.
Honest about TMC
For UART TMC drivers it tells you to set current in firmware instead, rather than giving a Vref that does nothing.
What people use it for
A new control board
Building or upgrading a printer with A4988 or DRV8825 drivers. Set each axis to a correct Vref before the first move.
Motors running hot
A stepper too hot to touch is over-current. Lower the target current and re-set Vref to cool it down.
Skipped steps or layer shifts
Layer shifts under load can be too little current. Nudge the running fraction up and re-set.
Good to know
- Measure Vref with the board powered but motors idle, probing between the driver's pot and a ground point. A tiny ceramic screwdriver avoids shorting.
- Sense resistor value is printed on the driver or in its datasheet. Getting it wrong is the most common Vref mistake, so check it.
- Set current to the motor rating, not the driver maximum. A driver rated higher than the motor will happily cook it.
- After setting Vref, run the axis and feel the motor after a few minutes: warm is fine, too hot to hold is too much.
Questions
What is Vref?
A small reference voltage on a stepper driver that sets the motor current. Measure it between the driver's adjustment pot and ground, and turn the pot to reach the target this tool gives.
What is the formula?
It depends on the driver. For an A4988, Vref equals current times 8 times the sense resistor. For a DRV8825, Vref equals current times 5 times the sense resistor. Enter your driver and the tool uses the right one.
What current should I set?
Not the motor's full rated current. Set the running current to about 70 to 90 percent of the rating, so the motor runs cool and does not lose steps from overheating.
What about TMC drivers?
On TMC2208 and TMC2209 in UART mode, current is set in firmware with M906, not by a Vref pot. If your board uses these in UART, set the current in your config, not with a meter.
Also called
Known as a stepper Vref calculator, A4988 Vref calculator, DRV8825 current calculator, or stepper current calculator. It sets motor current safely by giving you a target voltage.